I will summarize the status and some recent highlights of our Chandra mission and take a look forward to events over the next 2 years, including our plans to celebrate our 20th anniversary year in 2019.
A brief epoch of rapid mass transfer is expected to occur in most accreting binary systems, and can readily exceed the Eddington mass accretion rate by up to several orders of magnitude. We should therefore expect super-Eddington accretion to be wide-spread. I will discuss the observational signatures of these systems, how it can explain ultraluminous X-ray sources (ULXs) and the exciting physics we can glean from their study.
We have obtained near-simultaneous Swift/XRT imaging and Gemini GMOS spectroscopy for the ultraluminous X-ray source (ULX) NGC 300 ULX-1 (formerly designated SN 2010da). The observed X-ray emission is consistent with an inhomogeneous wind that partially obscures a central, bright inner accretion disk. We simultaneously fit eleven 0.3-10 keV spectra obtained over a ~1 year time period (2016 April to 2017 July) using a partial covering model, and find that although the covering fraction varies significantly (from 78% to consistent with 0%), the unabsorbed luminosity remains essentially constant across all observations (2-6 × 1039 L☉). This is significantly higher than the variable X-ray emission previously observed with Chandra. A relatively high 0.3-10 keV fractional variability amplitude (Fvar) of ~30% is observed in all eleven observations. Optical spectra from Gemini exhibit numerous emission lines (e.g., Hα, Hβ, He II λ4686) which suggest that the neutron star primary is photoionizing material in the immediate vicinity of the binary. By comparing the He II λ4686 line luminosity (~7-9 x1035 L☉) to the contemporaneous soft X-ray emission, we find the X-ray emission is broadly consistent with the observed He II line luminosity. The combination of our X-ray observations and optical spectroscopy suggest that geometric beaming effects in the ULX-1 system are minimal, making ULX-1 one of only a few bona fide ULXs to be powered by accretion onto a neutron star.
Ultracompact X-ray binaries (UCXBs) are binaries with a neutron star accretor and an orbital period less than 80 minutes. While these systems have been relatively well-studied from observational and theoretical standpoints over the past two decades, there are a few UCXBs with X-ray fluxes several orders of magnitude higher than their predicted long-term average accretion rates would imply. In this talk I will focus on the unique, bright UCXB 4U 1626-67. The donor star in this system has been variously suggested to be a highly evolved helium star or a white dwarf, with a CO or ONeMg composition. Our results from the Chandra-HETGS and LETGS unambiguously confirm previous reports of anomalously high neon abundance in the system while simultaneously ruling out the presence of magnesium in the X-ray spectrum, a state which is in conflict with all previously-suggested donor compositions for this source. I will review the existing literature on this source and UCXBs in general and present some possible paths forward to reconcile these apparently-contradictory results.
Ultraluminous X-ray sources (ULXs) probe the accretion physics in the extreme accretion rate regime. Common suspects for the mechanism that powers the most luminous X-ray stellar sources, are super-Eddington accretion, beaming and sub-Eddington accretion on intermediate-mass black holes. Understanding these scenarios depends on pinning down the nature and evolution of the binary systems. The importance of studying ULXs is further highlighted by the fact that (i) a fraction of them are considered progenitors of gravitational wave sources and (ii) are seen as important players in the heating of the universe during the epoch of reionization. Individual studies are challenged by the difficulty of measure the mass of the compact object (and hence its nature) and the detection of the donor star (and hence the evolution of the system). Statistical studies though, offer a chance to study the ULXs as a class through (i) the connection of the populations with the bulk properties of the host galaxies, and (ii) the comparison with binary population synthesis studies. Aiming for the largest statistical study of ULXs, we create a tabulation of galaxies in the local universe (<200 Mpc) with robust distances and a large set of multiwavelength data, ultimately providing estimates on star formation rates, stellar masses and metallicities. After cross-matching with the Chandra Source Catalog 2.0, we find ~900 galaxies that host ULXs. We study the connection of the number of ULXs and their luminosities with the host galaxy properties and discuss them in the context of population synthesis studies.
Ultraluminous X-ray sources with a Wolf-Rayet donor star are the most likely progenitors for LIGO merger events. They are accreting at a super-Eddington rate for ~1e5 yr, likely via Roche lobe overflow; after core collapse of the donor star, the system can turn into a double black hole binary with an initial orbital period of a few hours. By contrast, HMXBs with a supergiant or main sequence donor can also produce black hole-black hole binaries, but with binary separations that are too large for inspiral and merger within a Hubble time. Only a handful of ultraluminous Wolf-Rayet HMXBs have been identified so far. We will present the X-ray spectral and timing properties of the best-known example, located in the Circinus Galaxy. Its intriguing dipping and eclipse behaviour is different from the type of orbital modulations seen in other classes of HMXBs. We argue that such lightcurves are a defining property of this class of super-Eddington HMXBs, in which both the primary and the secondary launch dense, fast outflows with similar kinetic power. We suggest that the occulting material is dense shocked gas between black hole and donor star, and in a bow shock ahead of the black hole.
Novae in our neighbour galaxy Andromeda (M31) have always been at the forefront of research on accreting white dwarfs. From the first discoveries a century ago, enriching the debate on the nature of the nebulae, until the various professional and amateur surveys today we have found more than one thousand of these fascinating stellar eruption. This unparalleled sample allows us to study the physical properties and distributions of various sub-classes of novae from a population point of view. In this talk I will summarise recent results from multi-wavelength observations of M31 novae and discuss their relevance and connection to new theoretical nova models in the context of extragalactic nova science. I will give a special focus to Recurrent Novae (with more than one recorded eruption) and outline their population properties, rates, and their importance for the single-degenerate channel of type-Ia supernova progenitors.
Magnetically driven accretion in white dwarfs (WDs) provide a unique understanding of highly magnetized environments. Polars, a subclass of cataclysmic variables, have accretion columns formed when the magnetic WD channels the partially ionized outer atmosphere from a red-dwarf star towards the WD's surface via Roche lobe overflow. The accretion column emission dominates a wide wavelengh range: from X-ray to the IR domain. The optical and IR emission is mainly produced by cyclotron emission, resulting in high degrees of polarization, typically up to 20%. Here I present the results of broadband optical polarimetry models of 5 polars from which important physical parameters are derived: the electronic density and temperature along the column, the magnetic field in the region and its geometry. Additionally, to assess the optical polarimetry modeling performed, I simulate Spectral Energy Distribution covering X-ray to optical/IR ranges and compare them with available measurements in the literature.
The galactic novae are a direct consequence of accretion onto a white dwarf in a close binary system. In most (if not all systems) the accretion resumes following the nova event. In some cases the disk emission dominates the light from the system as the nova fades. I shall present and discuss the evidence for high mass accretion rate disks in two recent novae. NR TrA (Nova TrA 2008), an optically-thick (Fe II type) nova developed eclipses and a hot spectrum optical spectrum reminiscent of the persistent super-soft sources (SSS) in the optical high state, about 3 years after peak. For the past 6 years it has remained more-or-less static. The orbital period is 5.25 hours; the eclipse is deepest and narrowest in the UV. There is no X-ray eclipse. The X-ray spectrum is well-described by an 11 eV comptonized blackbody, but an active accretion disk with inner disk temperature 25 eV, or a simple BB of similar temperature, also fit well. Time-resolved optical spectra reveal the orbit of the white dwarf. The optical-UV spectral energy distribution (SED) is well-fit by an active accretion disk, with a mass accretion rate of 10-7 solar masses/yr at 10 kpc. This may suffice to support steady nuclear burning on the surface of the white dwarf. After the ejecta of the fast He-N type nova V1535 Sco (N Sco 2015) became optically-thin (about 28 days after peak), the OIR SED (B through K) could be described as the sum of an active accretion disk and the K5 III donor star. Over the past 3 years the mass accretion rate has steadily decreased from about 5x10-6 to 3x10-8 solar masses per year. In these two very different novae, accretion disks have re-established themselves and started to dominate the light curve. In both cases, irradiation of the donor star is probably critical for establishing the disk accretion rate. NR TrA has established a precarious equilibrium: in V1535 Sco the K giant is again establishing dominance. I shall present the observational evidence that supports these scenarios, discuss the models, and make the case that recent novae are not only individually interesting, but are laboratories for the study of high mass accretion rate disks.
Low-mass X-ray binaries (LMXB) are binary systems harbouring a neutron star or a black hole accreting mass from a companion star less massive than the Sun. They provide us a unique scenario to study in great detail both the accretion process and geometry, and the fundamental properties of the compact objects. Its optical counterparts are typically faint and highly variable sources: ~20th. mag in quiescence, varying on time scales from milliseconds to years. Therefore, very detailed optical studies of these objects are only at the reach of large telescopes. We will present GTC-10.4m spectroscopy of the optical counterpart of the neutron star transient system Aquila X-1 during three consecutive outburst in 2011, 2013 and 2016. We observed strong high excitation emission lines arising from reprocessing on the donor star. We carried out Doppler mapping in order to determine the radial velocity of these features (Kem). Since this velocity traces the motion of the irradiated, inner side of the companion, Kem is smaller than the radial velocity of its centre of mass (K2). We determine the so-called K-correction, by combining Kem with K2 (previously obtained by our group from VLT-8.2m infrared spectroscopy; Mata Sanchez et al. 2017). The K-correction is closely related with fundamental parameters of the system and can be expressed as function of the mass ratio of components and the accretion disc flaring angle. For first time, we placed strong constraints to the accretion disc vertical size (opening angle) using direct measurements and detailed modelling (Jimenez-Ibarra et al. 2018, MNRAS). Our results are consistent with theoretical predictions for highly irradiated accretion discs.
In the magnetospheric accretion model for T Tauri stars, a shock is expected to form on the stellar surface when matter lifted from the disk free-falls onto the star following magnetic field lines. For typical free-fall velocities, soft X-rays form in the shock, some of which are expected to escape and correspond to cooler emission observed in X-rays. Observations of T-Tauri stars have broadly supported this picture, where the X-ray emission of a young star could be modeled with two components. One of them corresponding to emission from coronal gas with temperature T ~ 1e7 K and electron density ne ≤ 1010 cm-3. The other component corresponding to a cooler denser plasma attributed to the accretion shock (T ~ few x106 K; ne~1011-1012 cm-3). However, high resolution Chandra grating observations of accreting T-Tauri stars have demonstrated the X-ray emission to be more complex and it is no longer clear that this soft component originates in the accretion shock. In order to probe the origin of the X-ray emission from T-Tauri objects, we have undertaken a program to study the temporal evolution of the accretion flow on the archetypal T-Tauri star - TW Hya - with Swift. These observations enable a study of the hard and soft components detected in X-rays simultaneously with the UV flux. As the UV flux is directly related to the accretion shock emission, these observations provide a direct test of the X-ray/accretion connection in TW Hya. Herein, we will present the results of this program and discuss the results in the context of the nature of the accretion flow onto T-Tauri stars.
Accretion disks and associated outflows are ubiquitous in astrophysics. They are found in systems ranging from young stellar objects, to compact binaries containing white dwarfs, neutron stars and black holes, to active galactic nuclei and quasars. Remarkably, all of these classes share a rich and common phenomenology. This phenomenology includes "outbursts" -- transient increases in the accretion rate -- during which systems cycle through distinct spectral "states". These states are also associated with other key properties, such as mass loss in the form of collimated jets or bipolar disk winds, and both periodic and aperiodic variability. In this talk, I will explore the similarities between disk-accreting systems on all scales and propose a way to turn this phenomenology into physics. Doing so will require an ambitious multi-wavelength observational campaign. I will outline the requirements of this campaign, which we have provisionally dubbed GOALS: The Great Observatories Accretion Legacy Survey.
Essentially all low-mass X-ray binaries (LMXBs) in the soft state appear to drive powerful equatorial disc winds. A simple mechanism for driving such outflows involves X-ray heating of the top of the disc atmosphere to high temperatures. At large radii, the thermal speed exceeds the escape velocity, and mass loss is inevitable. Here, we present the first coupled radiation-hydrodynamic simulation of such thermally-driven disc winds. Initially, we have adopted parameters representative of the wind-driving LMXB GRO J1655-40 and our model yields a mass-loss rate that is more than twice the accretion rate. This agrees well with the mass-loss rate inferred from Chandra/HETG observations of the source at a time when the system had a similar luminosity to that adopted in our simulations. We also present synthetic line profiles for hydrogen and helium like Iron, which show similar line equivalent widths to the observations, although the outflow velocities are much lower than inferred from observations. We then increase the luminosity towards the Eddington rate, and note that the wind efficiency (wind mass-loss rate/accretion rate) tends to a constant value, although the velocity of the outflow increases with increasing source luminosity.
The rich phenomenology characterizing low mass X-ray binaries (LMXBs) --- including different spectral states, state transitions, and quasi-periodic oscillations --- poses many theoretical challenges. Prime among these are the role of magnetic fields and X-ray irradiation. Focusing on the physics of disk winds launched in the high/soft state, in this contribution we present our recent work on combining thermal and magnetic driving in simulations performed with the new MHD code Athena++. These magnetothermal simulations are the first of their kind, as the heating and cooling rates associated with X-ray irradiation can be tailored to a given system if its spectral energy distribution is known. Our first paper (Waters & Proga, submitted to MNRAS) addresses the effect of adding a large scale poloidal magnetic field to a Compton heated disk wind in low-luminosity systems where the radiation force can be neglected, leaving only thermal and magnetic driving as candidate wind launching mechanisms. Our solutions show, contrary to expectations, that adding magnetic fields suppresses rather than enhances the thermal wind. This result further supports the analysis of the LMXB system GRO J1655-40, which is highly suggestive of a magnetic origin for the wind well within the Compton radius where thermal driving is strongly inhibited by gravity.
RW Aur is a binary system composed of two young, low-mass stars. The primary, RW Aur A, has undergone visual dimming events (Δ V =2-3 mag) in 2011, 2014-16, and 2017-2018. Visual and IR observations indicate a gray absorber that moved into the line-of-sight. This dimming is also associated with changes in the outflow. In 2017, when the optical brightness was almost 2 mag below the long-term average we triggered a Chandra observation to measure the absorbing column density NH and to constrain dust properties and the gas-to-dust ratio of the absorber. In 2017, the X-ray spectrum is more absorbed than it was in the optically bright state (NH = (4±1) × 1023 cm-2) and shows significantly more hot plasma than in X-ray observations taken before. Also, a new emission feature at 6.63±0.02 keV (statistic) ±0.02 keV (systematic) appeared indicating an Fe abundance an order of magnitude above Solar, in contrast with previous sub-Solar Fe abundance measurements. Comparing X-ray absorbing column density NH and optical extinction AV, we find that either the gas-to-dust ratio in the absorber is orders of magnitude higher than in the ISM or the absorber has undergone significant dust evolution. Given the high column density coupled with changes in the X-ray spectral shape, this absorber is probably located in the inner disk. We speculate that a break-up of planetesimals or a terrestrial planet could supply large grains causing gray absorption; some of these grains would be accreted and enrich the stellar corona with iron which could explain the inferred high abundance.
I review the key physical processes that operate in accretion flows with the emphasis on those processes that are important in driving mass outflows. Numerical simulations can help us not only to better understand these processes but also their interplay which is typically very complex. I will show some examples of some recent simulations and discus how they inform us about physics of mass accretion and ejection and also how they help us to interpret observations.
Accretion is a fundamental physical process that occurs on many different scales throughout the universe. Accretion on to black holes and neutron stars is known to produce strong outflows in the form of collimated radio jets and dense disk winds. Understanding the detailed properties of these different types of outflows, and their connection with the accretion properties, is important for a variety of science goals. For instance, these outflows can carry away a significant fraction of the accretion power and can strongly impact their environment. Furthermore, the loss of energy through outflows can significantly alter the evolution of interacting binary systems. In this talk, I will review recent observational progress in understanding jets and winds from accreting black holes and neutron stars in X-ray binaries.
The most sensitive HETG observations of stellar-mass black holes in outburst deliver excellent spectra of disk winds. In some cases, third-order spectra can be utilized; with a resolution of 15 eV in the Fe K band, these spectra sit halfway between the resolution of standard first-order HETG spectra and the calorimeter data anticipated with XARM. Much faster and more highly ionized outflows are detected with the aid of third-order data, increasing by orders of magnitude the mass outflow rates and kinetic power inferred from disk winds. This talk will present an overview of these spectra, methods of inferring launching radii, and consequences for wind driving mechanisms, fundamental disk physics, and binary evolution.
X-ray observations performed during the last few decades have provided a rich data base on accreting black holes X-ray binaries. A strong coupling between the properties of the accretion flow and the presence of outflows, such as radio-jets and hot X-ray winds, has been found to be a fundamental characteristic of black hole systems. In 2015 we discover a sustained optical accretion disc wind, simultaneous with the radio jet, in the prototypical black hole transient V404 Cygni (Munoz-Darias et al. 2016, Nature 2017 MNRAS). Here, I will present optical spectroscopic observations of black hole systems in outbursts, which reveal that cold optical winds with terminal velocities above 1000 km/s might be a common feature in these objects. I will discuss the nature of these winds as well as their impact in the general context of black hole accretion.
We developed a functional 3D astrophysical model to produce synthetic light curves of IC10 X-1 a close X-ray binary system consisting of a Wolf-Rayet (WR) star and a compact object accreting material in presence of radiatively-driven stellar wind of the WR. This system has long been a keen interest to astronomers due to its wide dip at mid-eclipse in broad X-ray band showing evidence of an extended corona of the emitter region. We produced light curves in two different energy bands using column integral values of the absorbing material in the stellar wind at different phase values. We explored two steady state wind density structures that of a constant velocity wind and a constant acclerating wind. Different parameters were explored in addition including but not limited to, orbital inclination, physical size ratio, density structure, wind composition, etc. The resulting light curves from the model showed energy dependence and evidently seemed to confirm that the source is being eclipsed by the thick core of the WR wind instead of a physical eclipse
Evidence is growing that the interaction between outflows from active galactic nuclei (AGN) and their surrounding medium may play an important role in galaxy evolution, i.e. in the regulation of star formation in galaxies, through AGN feedback processes. Indeed, powerful outflows, such as the ultra-fast outflows (UFOs) that can reach mildly relativistic velocities of 0.2-0.7c, could blow away a galaxy's reservoir of star-forming gas and hence quench the star formation in host galaxies. The low-redshift (z=0.184) radio-quiet quasar PDS 456 has showed the presence of a strong and blueshifted absorption trough in the Fe K band above 7 keV, that has been associated with the signature of such a fast and highly ionized accretion disk wind of a velocity of 0.25-0.3c. This persistent and variable feature has been detected in many observations of PDS 456, in particular by XMM-Newton, Suzaku and NuSTAR, together with other blueshifted absorption lines in the soft energy band (e.g. Nardini et al. 2015, Reeves et al. 2016) and a possible highly blueshifted line around 11 keV from an even faster wind of a velocity of 0.45c (Reeves et al. 2018). I will present here the results of the analysis of recent and contemporaneous high-resolution Chandra/HETGS and NuSTAR observations of PDS 456. We confirm the presence of two UFOs in PDS 456, considering our dataset and previous observations, that may impact the host galaxy.
I will present recent results from our study of the highly-absorbed high-mass X-ray binary GX 301-2 using two NuSTAR observations. We find that the best-fit model requires two separate cyclotron resonant scattering features (CRSFs, cyclotron lines), i.e., broad absorption throughs in the hard X-rays, at ~35keV and ~50keV. These CRSFs are overlapping strongly and could only be disentangled thanks to NuSTAR's superior energy resolution. As the energies of the lines are not harmonically related, we interpret them as being formed in the same accretion column, but at different heights above the neutron star surface, where they sample different magnetic field strengths. By performing phase-resolved analysis, we find strong variations of the energy of the 35keV line. Using a newly developed code describing relativistic light-bending around neutron stars as well as cyclotron resonant scattering in a thin layer around the accretion column we explain this phase-dependent behavior of the cyclotron line purely as a function of viewing angle over which the projected velocity and beaming factor towards us changes. For this model we assume a certain emission pattern of the accretion column, based on physical modeling of the in-falling material in a strong magnetic field. I will put our results in the context of other known CRSF sources and provide a short outlook on how the model can be applied to other sources as well.
Relativistic disk lines in neutron star low-mass X-ray binaries provide a valuable tool to determine the neutron magnetic field strength, the extent of a boundary layer, and even place a limit on the radius of the compact object itself. Using NuSTAR, we have recently been able to obtain measurements of the inner disk around neutron stars that are unbiased by pile-up effects. Now, NICER affords the opportunity to search for low-energy relativistic lines down to 0.25 keV using detectors that are also free of distortions at high flux levels. The combined bandpass and sensitivity of NuSTAR and NICER opens a new opportunity to capture multiple emission features, and to utilize them to measure and map out different observables within the disk. I will present results from the first simultaneous NuSTAR and NICER observations of the neutron star LMXBs Serpens X-1 and 4U 1735-44.
High resolution X-ray spectroscopy is a powerful means to investigate the accretion process in young low-mass stars. X-rays are in fact expected from the accretion-shock region, where the accreting material, because of the high infall velocity, is heated to temperatures of a few million degrees as it continues its inward bulk motion in the stellar atmosphere. To detect for the first time the predicted motion of this X-ray emitting post-shock material, we searched for Doppler shifts in the deep Chandra High Energy Transmission Grating observation of TW Hya. This test allows us to definitely assess the nature of this X-ray emitting plasma component in young accreting stars, and constrain the accretion stream geometry. We searched for Doppler shifts in the X-ray emission from TW Hya with two different methods: by measuring the position of a selected sample of emission lines and by fitting the whole X-ray spectrum. To check the absolute wavelength calibration of the Chandra gratings we also analyzed a sample of Chandra/HETG spectra of non-accreting stars. Both methods show that on TW Hya the plasma at T~2-4 MK has a radial velocity of 38±5 km s-1 in the stellar reference frame. Conversely the X-ray spectra of non-accreting stars do not show any shift, confirming the excellent accuracy of the Chandra gratings. This result certifies that the X-ray-emitting material at 2-4 MK originates in the post-shock region, at the base of the accretion stream, and not in coronal structures. The observed infall velocity is very similar to that of the narrow component of the Civ resonance doublet at 1550 Å, also ascribed to the post-shock region. Therefore the plasma at 2-4 MK and that at 0.1 MK likely originate in the same post-shock. Finally, these results provide strong constraints on magnetospheric accretion geometry. In fact, the comparison of the observed radial velocity with the inferred intrinsic post-shock velocity, 110-120 km s-1, suggests that the footpoints of the accretion streams on TW Hya are located at low latitudes on the stellar surface, indicating that complex magnetic field geometries, such as that of TW Hya, permit low-latitude accretion spots.
We present a new technique for the prediction of Fe Kα profiles directly from general relativistic magnetohydrodynamic (GRMHD) simulations. Data from a GRMHD simulation are processed by a Monte Carlo global radiation transport code, which determines the X-ray flux irradiating the disk surface and the coronal electron temperature self-consistently. With that irradiating flux and the disk's density structure drawn from the simulation, we determine the radiation field within the accretion disk from photoionization equilibrium and solution of the radiative transfer equation, including all relevant physical processes---this is accomplished via a new disk reprocessing code, PTRANSX, in conjunction with XSTAR. This yields an outgoing spectrum at each point on the disk surface including the Fe Kα photons, which are then ray-traced to an observer at infinity to produce a line template ready for observational comparison; for the 10 M☉ case we have examined already, both the shapes of the line profiles and the equivalent widths of our predicted Kα lines are qualitatively similar to those typically observed from accreting black holes. Most importantly, this technique allows for the translation of state-of-the-art global simulation data into readily observable spectral data, allowing for more direct comparison between theory and observation.
The gas dynamics of very weakly ionized protoplanetary disks is largely governed by non-ideal MHD effects including Ohmic, Hall and ambipolar diffusion. I will briefly review the theoretical and computational studies on PPD gas dynamics over the past years, and present global disk simulations aiming to incorporate the most realistic disk microphysics, which include ionization, chemistry and radiation. Net poloidal magnetic flux is the key to drive disk accretion and evolution. The bulk part of PPDs are likely to be largely laminar/weakly turbulent, with complex internal flow structures. Accretion is accompanied by outflows that are magneto-thermal in nature with significant mass loss comparable to the accretion rates. Observational evidence and implications on planet formation will also be briefly discussed.
In the sub-Eddington regime the X-ray luminosity function of high-mass X-ray binaries in star forming galaxies can be universally described by a nearly perfect power law stretching over ~3 decades in luminosity. This can be used to infer the slope of the underlying distribution of massive binaries over the mass transfer rate Ṁ. As mass transfer is regulated by the binary system parameters and properties of the donor star, one may expect that the Ṁ-distribution does not change at the Eddington limit of the compact object and continues with same slope towards higher mass transfer rates. Knowing the LX and Ṁ distributions, we can compute the population average radiative efficiency of accretion in HMXBs. The so determined radiative efficiency is virtually constant at low LX, in accord with our initial assumption. It starts to decline at log(LX)≈ 38.5, roughly the Eddington luminosity limit of a neutron star, and drops down by a factor of ~10 in the ULX regime, at log(LX)~40. The overall dependence of the luminoisty on the mass transfer rate is well described by the ∝ 1+ln(ṁ) law, predicted in a broad class of models of super-Eddington accretion. The consequence of the low radiative efficiency of accretion in the ULX regime is that typically ~90% of mass in such systems must be lost in outflows, in agreement with detection of photoionized nebulae around many ULXs.
The accretion process in the most massive binary stars is of great interest, partly because they represent the leading channel for production of double-degenerate systems (e.g. binary black holes), such as those observed merging as gravitational wave events. In young starburst galaxies, the relics of the most short-lived and hence most massive stars could dominate the X-ray binary (XRB) population. In the closest such galaxy, IC 10, we have made a multi-wavelength study of these objects. Employing a decade of Chandra observations, and optical imaging, we report an 8-sigma correlation between the celestial coordinates of the X-ray and optical catalogs. Applying color-magnitude selections to isolate blue supergiant (SG) stars, we find 16 SG-XRB candidates. These objects exhibit systematically higher X-ray/optical flux ratios than other stars in the same magnitude range, and several are X-ray variables. The sample includes the famous eclipsing WR+BH binary IC 10 X-1, which has been viewed as an exemplar of the progenitors for BH+BH mergers. Somewhat disturbingly the periodic doppler-shift of the star's optical He II line shows a phase-relationship to the X-ray eclipse that is contrary to that expected for binary motion. A similar picture has emerged for its near twin, NGC 300 X-1, casting doubt on the mass-functions of all WR+BH binaries. We report progress in characterizing young massive XRBs and understanding the complex interactions between the radiation field of the compact object, and the stellar-wind of the companion.
Chandra observations of nearby galaxies have given us new insights into the populations of accreting binaries. They have allowed us to probe X-ray binaries down to unprecedented luminosities and in very diverse environments. The wealth of these data provide a direct measurement of the formation rate of X-ray binaries as function of their age and their X-ray luminosity functions. Both are key components for constraining the formation and evolution pathways of accreting binary systems and their end-points (e.g. binary compact object systems). I will present results from recent studies of X-ray binaries in nearby galaxies focusing on: their formation rate in different environments, the shape of their X-ray luminosity functions, and the constraints we can set on the nature of these systems based on X-ray and multi-wavelength observations. Finally, I will discuss prospects for this field with future X-ray observatories.
There has been long-standing debate on how to decompose the continuum X-ray spectra from neutron-star (NS) low-mass X-ray binaries (LMXBs), due to the complication by the presence of emission from the boundary layer between the accretion disk and the NS surface. Here I briefly review the long-term progress that we have achieved in understanding the spectral evolution of these objects from super-Eddington accretion to near quiescence, based on modeling of X-ray spectra from thousands of RXTE observations of Galactic objects, megasecond Chandra observations of nearby galaxies, and the recent survey of M31 by NuSTAR. Based on our model, the spectral evolution of NS LMXBs is similar to black hole X-ray binaries in many aspects, e.g., a dominant thermal accretion disk with a constant inner radius at the ISCO in the thermal state. One main difference is that NS LMXBs has an optically thick boundary layer of a small, constant emission area and the temperature increasing with the accretion rate. The bright sub-class of NS LMXBs, the so-called Z sources, is characterized by the super-Eddington accretion state, with the inner accretion disk radius and the boundary layer emission area increasing with the accretion rate due to the reach of the local Eddington limit. Different subclasses of NS LMXBs are thus explained by different accretion rates, not by different inclinations or magnetic fields.
An X-ray pulsar (XRP) is a highly-magnetized neutron star (NS) that rotates while emitting beams of radiation produced primarily in the vicinity of its magnetic poles. If these beams happen to cross our line of sight and the NS's spin and magnetic axes are not aligned, then our telescopes detect it as a periodically pulsating source. With the introduction of a new class of orbit-based observatories over the last quarter of a century the field of X-ray pulsar astronomy has seen an influx of high-resolution data. This windfall demands new models of pulsar behavior and emission geometry be created and subsequently fit to this high-quality data. We have written a model (Polestar) in Python that mathematically represents a simplified XRP. The code has ten different, tunable geometric parameters, born from physical considerations, which may be individually incorporated or suppressed. Any given XRP has a unique pulse profile which is often energy-dependent, and changes with different luminosity states. A change in luminosity coincides with a change in the system (e.g. a periodic Type-1 outburst is triggered following periastron passage, or the orientation of the decretion disk around the donor star has changed), and as such an increase in luminosity tends to produce an increase in complexity of the accompanying pulse profile. If a particular source in a low-luminosity state can be fit well with Polestar incorporating only a few parameters then an underlying geometry may be inferred. Further, if profiles from the same source in higher-luminosity states can be fit with the addition of only one or two additional parameters it will serve to further solidify current XRP theory (e.g. the emergence of fan-like emission patterns, or the vertical growth of the accretion column). Specifically, this can serve as an independent means of determining the critical luminosity (when the accretion column is high enough to produce significant fan-beam emission) for a given source. Our initial fitting campaign was directed at the ~100 XRPs in the Small Magellanic Cloud. The preliminary results from this campaign will be presented along with an overview of the model and the fitting procedures we employed.
The population of very faint X-ray binaries (VFXBs) display accretion luminosities of Lx \0x223C1034\0x221236 ergs/s and never become brighter. They have opened new, exciting windows for accretion studies but their existence also raises some intriguing questions. In particular, the persistent systems accreting at very low accretion rates (persistent VFXBs) challenges the standard and well tested transient/persistent paradigm for X-ray binaries explained by the disc instability model (DIM); a possible solution requiring they having very short orbital periods, even within the ultra-compact regime. Optical studies carried out so far on these sources are compressed by only 2 cases, which results have provided contradictory conclusions. I will review the state-of-the-art of the field and will present a global X-ray analysis of a sizable fraction of the known population of Ultra-compact X-ray binaries. In the second part of my talk I will focus on the persistent VFXB 4U1802-12. I will present optical (GTC-10.4m spectroscopy) and X-ray (Suzaku) observations, which are able to test the possible connection between sub-luminous behaviour and ultra-compact nature.
Transitional millisecond pulsars (tMSPs) are a newly recognized population of compact neutron star binaries that switch between clearly distinct accreting and rotation-powered pulsar states. A deluge of observational studies within the past few years have revealed intriguing behavior from tMSPs spanning from radio frequencies to the GeV gamma-ray range. I will present an overview of our extensive coordinated multi-wavelength observing campaigns of tMSPs and how they offer fresh insight into the physics of accretion onto magnetized objects, particle acceleration in shocks, and compact binary evolution.
An extremely promising probe into compact binary formation and evolution will be the populations observed by both gravitational wave (GW) and electromagnetic (EM) telescopes, enabling high precision measurement of key properties such as orbital periods, masses, etc. The two upcoming ESA L-class high energy astrophysics missions, which both have NASA contribution, will have a great deal to contribute to our understanding of white dwarf (WD), neutron star (NS) and black hole (BH) systems. For instance, Athena WFI observations will permit constraint of X-ray binary orbital periods to allow searches for the important, yet rare, examples of short-period massive binaries that are on the progenitor path for forming double-compact-object binaries giving rise to gravitational waves. Also in the time domain, Athena WFI's timing resolution will allow study of ULX pulsars out to z~0.1, permitting a detailed analysis of both the accreting column as well as the region of the accretion flow outside the magnetosphere. This may be combined with the independent probe of BH and NS populations enabled by LISA, which should detect hundreds of NS systems as well as many tens of stellar-mass BH systems with constraint on parameters such as eccentricity and masses. At lower masses still, LISA will give us a new probe for finding mass-transferring WD binaries, and is expected to characterize in detail thousands of WD binary systems. Finally, we will also review how the combined power of Gaia and LISA will cover current blind spots in our understanding of the stellar mass BH population.
Globular clusters are expected to have large populations of stellar-mass black holes at early stages in their lifetimes. These stellar-mass black holes were long predicted to have been kicked out of globular clusters through gravitational interactions during the clusters' evolution, with some clusters retaining only one or two stellar-mass black holes and most clusters not retaining any. However, recent discoveries of stellar-mass black hole candidates in globular clusters have called this narrative into question. With the goal of assessing the frequency of accreting stellar-mass black hole systems in globular clusters, and understanding the physics of low-luminosity accretion, we have undertaken a deep radio continuum survey of 50 Milky Way globular clusters using the Karl G. Jansky Very Large Array and the Australia Telescope Compact Array. Here we present our method of selecting candidates, preliminary results, and implications for the dynamical formation of binary black holes observable as gravitational wave sources.
Variability is extremely common among young stars, yet the processes responsible for driving that variability are not well understood. One underlying mechanism for the variability seen in the ultraviolet to optical emission from these stars may be linked to variable rates of mass accreting onto the star via the stellar magnetosphere. Understanding the changes in mass accretion rate gives us insight into the physical conditions in the innermost regions of the disk where the mass is being loaded onto the star. To study this star-disk connection, here we present multi-epoch HST UV to optical spectra for a subset of accreting, young stars. We extract the excess UV to optical emission above the stellar photosphere to measure the accretion luminosity, from which we infer the mass accretion rate onto the star as well as the surface coverage of magnetically funneled accretion hotspots. We find that the accretion rates vary significantly and may be linked to inhomogeneities in the innermost disk. We will discuss the impact of these changing radiation fields on the heating and chemistry of the surrounding disk, which has important implications on planet formation.
With the increasing number of observed magnetic WDs, the role of magnetic field of the WD in both single and binary evolutions should draw more attentions. In this study, we investigate the WD/main-sequence star binary evolution with MESA code, by considering non-, intermediate and high magnetic field WD in the binary. Mainly focus on how the high magnetic filed of the WD (in a polar- like system) affect the binary evolution towards to the type Ia supernovae (SNe Ia). The accreted matter goes along the magnetic field lines and falls down onto polar caps, and it can be confined by the high magnetic field of the WD, so that the enhanced pole-mass transfer rate can let the WD grow in mass even with low mass donors (low RLOF mass transfer rate). The results under the magnetic confinement model show that both initial parameter space for the SNe Ia and characteristics of the donors after SNe Ia are more challenge and compatible with the observations and pervious SNe Ia progenitor models.
Virtually all gravitational mergers, from black-hole/black-hole mergers to white-dwarf/white-dwarf mergers producing Type Ia supernovae, pass through a prior epoch during which mass is transferred onto a compact object. I will report on recent work in which we explore a new class of accretion systems. Mass is donated by a third star in a wide orbit to a close-orbit compact-object binary. The results are typically to shorten the time-to-merger, increase the mass (and possibly change the nature) of the accreting compact objects, and to occasionally produce electromagnetic signatures of the merger. During mass transfer there are observational signatures at X-ray wavelengths that may help us to identify hierarchical X-ray triples. A search of archived Chandra data is under way.
One of the most important problems in the context of cataclysmic variables (CVs) is the lack of observations of systems with periods between 1 and 3.12 hours, known as the period gap. The orbital evolution of CVs with periods shorter than those in the gap is dominated by gravitational radiation while for periods exceeding those of the gap it is dominated by magnetic braking of the secondary star. Spruit & Ritter (1983) have shown that a sharp decline in magnetic dynamo and braking efficiency as periods approach 3 hours and secondary stars become fully convective would result in such a gap. Recent X-ray observations showing coronal magnetic energy dissipation is similar in fully convective and partly radiative M dwarfs cast this theory into doubt. In this work, we use Zeeman-Doppler observations culled from the literature to show that the complexity of the surface magnetic fields of rapidly rotating M dwarfs increases with decreasing rotation period. Garraffo et al. (2018) have shown that the efficiency of angular momentum loss of cool stars declines strongly with increasing complexity of their surface magnetic field. By generating synthetic CV populations, we show that the CV period gap can naturally arise as a consequence of a rise in secondary star magnetic complexity near the long period edge of the gap that renders a sharp decline in their angular momentum loss rate.
I will review what is known about stable mass transfer -- how do we find the mass transfer rates, and what we know about the angular momentum loss processes. I will also pay attention to the conditions that separate the systems evolving via stable mass transfer and the systems starting unstable mass transfer (common envelope event), and how this partitioning affects the resulting population of stellar accretors. The examples of specific stably evolving binary systems will include binaries with pre-main sequence donors, ultraluminous X-ray sources, and Roche lobe overflow binaries with very massive donors. I will also present a new method for constraining the mass transfer evolution of low and intermediate-mass X-ray binaries - a reverse population synthesis technique.
Common envelope phases occur in binary systems when one star evolves to engulf its companion. These interactions transform binaries through the tightening of orbits by drag forces, and they may transform the objects themselves through accretion. This talk will summarize recent findings from hydrodynamic simulations about the nature of gas flow around objects embedded within a common envelope and use these results to discuss the degree of accretion that results. Accretion during the common envelope phase is significant in that it may modify the masses and spins of compact objects passing through such an interaction, potentially with observable signatures for gravitational wave detectors like the LIGO-VIRGO network.
In the low state and quiescent state, black hole X-ray binaries show evidence for a hot accretion flow. X-ray observations indicate the presence of an advection-dominated accretion flow (ADAF) at small radii, surrounded by a thin accretion disk at larger radii. The ADAF naturally explains the presence of jets in these systems.
Although blackbody based models predominate in the spectral modeling of continuum from X-ray binaries and AGN, it is well understood that opacity effects and electron scattering will generally lead to deviations from blackbody emission. These deviations are often formulated in terms of a color corrected blackbody with spectral hardening factor f. Previous theoretical evidence generally suggest f spans a narrow range around f ~ 1.7, but there are some claims that larger values of f might be merited in certain circumstances. We extend TLUSTY based calculations of accretion disk annuli spectra to AGN masses and explore the variation of the spectral hardening factor over a wide range of black hole mass and mass accretion rate, delineating the regions where large spectral hardening factor is present and relating it to "photon starvation" in the underlying disk models.
Even after two decades of observations of the rapidly-spinning stellar mass black hole GRS 1915+105, new data continue to provide payoffs in our efforts to understand the physics of winds and accretion variability around black holes. To that end, I will present new results on GRS 1915+105 from the last year, including a coordinated observing campaign with data from NICER, NuSTAR, and Chandra, as well as an extended series of NICER observations covering a wide range of spectral variability. I will focus on how winds change in response to the X-ray variability and the implications for links between the inner and outer accretion flow.
With a few exceptions, the masses of neutron stars and black holes in X-ray binaries are measured without using distance information. Radial velocities of the companion stars derived from optical and/or infrared spectra are combined with optical and/or infrared light curves to place constraints on the geometry of the system. In favorable cases, the period, mass ratio, and inclination of the binary and the radial velocity amplitude of the companion star can be measured, leading to measurements of the mass of the compact object and the mass and radius of the companion star. If the radius and the effective temperature of the star are known, one can compute its absolute magnitude. The distance to the system then follows from the apparent brightness, after suitable corrections for interstellar extinction and contaminating sources of light are applied. For some X-ray binaries that contain a black hole, the distance to the system (along with the inclination of the binary) can be used to estimate the black hole spin from models of the X-ray spectra. There are a few high-mass X-ray binaries where independent distance measurements are available. For example, the parallax of Cyg X-1 has been measured using VLBI observations, and the distances to M33 X-7, LMC X-1, and LMC X-3 are known by virtue of their memberships in nearby galaxies. For these systems the absolute magnitude of the companion star can be found from the apparent magnitude (once corrections are made for interstellar extinction). The radius of the companion star follows from the luminosity and temperature of the star. The geometrical model that one constructs from all of the observational data (e.g. radial velocity and light curves of the companion star, the rotational velocity of the companion star, etc.) must be consistent with the measured radius of the companion star. At optical wavelengths, the Gaia mission is expected to provide parallax measurements of an enormous number of stars in the Galaxy, including many X-ray binaries. At radio wavelengths, upcoming projects like the Next Generation Very Large Array (ngVLA) could potentially provide astrometric measurements that are a few to several times better than what is currently available. I will discuss how these distance measurements can be used to improve on measurements of the component masses in a few of the well-studied X-ray binaries. In addition to parallax information, these survey missions will also provide proper motion measurements for many X-ray binaries. The availability of distances and three-dimensional space motions for a large sample of X-ray binaries should inform models of the formation of these interesting binary systems.
Over the last twelve years of his life, Jeff McClintock championed efforts to measure the spins of stellar-mass black holes via X-ray spectroscopy. He described this ground-breaking work as the highlight of his career. This is because, spin and mass together - by the no-hair theorem - provide a complete physical description of an astrophysical black hole. This property is stunningly elegant in its simplicity and in stark contrast to the comparatively enormous complexity of the black hole's stellar progenitor. Further, knowledge of black-hole spin is crucial for assessing fundamental questions in astrophysics including understanding how black holes form, the mechanism by which relativistic jets are launched, and more. I will present an overview of X-ray measurements of stellar-mass black hole spins, emphasizing work from the continuum-fitting technique of which Jeff was a principal architect and pioneer.
Accretion into compact objects such as black holes and neutron stars displays a common phenomenology as most physical processes scale with their mass. This is particularly true for their X-ray emission, which is often accompanied by a reflection spectrum produced by the reprocessing of the hard photons in the accretion disk. Detailed modeling of the X-ray spectra provides direct access to the accretion physics on these systems, such as the black hole spin, inner-disk radius, ionization stage, among others. Here we discuss the current state of modern relativistic reflection models that have been recently computed and tailored specifically for three kinds of accreting sources: (1) black hole binary systems in the hard state, where the continuum is now produced by a physical thermal Comptonization model; (2) neutron star X-ray binaries, for which the continuum is better described by a black body emission, thus affecting the ionization state of the reflector; and (3) ultra-compact binaries, were the companion is a carbon/oxygen rich white dwarf, which drastically changes the elemental abundances in the disk. We present the implementation of our new models and their application to several of the systems listed above. We also discuss current outstanding issues in the interpretation of the observational data, with particular emphasis on the advances and limitations of current models for ionized X-ray reflection in strong gravitational fields.
Stellar mass black holes in the Galaxy have all been first detected by their luminous X-ray emission -- either as very bright persistent emission in black hole high mass X-ray binaries (BH-HMXBs), as epitomized by Cyg X-1, or as very luminous (but rare) outbursts of black holes accreting from low mass companions (BH-LMXBs), as first discovered by McClintock and Remillard (1986) for A0620-00. Luminous BH-HMXBs are rare, with only Cyg X-1 as incontrovertible and quite possibly also MWC 656 as well. This is due to the very short lifetimes of their giant or super-giant massive companions that supply the high mass-loss winds required for their luminosity. We report the discovery of a "Cyg X-1 progenitor", the single-line spectroscopic binary (SB1) HD96670 with a "weak wind" O8.5V-IV star feeding a ~6 M☉ BH as required by both its hard X-ray emission detected with NuSTAR (Grindlay et al 2018a) and detailed optical spectroscopy (Gomez and Grindlay 2018). In this talk, we estimate the total population of such SB1 systems in the Galaxy. BH-LMXBs are similar in that most are missed in both X-ray and optical surveys, since they spend >99% of their time in deep quiescence as very low luminosity objects. Their typical <1% Duty Cycle for being in a high accretion state is derived both from X-ray outburst data (Teterenko et al 2016) and now the rapidly emerging ~100 year photometric database being released by the DASCH survey, which has discovered historic transient outbursts for most of the dynamically confirmed BH-LMXBs with low optical extinction (Grindlay et al 2018b). We report on the likely total population of BH-LMXBs within moderately low Av regions and extrapolate to a plausible full population in the Galaxy and implications for their formation vs. NS-LMXBs. This talk is dedicated to Jeff McClintock, who was following the early development of this work.
We report twenty years of optical/IR data from the prototype "dynamically confirmed black hole candidate" A0620-00, for which McClintock & Remillard first reported a mass function greater than 3 solar masses in 1986. We have observed this source at least once per night (weather and season permitting) since 1998, in at least three bandpasses (sometimes as many as six), with the ANDICAM instrument on the 1.0m and 1.3m telescopes at CTIO. We find that the alternation of passive and active states identified by Cantrell et al. (2008, 2010) has continued. One possible explanation for these two states is that flux from a jet is present in the O/IR during the active state. There is also an overall trend over the years toward increased disk luminosity. We may thus be seeing the gradual build-up of the disk as the source progresses toward its next outburst. This buildup may explain the observed orbital period decrease, as angular momentum is transferred from the orbit to the disk.