Chandra's First Decade of Discovery

Compact Objects

Pulsars in X-rays

Werner Becker, MPE-Garching

In February 2009 it was 10 years ago that the ROSAT mission came to its end, whereas in July and December it is a decade that the Chandra and XMM-Newton observatories were launched to continue with the exploration of the X-ray sky. A decade is a good measure to review the progress made by these two observatories in the field of X-ray emission from pulsars. I will further discuss the prospects of studying pulsars with the future missions eROSITA and IXO.

Rapid Cooling of the Neutron Star in the Quiescent Super-Eddington Transient XTE J1701-462

Joel K. Fridriksson, MIT
Jeroen Homan (MIT), Rudy Wijnands (University of Amsterdam), Mariano Méndez (University of Groningen), Edward M. Cackett (University of Michigan), Diego Altamirano (University of Amsterdam), Tomaso M. Belloni (INAF/Osservatorio Astronomico di Brera), Edward F. Brown (Michigan State University), Nathalie Degenaar (University of Amsterdam), Walter H. G. Lewin (MIT)

In the past decade the observing of cooling neutron star transients after long-duration (year or longer) outbursts has entered as a new approach to constraining the properties of matter inside neutron stars. We present Chandra and XMM-Newton observations of the super-Eddington neutron star transient XTE J1701-426 in quiescence. The observations cover the first ≃700 days of quiescence following the end, in August 2007, of a very luminous ≃19-month-long outburst. This data set represents the best- sampled cooling curve of a neutron star transient to date. We also present Rossi X-Ray Timing Explorer and Swift observations made during the final three weeks of the outburst. The transition from active accretion to a quiescent phase dominated by thermal emission from the neutron star is resolved with unprecedented precision compared to other neutron star transients observed after extended outbursts. The observations of XTE J1701-426 represent new parameter space being covered in neutron star cooling, this source having accreted at a much higher level than other observed sources, and the length of the outburst being intermediate between regular transients (outbursts typically lasting weeks or months) and quasi-persistent transients (which accrete for many years or decades). The inferred effective surface temperature at the start of the quiescent phase is considerably higher for XTE J1701-426 than other observed sources, and the timescale of the cooling is much faster, strongly indicating a highly conductive neutron star crust. The quiescent spectra also show a prominent non-thermal component, which has exhibited large and irregular variability. The origin of this component is highly uncertain; one possibility is residual accretion from the companion star.

[PDF of this poster]

CI Cam: A Local Analog to Heavily Obscured AGN?

Michael Garcia, SAO
Elizabeth Bartlett (Southampton), Arvind Parmar (ESTEC)

CI Cam = XTE J04210+560 is an X-ray transient consisting of a supergiant B[e] star and a compact (black hole or neutron star) companion. In 1998 it underwent a 2 Crab x-ray outburst along with a 1 Jy radio outburst. Even in quiescence, it remains relatively bright at ~10-12 erg/cm2/sec (2-10 keV). The X-ray spectrum bears strong similarities to those of obscured AGN in that it is heavily absorbed and shows a strong Fe-Kalpha line (reaching ~1keV equivalent width!). Unlike AGN, a dusty torus around CI Cam can be spatially resolved with IR interferometry, and comparison of this to the radio maps sheds light on the dynamics of the outburst. We report on XMM-Newton observations during quiescence which allow Fe-Kalpha reverberation mapping and an analysis of the quiescent spectrum.

Shock Heating by the SS 433 Relativistic Jets

Herman Marshall, MIT Kavli Institute
Claude Canizares, Norbert Schulz (MIT)

We examine the hypothesis that the X-ray emission from the SS 433 jets results from shock heating by relativistic jets. The main body of evidence comes from high resolution X-ray spectra using the Chandra HETGS. The X-ray spectra show many signatures of shock heating, by analogy to other systems where lower velocity shocks are involved. From density and temperature estimates from the X-ray spectra, the X-ray emission must originate very close to these shocks and the hot gas is advected downstream, cooling significantly via radiative and adiabatic losses. Comparing spectra taken during primary eclipse, it is clear that only the highest temperature portion of the X-ray emitting regions are blocked, placing an additional constraint on the location of the shocks. By relating contemporaneous X-ray and optical observations, we demonstrate that the optical emission originates much further downstream, past the point where the jet starts to clump.

The True Nature of Cygnus X-3's “Little” Friend

Michael McCollough, CXC/SAO

In 2006, Chandra observations of Cygnus X-3 reveled our best view to date of an unique feature associated with this well know microquasar. Extended emission located 16" from Cygnus X-3 was found to be varying in (anti)phase with Cygnus X-3. Also from previous Chandra observations it is shown that the total flux from the feature varies with Cygnus X-3's total flux. If both this feature and Cygnus X-3 lie at the same distance the separation between the two would be 2.4 light years (assume a distance of 9 kpc).From a study of the spectra, flux and time variations of this feature we believe that this feature is a dust cloud that is located along our line of sight to Cygnus X-3. From these observations we are able to deduce the location, size, and properties of this cloud. We will present this analysis and the insights that this give us into the nature of Cygnus X-3 and its environment. We will also present models which will explain why this phenomenon has only been observed, by Chandra, for one X-ray binary. This object represent a discovery that was only possible due to the unprecedented spatial resolution that is only possible with Chandra.

[PDF of this poster]

X-ray observations of the recurrent nova RS Oph in quiescence

Thomas Nelson, NASA/GSFC and UMBC
Thomas Nelson (NASA/GSFC and UMBC), Koji Mukai (NASA/GSFC and UMBC), Marina Orio (UW-Madison and INAF-OAPD), Juan Luna (SAO) and Jeno Sokoloski (Columbia University)

We present Chandra and XMM-Newton observations of the recurrent nova RS Ophiuchi (RS Oph) carried out 1.5 and 2 years after the 2006 outburst. The observed X-ray emission is faint (Lx < 1032 erg s-1) and dominated by soft emission at energies <2 keV. This component is well described by a two temperature plasma model with non-solar abundances consistent with those derived by Ness et al. (2009) from high resolution spectra obtained during outburst. A faint hard component is also detected, and is consistent with a cooling flow typical of accreting white dwarfs, although with a low maximum temperature and implied accretion rate. We discuss the physical mechanisms that are likely responsible for these emission components. We find evidence of high intrinsic absorption in the system which may resolve the apparent contradiction between the low observed X-ray luminosity and the high mass accretion rate onto the white dwarf required to power a nova outburst every ~20 yrs.

Initial identifications of hard and red X-ray sources in the ChIcAGO survey

Bettina Posselt, SAO
the ChIcAGO collaboration

We introduce the “ChIcAGO” (Chasing the Identification of ASCA Galactic Objects) survey, designed to identify the unknown X-ray sources discovered during the ASCA Galactic Plane Survey (AGPS). Little is known about most of these objects, especially those that emit primarily in hard X-rays (2-10 keV). In ChIcAGO, we are combining the subarcsecond localisation capabilities of Chandra with a detailed multi-wavelength follow-up program, with the goal of classifying the >100 unidentified sources in the AGPS.In this poster, we will present some of the initial identification results from optical and infrared follow-up observations of hard (X-ray) and red (optical/infrared) ChIcAGO sources.

New ULX in the collisional ring galaxy NGC 922

Andrea Prestwich, SAO
A Prestwich (CfA), J Galache (CfA), V Kalogera (Northwestern University), T. Linden (UC Santa Cruz), A. Zezas (University of Crete), T. Roberts (Durham University), A. Wolter (INAF, Osservatorio Astronomico di Brera), G Trinchieri (INAF, Osservatorio Astronomico di Brera), R Kilgard (Wesleyan University)

We present a new Chandra observation of the nearby collisional ring galaxy NGC 922. NGC 922 is undergoing a violent burst of star formation as density waves are driven through the disk as a result of the collision. It is similar to the famous Cartwheel galaxy, except that the metallicity is somewhat higher and star formation rate lower. Our primary science goal is to determine whether a low metallicty environment is required to form the most luminous X-ray sources (Lx>1040 ergs s-1). We find a total of 14 bright X-ray sources, 7 of which are ULX ((Lx>1039 ergs s-1). One source has Lx~ 1040 ergs s-1. The X-ray sources are associated with the H-alpha ring. However, they are NOT associated with the brightest H-alpha patches, suggesting that the ULX phase starts >1 x 107 years after the starburst. Like the Cartwheel, the X-ray luminosity function of NGC 922 has a slope close to that of the “cannonical” HMXB slope of Gilfanov et al 2004. We conclude that NGC 922 is forming ULX as efficiently as the Cartwheel (after allowing for the difference in the star formation rate). We do not see a strong metallicity effect.

Chandra Observations of Classical Novae in Outburst: The Demise of the Constant Bolometric Luminosity Paradigm

Sumner Starrfield, Arizona State University
J. Krautter (LSW), J. U. Ness (ESA), R. M. Wagner (LBT), G. Schwarz (AAS), J. Drake (CFA), J. Osborne (Leicester), K. Page (Leicester), A. Beardmore (Leicester), N. Gehrels (GSFC)

One of the first predictions of the thermonuclear runaway theory of the nova outburst was that at later stages in the outburst there would be a phase of constant bolometric luminosity. The cause was predicted to be emission from the accreted layers that were not ejected in the explosive phase and were steadily burning hydrogen to helium in the surface layers of the white dwarf. It was predicted to be hot and last for years. This is now called the Super Soft Source (SSS) phase because the spectrum and luminosity of a nova in outburst resembles the SSS discovered by ROSAT in the LMC. The existence of this phase for novae in outburst was confirmed by both ultraviolet and early X-ray observations with EXOSAT and ROSAT. However, the much larger amount of data obtained by Chandra shows that the typical amount of time spent in this phase by a nova is much shorter than predicted by the original calculations and depends on the mass of the white dwarf and its composition. More exciting, the Chandra X-ray observations show that this phase is neither constant on short or long time scales. Every nova, and at least one canonical SSS (Cal 83), have shown large fluctuations in count rate when observed in their SSS phase. We will show the X-ray light curves obtained for V382 Vul and V1494 Aql shortly after the launch of Chandra, V4743 Sgr which had oscillations of more than 50%, RS Oph with both a large amplitude oscillation and a 35-second oscillation, and more recent novae.

On Ultraluminous X-ray Source Correlations with Star-Forming Regions

Douglas Swartz, USRA NSA/MSFC
Allyn Tennant (NASA/MSFC), Roberto Soria (MSSL/UCL)

Maps of low-inclination nearby galaxies in Sloan Digitized Sky Survey u-g, g-r and r-i colors are used to determine whether Ultraluminous X-ray sources (ULXs) are predominantly associated with star-forming regions of their host galaxies. An empirical selection criterion is derived from colors of HII regions in M81 and M101 that differentiates between the young, blue stellar component and the old disk and bulge population. This criterion is applied to a sample of and verified through an application of Fisher's linear discriminant analysis. It is found that 60% (49%) of ULXs in optically-bright environments are within regions blueward of their host galaxy's HII regions compared to only 27% (0%) of a control sample according to the empirical (Fisher) criterion. This is an excess of 3-sigma above the 32% (27%) expected if the ULXs were randomly distributed within their galactic hosts. This indicates a ULX preference for young, approximately <10 Myr, OB associations. We show that the environments surrounding the brightest ULXs are consistent with the onset of an accretion phase as the donor star ascends the giant branch if the donor is a <20 solar-mass star.

Variability in N10, the brightest ULX in the Carthweel ring

Anna Wolter, INAF-OABrera
Fabio Pizzolato, Ginevra Trinchieri (INAF-OABr)

The compact X-ray source N10 detected in the Cartwheel ring is one of the brightest ULXs in the sky. I will present results on the variability of this source, and its spectrum. I will discuss the inferred parameters in the context of the current theoretical models. Due to the low statistics, we cannot clinch down the argument on the nature of N10. Nonetheless, I will show that the available information allow us to constrain the parameter space for black hole accretion.