Symposium Proceedings

Black Hole and Neutron Star Binaries

Optical Counterparts of Faint X-ray Sources in the SMC

Valsamo Antoniou (Harvard-Smithsonian Center for Astrophysics/Univ. of Crete, Greece), Andreas Zezas (Harvard-Smithsonian Center for Astrophysics), Despina Hatzidimitriou (Univ. of Crete, Greece), Jonathan C. McDowell (Harvard-Smithsonian Center for Astrophysics), Paul Taylor (Harvard-Smithsonian Center for Astrophysics/Boston College), Giuseppina Fabbiano (Harvard-Smithsonian Center for Astrophysics), Vicky Kalogera (Northwestern Univ.),

We present optical counterparts of the X-ray sources detected in our Chandra X-ray observations of the central region of the SMC. The survey yielded a total of 122 significant sources (at $3\sigma$ level) down to a luminosity limit of $4.3\times10^{33}$ erg $\rm {s^{-1}}$. Comparisons were made with the OGLE-II and MCPS catalogs of stars in the SMC. We find secure optical counterparts for 40 sources of which 18 are early type stars suggesting that their associated X-ray sources are High Mass X-ray Binaries. We also identify potential counterparts for 63 additional sources. We further discuss the implications of these results for the observed overabundance of the X-ray binaries in the SMC.

A Comparison of X-ray Spectral Properties of ULXs and Lower-Luminosity Point Sources in Nearby Galaxies

Ciprian T. Berghea (Catholic Univ. of America), Edward J. M. Colbert (Johns Hopkins Univ.), Timothy P. Roberts (Univ. of Leicester, UK), Kimberly A. Weaver (NASA)

We investigate the spectral properties of ultraluminous X-ray sources (ULXs) using the highest quality data available in the Chandra archive. This is defined as sources with at least 1000 observed counts, sufficient for reliable spectral analysis. We found 49 ULXs with X-ray luminosities L$_X$ $\ge$ 10$^{39.0}$ erg s$^{-1}$ in the energy range 0.3$-$8.0 keV that fulfill this criterion. We have also selected a comparison sample of 22 point-like X-ray sources with luminosities between 10$^{38.3}$ erg s$^{-1}$ and 10$^{39.0}$ erg s$^{-1}$ and a similar data quality. For our well-defined sample of ULXs and the comparison sample, we fitted the spectra with one-component models (cold absorption with power-law, and cold absorption with multi-colored disk blackbody) and two-component models (absorption with both a power-law and a multi-colored disk blackbody). The results are then used to determine if spectral properties of the ULXs are statistically distinct from those of the lower luminosity objects.

High Resolution Fe Lyman and Balmer Band Spectroscopy of the Bursts in EXO 0748-676

Jean Cottam (NASA/GSFC), Frits Paerels, Gisela Telis, Marc Audard (Columbia Astrophysics Laboratory), Mariano Mendez (SRON), Thierry Lanz (Univ. of Maryland), Walter Lewin, Herman Marshall (MIT), Lars Bildsten, Phil Chang (UCSB)

We present the results of high resolution spectroscopy of the X-ray bursts in the LMXB EXO 0748-646. Early observations with XMM/RGS showed evidence for gravitationally redshifted absorption lines, which can be identified with the n=2-3 transitions in H- and He-like Fe arising in the stellar photosphere at z=0.35. We will discuss the results of spectroscopy of the Fe Lyman band with Chandra/HETGS, and a reobservation in the Fe Balmer band with XMM/RGS.

Spectral Changes During Dipping in Low-mass X-ray Binaries due to Highly-ionized Absorbers

Maria Diaz Trigo, Arvind Parmar (ESTEC-ESA, The Netherlands), Laurence Boirin (Observatoire Astronomique de Strasbourg, France), Mariano Mendez, Jelle Kaastra (SRON, The Netherlands)

X-ray observations have revealed that many low-mass X-ray binaries (LMXBs) exhibit narrow absorption features identified with Fe XXV and Fe XXVI. We successfully model the changes in both the X-ray continuum and the Fe absorption features during dips from all the bright dipping LMXBs observed by XMM-Newton (EXO 0748-676, XB 1254-690, X 1624-490, MXB 1659-298, 4U 1746-371 and XB 1916-053) as resulting primarily from an increase in column density and a decrease in the ionization state of a highly-ionized absorber in a similar way as was done for XB 1323-619. This implies that complex spectral changes in the X-ray continua observed from the dip sources as a class can be most simply explained primarily by changes in the highly ionized absorbers present in these systems. We observe also small changes in the equivalent hydrogen column of neutral material, which may be related to the inclination of the system. Since the ionized plasma has a cylindrical geometry with a maximum column density close to the plane of the accretion disk and dipping sources are simply normal LMXBs viewed from close to the orbital plane this implies that ionized plasmas are a common feature of LMXBs.

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Evidence for Enhanced Neutrino Cooling from a NS SXT in Quiescence

Peter Jonker (SRON & Harvard-Smithsonian Center for Astrophysics) , Deepto Chakrabarty (MIT), Ed Brown (Michigan State Univ.), Gijs Nelemans (Nijmegen Univ.), Adrienne Juett (Univ of Virginia)

Observations of black hole and neutron star Soft X-ray Transients (SXTs) with Chandra and XMM-Newton turned out to have a profound impact on two important areas of high energy astrophysics. First of all, comparing the quiescent luminosity of neutron star SXTs with that of black hole SXTs it was found that black hole (BH) SXTs are systematically fainter in quiescence than neutron stars. This has been interpreted as evidence for advection of energy across a BH event horizon. Secondly, in observations of neutron star SXTs in quiescence which allow for a spectral study, the spectrum was found to be well-fit by a neutron star atmosphere model (NSA) sometimes supplemented with a power-law component. Well established theories about the time averaged mass accretion rates in neutron star SXTs, the pycnonuclear reactions taking place in the neutron star crust combined with neutron star cooling theory predictions, yield a neutron star core temperature. This hot neutron star core, moderated by the neutron star atmosphere, is thought to be observed during the quiescent phase of neutron star SXTs. In theory, a NSA-fit provides means to measure the mass and radius of the neutron star and hence constrain the equation of state (EoS) of matter at supranuclear densities. The description of the relations between pressure and density of matter (the EoS) under the extreme conditions encountered in neutron stars is one of the ultimate goals of the study of neutron stars. We recently observed the neutron star SXT 1H1905+000 in quiescence with ACIS-S. However, the source was not detected even though the distance and interstellar extinction are well known. This means that the source (thermal) luminosity in the 0.5-10 keV band is lower than $\sim~10^{31}$ erg s$^{-1}$. From this and from the fact that it is known from binary evolution theory that the time averaged mass accretion rate cannot be much less than $10^{-12}$ Msun per year, we conclude that the neutron star must have access to enhanced neutrino emission processes. Furthermore, the low luminosity puts these sources in the midst of the quiescent BH SXTs removing the evidence for the event horizon.

Chandra HETG Observations of Fe K in Cygnus X-3

Michael McCollough (SAO), Norbert Schulz (MIT), Guy Pooley (MRAO)

We present an analysis of HETG Chandra observations of the Fe K region of the spectrum of Cygnus X-3. Cygnus X-3 is a HMXB which has two major states (hard/low and soft/high) and shows correlative activity in the radio. Chandra has observed Cygnus X-3 with the HETG on five occasions. These observations include a hard/low state (radio quiescent), soft/high (major radio flares), and a transitional period between these states. Lines of H-like, He-like and neutral Fe are observed. A discussion of the nature of these lines and how they vary as both a function of state and orbital phase will be given. Preliminary modeling of the data with XSTAR will be provided.

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The Broadband Spectrum of Centaurus X-4

Manuel Perez Torres, Michael Garcia, Jeffrey McClintock (Harvard-Smithsonian Center for Astrophysics), Ronald Remillard (MIT), Jon Miller (Univ. of Michigan),

We present the X-ray/UV/optical spectrum of the neutron star X-ray transient Centaurus X-4 in quiescence. HST/STIS and Magellan-Baade observations were performed simultaneously with Chandra/ACIS-S observations to avoid uncertainties in the broadband spectrum caused by short-term brightness variability. We show new results for the quiescent state, such as the first far-UV spectrum. This spectrum has sufficient signal-to-noise ratio to reveal broad emission lines of NV, SiIV, CIV and HeII superimposed on a flat continuum. The near-UV flux is a factor 2 lower than observed in a previous HST/STIS observation and proves UV variability. However, the centerpiece of this work is the quiescent multiwavelength spectrum of Centaurus X-4, which is discussed in terms of different spectral models.

Clouds, Winds, and Jets in the Luminous X-ray Source Circinus X-1

Norbert S. Schulz (MIT), William N. Brandt (PSU), Duncan Galloway (Melbourne), Deepto Chakrabarty, Sebastian Heinz (MIT)

One of the early highlights of high resolution spectroscopy was the discovery of P Cygni lines in the highly variable and luminous X-ray source Circinus X-1 during the early days of the Chandra mission. By adapting the preceding paradigm established from ASCA observations that the accretion disk around the neutron star is viewed edge-on, the for the complex line structure was interpreted as a combination of accretion disk coronal line emission and absorption from a radiatively driven equatorial disk wind. The detection of ultra-relativistic radio jets at about the same time seemed to support the image of Circinus X-1 as a powerful microquasar, but also casted some doubt on the assumption that the source is viewed edge-on, but instead featured a face-on view. This not only challenges the current model for the observed X-ray emission, it could also force a complex scenario involving jets, winds and clouds that seems quite unique among accreting compact sources. Furthermore the X-ray emission of Circinus X-1 has undergone radical changes since then as it gradually slipped into a low intensity state which is even devoid of the typical outburst pattern with its binary orbit. We observed Circinus X-1 several times with the Chandra HETG spectrometer during this transition. These observations resulted not only in the first X-ray image of the source, they also show a unique pattern of changing X-ray continuum as well as line emission and absorption. We discuss the results in conjunction with its previous lightcurve and investigate possible connections of to the relativistic jet.

High-speed Optical Observations of Black Hole X-ray Transients in Quiesence

Tariq Shahbaz (Instituto de Astrofisica de Canarias) Vik Dhillon (Sheffied University), Tom Marsh (Warwick University), Jorge Casares, Cristina Zurita (Instituto de Astrofisica de Canarias), Rob Hynes (Louisiana State University), Carole Haswell (Open University),

We present high time-resolution multicolour observations of the quiescent soft X-ray transients V404 Cyg and XTE J1118+480 obtained with ULTRACAM. The power density spectrum (PDS) of the V404 Cyg data shows a quasi-periodic oscillation (QPO) feature at 0.78 mHz (=21.5 min), whereas the PDS of the XTE J1118+480 data shows a 2 mHz break or QPO. We discuss the possible origins for the QPO feature and break-frequency in the context of the advection-dominated accretion flow model.

Distance Determination Using Variability in X-ray Halos

Thomas Thompson, Richard Rothschild, John Tomsick (CASS, UCSD)

With the unprecedented angular resolution of the Chandra X-ray Observatory, it is now possible to geometrically determine the distance to variable galactic sources, based on the phenomenon that radiation scattered into the X-ray halo has to travel along a slightly longer path than the direct, unscattered radiation. By measuring the delayed variability, the source distance can be obtained if the halo brightness (scattering optical depth) is large enough to dominate the PSF and to provide sufficient statistics. In addition, the variability must be on time scales shorter than $\sim~10^4$ s, depending on the source distance, optical depth, and the halo angles investigated. although this method is appealing, the accuracy of distances measured in this manner is limited, among other things, by knowledge of the dust distribution along the line of sight. Here we examine the applicability of this technique to two sources with different types of variability: (1) Gradual variability from the X-ray binary, Cyg X-3. Currently, this is the only source whose distance has been measured with this method. (2) Sharp variability from type I X-ray bursts from GS 1826-238.