Binary X-ray Sources Research Articles

THE BLACK HOLE MASS DISTRIBUTION IN THE GALAXY

We use dynamical mass measurements of 16 black holes in transient low-mass X-ray binaries to infer the stellar black hole mass distribution in the parent population. We find that the observations are best described by a narrow mass distribution at 7.8 +/- 1.2 Msolar. We identify a selection effect related to the choice of targets for optical follow-ups that results in a flux-limited sample. We demonstrate, however, that this selection effect does not introduce a bias in the observed distribution and cannot explain the absence of black holes in the 2-5 solar mass range. On the high mass end, we argue that the rapid decline in the inferred distribution may be the result of the particular evolutionary channel followed by low-mass X-ray binaries. This is consistent with the presence of high-mass black holes in the persistent, high-mass X-ray binary sources. If the paucity of low-mass black holes is caused by a sudden decrease of the supernova explosion energy with increasing progenitor mass, this would have observable implications for ongoing transient surveys that target core-collapse supernovae. Our results also have significant implications for the calculation of event rates from the coalescence of black hole binaries for gravitational wave detectors.

STRUCTURE AND MAGNETIC FIELDS IN THE PRECESSING JET SYSTEM SS 433. II. INTRINSIC BRIGHTNESS OF THE JETS

Deep Very Large Array imaging of the binary X-ray source SS 433, sometimes classified as a microquasar, has been used to study the intrinsic brightness distribution and evolution of its radio jets. The intrinsic brightness of the jets as a function of age at emission of the jet material tau is recovered by removal of the Doppler boosting and projection effects. We find that intrinsically the two jets are remarkably similar when compared for equal tau, and that they are best described by Doppler boosting of the form D^{2+alpha}, as expected for continuous jets. The intrinsic brightnesses of the jets as functions of age behave in complex ways. In the age range 60 < tau < 150 days, the jet decays are best represented by exponential functions of tau, but linear or power law functions are not statistically excluded. This is followed by a region out to tau ~ 250 days during which the intrinsic brightness is essentially constant. At later times the jet decay can be fit roughly as exponential or power law functions of tau.

Jets from Compact X-ray Sources

AbstractJets have been observed from both neutron stars and black holes in binary X-ray sources. The neutron star jets are typically 30 times weaker than the black-hole ones. Thus, the second have been studied more extensively. Contrary to common belief, jets from compact X-ray sources are not simply “fireworks” that emit radio waves. I will demonstrate that they play a central role in the observed phenomena in both neutron star and black-hole systems. In particular, for black-hole jets, a simple jet model can explain the very stringent correlations that have been found between the power-law X-ray spectrum and a) the time lag between hard and soft X-rays and b) the characteristic frequencies observed in the power spectra. Up to now, no other model has even attempted to explain these correlations. I will present the weaknesses of the model and the improvements that need to be done to it.

The viscous circle: the first steps toward modeling Galactic binary X-ray sources

The viscous circle: the first steps toward modeling Galactic binary X-ray sources

History of X-ray telescopes and astronomy

The development of X-ray telescopes over the last 45 years is briefly summarized. The first applications to the study of solar X-ray emission are described up to the 1973 Skylab mission. The rather convoluted path that led to the first stellar X-ray orbiting telescope Einstein (1978) and later to Chandra (1999) are discussed. During this 45 years period the angular resolution improved from 20 to 0.5 arc sec and the sensitivity by ten billions. X-ray observations have discovered new types of stellar systems (binary X-ray sources containing neutron stars and black holes) and intergalactic high temperature plasmas containing most of the baryonic mass of the universe. They have become an indispensable tool to study the role of energetic phenomena in the creation and dynamic evolution of cosmic structures. The methodology introduced by X-ray astronomy has influenced all of astronomy.

A decade of radio imaging the relativistic outflow in the peculiar X-ray binary Circinus X-1

We present observations of the neutron star X-ray binary and relativistic jet source Circinus X-1 made at 4.8 and 8.6 GHz with the Australia Telescope Compact Array during a time interval of almost 10 yr. The system shows significant variations in the morphology and brightness of the radio features on all time-scales from days to years. Using the time delay between the successive brightening of the different components of the radio emission we were able to provide further evidence for the relativistic nature of the arcsec-scale outflow, with an apparent velocity beta(app) >= 12. No compelling evidence for an evolution of the orientation of the jet axis was found. We also place an upper limit on the proper motion of the system which is consistent with previous optical studies. Besides the previously reported radio flares close to the orbital phase 0.0 (interpreted as enhanced accretion at periastron passage), we identified outbursts with similar properties near the orbital phase 0.5. The global spectral index revealed a preferentially steep spectrum over the entire period of monitoring with a mean value and standard deviation alpha = -0.9 +/- 0.6(F-nu proportional to nu(alpha)), which became significantly flatter during the outbursts. Polarization was detected in one third of the epochs, and in one case Faraday rotation close to the core of the system was measured.

V723 CASSIOPEIA STILL ON IN X-RAYS: A BRIGHT SUPER SOFT SOURCE 12 YEARS AFTER OUTBURST

We find that the classical nova V723 Cas (1995) is still an active X-ray source more than 12 years after outburst, and analyze seven X-ray observations carried out with Swift between 2006 January 31 and 2007 December 3. The average count rate is 0.022 ± 0.01 cts s–1, but the source is variable within a factor of 2 of the mean and does not show any signs of turning off. We present supporting optical observations which show that between 2001 and 2006 an underlying hot source was present with steadily increasing temperature. In order to confirm that the X-ray emission is from V723 Cas, we extract a ROSAT observation taken in 1990 and find that there was no X-ray source at the position of the nova. The Swift XRT spectra resemble those of the super soft X-ray binary sources (SSS) which is confirmed by RXTE survey data which show no X-ray emission above 2 keV between 1996 and 2007. Using blackbody fits we constrain the effective temperature to between T eff = (2.8 – 3.8) × 105 K and a bolometric luminosity 5 × 1036 erg s–1 and caution that luminosities from blackbodies are generally overestimated and temperatures underestimated. We discuss a number of possible explanations for the continuing X-ray activity, including the intriguing possibility of steady hydrogen burning due to renewed accretion.

SwiftX‐Ray Observations of Classical Novae

The new γ-ray burst (GRB) mission Swift has obtained pointed observations of several classical novae in outburst. We analyzed all the observations of classical novae from the Swift archive up to 2006 June 30. We analyzed usable observations of 12 classical novae and found 4 nondetections, 3 weak sources, and 5 strong sources. This includes detections of two novae exhibiting spectra resembling those of supersoft X-ray binary source spectra (SSS), implying ongoing nuclear burning on the white dwarf surface. With these new Swift data, we add to the growing statistics of the X-ray duration and characteristics of classical novae.

Accretion Disk Spectra of the Brightest Ultraluminous X‐Ray Source in M82

Emission spectra of hot accretion disks characteristic of advection-dominated accretion flow (ADAF) models are investigated for comparison with the brightest ultraluminous source, X-1, in the galaxy M82. If the spectral state of the source is similar to the low-luminosity hard state of stellar mass black holes in our Galaxy, a fit to the Chandra X-ray spectrum and constraints from the radio and infrared upper limits requires a black hole mass in the range of 9 x 10(4)-5; 10(5) M-circle dot. Lower black hole masses (<= 10(4) M-circle dot) are possible if M82 X-1 corresponds to the highluminosity hard state of Galactic black hole X-ray binary sources. Both of these spectrally degenerate hot accretion disk solutions lead to an intermediate-mass black hole interpretation for M82 X-1. Since these solutions have different spectral variability with X-ray luminosity and predict different radio/infrared emission, they could be distinguished by future off-axis Chandra observations or simultaneous sensitive radio/infrared detections.

Discovery of a New X-Ray Burst/Millisecond Accreting Pulsar, HETE J1900.1-2455

Abstract A class of low-mass X-ray binary sources is known to be both X-ray burst sources and millisecond pulsars at the same time. A new source of this class was discovered by High Energy Transient Explorer 2 (HETE-2) on 2005 June 14 as a source of type-I X-ray bursts, which was named HETE J1900.1$-$2455. Five X-ray bursts from HETE J1900.1$-$2455 were observed during the summer of 2005. A time-resolved spectral analysis of these bursts has revealed that their spectra are consistent with the blackbody radiation throughout the bursts. The bursts show an indication of radius expansion. The bolometric flux remained almost constant during the photospheric radius expansion, while blackbody temperature dropped during the same period. Assuming that the flux reached the Eddington limit on a standard 1.4 solar-mass neutron star with a helium atmosphere, we estimate the distance to the source to be $\sim$ 4 Kpc.

Time-Dependent Synchrotron and Compton Spectra from Microquasar Jets

Jet models for the high-energy emission of Galactic X-ray binary sources have regained significant interest with detailed spectral and timing studies of the X-ray emission from microquasars, the recent detection by the HESS collaboration of very-high-energy γ-rays from the microquasar LS 5039, and the earlier suggestion of jet models for ultraluminous X-ray sources observed in many nearby galaxies. Here we study the synchrotron and Compton signatures of time-dependent electron injection and acceleration, and adiabatic and radiative cooling in the jets of Galactic microquasars.

The Spin of the Near‐Extreme Kerr Black Hole GRS 1915+105

Based on a spectral analysis of the X-ray continuum that employs a fully relativistic accretion-disk model, we conclude that the compact primary of the binary X-ray source GRS 1915+105 is a rapidly-rotating Kerr black hole. We find a lower limit on the dimensionless spin parameter of a* greater than 0.98. Our result is robust in the sense that it is independent of the details of the data analysis and insensitive to the uncertainties in the mass and distance of the black hole. Furthermore, our accretion-disk model includes an advanced treatment of spectral hardening. Our data selection relies on a rigorous and quantitative definition of the thermal state of black hole binaries, which we used to screen all of the available RXTE and ASCA data for the thermal state of GRS 1915+105. In addition, we focus on those data for which the accretion disk luminosity is less than 30% of the Eddington luminosity. We argue that these low-luminosity data are most appropriate for the thin alpha-disk model that we employ. We assume that there is zero torque at the inner edge of the disk, as is likely when the disk is thin, although we show that the presence of a significant torque does not affect our results. Our model and the model of the relativistic jets observed for this source constrain the distance and black hole mass and could thus be tested by determining a VLBA parallax distance and improving the measurement of the mass function. Finally, we comment on the significance of our results for relativistic-jet and core-collapse models, and for the detection of gravitational waves.

A Time-dependent Leptonic Model for Microquasar Jets: Application to LS I +61 303

The Galactic high-mass X-ray binary and jet source (microquasar) LSI +61 303 has recently been detected at TeV gamma-ray energies by the MAGIC telescope. We have applied a time-dependent leptonic jet model to the broadband spectral energy distribution and suggested (though not unambiguously detected) orbital modulation of the very high energy gamma-ray emission of this source. Our model takes into account time dependent electron injection and acceleration, and the adiabatic and radiative cooling of non-thermal electrons. It includes synchrotron, synchrotron self-Compton and external inverse Compton (with seed photons from the companion star), as well as gamma-gamma absorption of gamma-rays by starlight photons. The model can successfully reproduce the available multiwavelength observational data. Our best fit to the SED indicates that a magnetic field of B_0 ~ 5 X 10^3 G at ~ 10^3 R_g is required, and electrons need to be accelerated out to TeV energies (gamma_2 = 10^6) with a nonthermal injection spectrum with a spectral index of q = 1.7, indicating the operation of acceleration mechanisms beyond the standard first-order Fermi mechanism at relativistic or non-relativistic shocks. The orbital modulation of the VHE gamma-ray emission can be explained solely by the geometrical effect of changes in the relative orientation of the stellar companion with respect to the compact object and jet as it impacts the position and depth of the gamma-gamma absorption trough. Such a scenario predicts a trend of spectral hardening during VHE gamma-ray low orbital phases.

Time‐dependent Synchrotron and Compton Spectra from Jets of Microquasars

Jet models for the high-energy emission of Galactic X-ray binary sources have regained significant interest with detailed spectral and timing studies of the X-ray emission from microquasars, the recent detection by the HESS collaboration of very high energy γ-rays from the microquasar LS 5039, and the earlier suggestion of jet models for ultraluminous X-ray sources observed in many nearby galaxies. Here we study the synchrotron and Compton signatures of time-dependent electron injection and acceleration, and adiabatic and radiative cooling in the jets of Galactic microquasars. Synchrotron, synchrotron self-Compton, and external Compton radiation processes with soft photons provided by the companion star and the accretion disk are treated. An analytical solution is introduced to the electron kinetic equation for general power-law geometries of the jets for Compton scattering in the Thomson regime. We pay particular attention to predictions concerning the rapid flux and spectral variability signatures expected in a variety of scenarios, making specific predictions concerning possible spectral hysteresis, similar to what has been observed in several TeV blazars. Such predictions should be testable with dedicated monitoring observations of Galactic microquasars and ultraluminous X-ray sources using Chandra and/or XMM-Newton.

The X-ray properties of the dwarf Magellanic-type galaxy NGC 55

We present an analysis of the X-ray properties of the Magellanic-type galaxy NGC 55 based on two contiguous XMM-Newton observations. We detect a total of 137 X-ray sources in the field of view, 42 of which are located within the optical confines of the galaxy. On the basis of X-ray colour classification and after correcting for background objects, we conclude that our source sample includes ~ 20 X-ray binaries, 5 supernova remnants and 7 very soft sources (including 2 good candidate supersoft sources) associated with NGC 55. Detailed spectral and timing analysis was carried out on 4 of the brightest X-ray sources. One of these objects is identified with a Galactic foreground star and is a possible new RS CVn system. The other three are consistent with accreting X-ray binaries, though further evidence of short term variability is required to confirm this. We also find evidence of an underlying component, which is concentrated on the bar region but has an extent of at least 6' (3 kpc) in the plane of the galaxy and +/- 1' (+/- 500 pc) perpendicular to it. This emission is best fitted by a thermal plasma plus power-law model but with high intrinsic absorption consistent with its location in the central disc of the galaxy. We interpret the soft component as diffuse thermal emission linked to regions of current star formation, whilst the hard power-law component may originate in unresolved X-ray binary sources. The intrinsic luminosity of this residual disc emission may exceed Lx ~ 6e38 ergs/s (0.3-6 keV). A comparison with other Magellanic systems confirms that, in terms of both its discrete X-ray source population and its extended emission, NGC 55 has X-ray properties which are typical of its class.

X-Ray and Optical Eclipses in Ultraluminous X-Ray Sources as Possible Indicators of Black Hole Mass

Ultraluminous X-ray sources (ULXs) with 1039 ergs s-1 LX < 1041 ergs s-1 have been discovered in great numbers in external galaxies with ROSAT, Chandra, and XMM-Newton. The central question regarding this important class of sources is whether they represent an extension to the luminosity function of binary X-ray sources containing neutron stars and stellar-mass black holes (BHs), or a new class of objects, e.g., systems containing intermediate-mass black holes (100-1000 M☉). We suggest searching for X-ray and optical eclipses in these systems to provide another diagnostic to help distinguish between these two possibilities. The sense of the effect is that ULXs with stellar-mass black hole accretors should be at least twice as likely to exhibit eclipses as intermediate-mass black hole systems—and perhaps much more than a factor of 2. Among other system parameters, the orbital period would follow. This would provide considerable insight as to the nature of the binary.

Studies of Accretion onto Hot, Massive White Dwarfs: The Growth to the Chandrasekhar Limit?

We report on studies of accretion of material of solar composition at a variety of rates onto hot, massive white dwarfs (WDs). Evolutionary sequences for 1.25 M ⊙ and 1.35 M ⊙ WDs with mass accretion rates varying from 1.6 × 10 −10 M ⊙ yr −1 to 3.5 × 10 −7 M ⊙ yr −1 have now been studied and, in all these cases, tlie WDs grow in mass to near the Chandrasekhar Limit. The surface conditions of our simulations are similar to observations of Super Soft X-ray binary sources. Our results provide the first confirmation of previous suggestions that some of the Super Soft X-ray binaries may be the progenitors of thermonuclear supernovae. We observe that the abundances in the accreted layers (including carbon and oxygen) are sensitive to the accretion rate, potentially affecting the dynamics of the supernova explosion and observations of elemental distributions.

INTEGRAL high energy observations of 2S 0114+65

We report the first INTEGRAL timing and spectral analysis of the high mass X-ray binary source 2S 0114+65 at high energies (5-100 keV). The pulse period was found at 2.668 hr with a high pulsed fraction, ~80% in both the 20-40 keV and 40-80 keV energy bands. The spin-up trend over ~8 years was measured to be -8.9 * 10^{-7}. The hard X-ray spectrum obtained with JEM-X/ISGRI is well described by a high energy exponential cut-off power law model where the estimated luminosity is 1.8 * 10^{36} erg/s in the 5-100 keV energy band, for a source distance of 7.2 kpc. We tentatively identify a cyclotron resonance scattering feature at ~22 keV with one harmonic, implying a magnetic field of 2.5 * 10^{12} G.

A Broad Iron Line in the Chandra High Energy Transmission Grating Spectrum of 4U 1705-44

We present the results of a Chandra 30 ks observation of the low-mass X-ray binary and atoll source 4U 1705-44. Here we concentrate on the study of discrete features in the energy spectrum at energies below ~3 keV, as well as on the iron Kalpha line, using the High Energy Transmission Grating Spectrometer on board the Chandra satellite. Below 3 keV, three narrow emission lines are found at 1.47, 2.0, and 2.6 keV. The 1.47 and 2.6 keV lines are probably identified with Lyalpha emission from Mg XII and S XVI, respectively. The identification of the feature at ~2.0 keV is uncertain because of the presence of an instrumental feature at the same energy. The iron Kalpha line at ~6.5 keV is found to be intrinsically broad (FWHM~1.2 keV); its width can be explained by reflection from a cold accretion disk extending down to ~15 km from the neutron star center or by Compton broadening in the external parts of a hot (~2 keV) Comptonizing corona. We finally report here precise X-ray coordinates of the source.

Recurrent ∼24 h Periods in RXTE ASM Data

AbstractAnalysis of data from the Rossi X-ray Timing Explorer satellite's All Sky Monitor (ASM) instrument for several X-ray binary sources has identified a recurrent ∼24 h period. This period is sometimes highly significant, giving rise to the possibility of it being identified as an orbital or super-orbital period. Further analysis has revealed the same period in a number of other X-ray sources. As a result this period has been discounted as spurious, described variously as arising from daily variations in background levels, the scheduling of ASM observations and beating between the sampling period and long-term secular trends in the light curves. We present here an analysis of the spurious periods and show that the dominant mechanism is in fact spectral leakage of low-frequency power present in the light curves.