We have analyzed a Chandra HETGS spectrum of the Galactic black hole Cygnus X-1, obtained at a source flux that is approximately twice that commonly observed in its persistent low-intensity, ...spectrally hard state. We find a myriad of absorption lines in the spectrum, including H- and He-like resonance lines from Ne, Na, Mg, and Si. We calculate a flux-weighted mean redshift of 100 km s super(-1) and a flux-weighted mean velocity width of 800 km s super(-1) (FWHM) for lines from these elements. We also detect a number of transitions from Fe XVIII-XXIV and Ni XIX-XX in absorption; however, the identification of these lines is less certain and a greater range of shifts and breadth is measured. Our observation occurred at a binary phase of h 0.76; the lines observed are consistent with absorption in an ionized region of the supergiant O9.7 Iab companion wind. The spectrum is extremely complicated: a rather large range of temperatures is implied, and it is unlikely that a narrow range of ionization parameters can account for the lines observed. Prior Chandra HETGS spectra of Cygnus X-1 were obtained in a similar transition state (at h 0.93) and in the low/hard state (at h 0.84). Considered together, these spectra provide evidence for a companion wind that is focused as it flows onto the black hole primary in this system.
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 sub(*) > 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 a-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.
The 2003 X-ray outburst of the candidate black hole binary, H1743-322, was investigated in frequent pointed observations (2-250 keV) with the Rossi X-Ray Timing Explorer (RXTE). One particular ...program of 130 observations is organized into 111 time intervals and searched for the presence of high-frequency quasi-periodic oscillations (HFQPOs) in the range 50-2000 Hz. Only a single observation (2003 June 13) yields a detection above 4 s. The central frequency of 239 c 4 Hz is consistent with the 240 Hz QPO reported for this source on 2003 May 28 (Homan and coworkers). We group the observations in several different ways and compute average power-density spectra (PDS) in a search for further evidence of HFQPOs. Significant results are found for two groups defined by the presence of low-frequency QPOs (0.1-20 Hz) and an absence of "band-limited" power continua. (1) The nine time intervals with the highest X-ray flux yield a QPO at 166 c 5 Hz. (4.1 s; 3-35 keV). (2) The group with lower X-ray flux (24 time intervals) produces a QPO at 242 c 3 Hz (6.0 s; 7-35 keV). The ratio of these two frequencies is 1.46 c 0.05. This finding is consistent with results obtained for three other black hole systems that exhibit commensurate HFQPOs in a 3:2 ratio. Furthermore, the occurrence of H1743-322's slower HFQPO at times of higher X-ray luminosity closely resembles the behavior of XTE J1550-564 and GRO J1655-40.
We report the serendipitous detection of a very bright, very nearby microlensing event. In late 2006 October, an otherwise unremarkable A0 star at a distance of image1 kpc (GSC 3656-1328) brightened ...achromatically by a factor of nearly 40 over the span of several days and then decayed in an apparently symmetrical way. We present a light curve of the event based on optical photometry from the Center for Backyard Astrophysics and the All Sky Automated Survey, as well as near-infrared photometry from the Peters Automated Infrared Imaging Telescope. This light curve is well fit by a generic microlensing model. We also report optical spectra and Swift X- ray and UV observations that are consistent with the microlensing interpretation. We discuss and reject alternative explanations for this variability. The lens star is probably a low-mass star or brown dwarf, with a relatively high proper motion of image20 mas yr super(-1), and may be visible using precise optical/infrared imaging taken several years from now. A modest, all-sky survey telescope could detect image10 such events per year, which would enable searches for very low mass planetary companions to relatively nearby stars.
Abstract The tidal disruption of a star around a supermassive black hole (SMBH) offers a unique opportunity to study accretion onto an SMBH on a human timescale. We present results from our 1000+ ...days monitoring campaign of AT 2019avd, a nuclear transient with tidal-disruption-event-like properties, with NICER, Swift, and Chandra. Our primary finding is that approximately 225 days following the peak of the X-ray emission, there is a rapid drop in luminosity exceeding 2 orders of magnitude. This X-ray dropoff is accompanied by X-ray spectral hardening, followed by a plateau phase of 740 days. During this phase, the spectral index decreases from 6.2 ± 1.1 to 2.3 ± 0.4, while the disk temperature remains constant. Additionally, we detect pronounced X-ray variability, with an average fractional rms amplitude of 47%, manifesting over timescales of a few dozen minutes. We propose that this phenomenon may be attributed to intervening clumpy outflows. The overall properties of AT 2019avd suggest that the accretion disk evolves from a super-Eddington to a sub-Eddington luminosity state, possibly associated with a compact jet. This evolution follows a pattern in the hardness–intensity diagram similar to that observed in stellar-mass BHs, supporting the mass invariance of accretion–ejection processes around BHs.
We have observed the persistent but optically unidentified X-ray source X1908+075 with the Proportional Counter Array and High-Energy X-Ray Timing Experiment instruments on the Rossi X-Ray Timing ...Explorer (RXTE). The binary nature of this source was recently established by Wen et al., who found a 4.4 day orbital period in results from the RXTE All-Sky Monitor. We report the discovery of 605 s pulsations in the X-ray flux. The Doppler delay curve is measured and provides a mass function of 6.1 M sub( ), which is a lower limit to the mass of the binary companion of the neutron star. The degree of attenuation of the low-energy end of the spectrum is found to be a strong function of orbital phase. A simple model of absorption in a stellar wind from the companion star fits the orbital phase dependence reasonably well and limits the orbital inclination angle to the range 38-72. These measured parameters lead to an orbital separation of 660-80 lt-s, a mass for the companion star in the range 9-31 M sub( ), and an upper limit to the size of the companion of 622 R sub( ). From our analysis, we also infer a wind mass-loss rate from the companion star of 1.3 x 10 super(-6) M sub( )yr super(-1) and, when the properties of the companion star and the effects of photoionization are considered, likely 4 x 10 super(-6) M sub( )yr super(-1). Such a high rate is inconsistent with the allowed masses and radii that we find for a main-sequence or modestly evolved star unless the mass-loss rate is enhanced in the binary system relative to that of an isolated star. We discuss the possibility that the companion might be a Wolf-Rayet star that could evolve to become a black hole in 10 super(4) to 10 super(5) yr. If so, this would be the first identified progenitor of a neutron star-black hole binary.
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