Ronald A. Remillard

X-Ray Properties of Black-Hole Binaries

AbstractWe review the properties and behavior of 20 X-ray binaries that contain a dynamically-confirmed black hole, 17 of which are transient systems. During the past decade, many of these transien...

First Results from the All-Sky Monitor on the Rossi X-Ray Timing Explorer

The all-sky monitor on the Rossi X-Ray Timing Explorer has been monitoring the sky in the 1.5-12 keV band since late February. The instrument consists of three coded-aperture cameras that can be rotated to view different regions by a motorized drive assembly. Intensities of ~100 known sources are obtained via least-squares fits of shadow patterns to the data and compiled to form X-ray light curves. Six orbital periodicities and four long-term periodicities, all previously known, have been detected in these light curves. Searches for additional sources have also been conducted. X-ray light curves for the Crab Nebula, Cyg X-1, 4U 1705-44, GRO J1655-40, and SMC X-1 are reported. They illustrate the quality of the results and the range of observed phenomena.

The Murchison Widefield Array: The Square Kilometre Array Precursor at Low Radio Frequencies

The Murchison Widefield Array (MWA) is one of three Square Kilometre Array Precursor telescopes and is located at the Murchison Radio-astronomy Observatory in the Murchison Shire of the mid-west of Western Australia, a location chosen for its extremely low levels of radio frequency interference. The MWA operates at low radio frequencies, 80–300 MHz, with a processed bandwidth of 30.72 MHz for both linear polarisations, and consists of 128 aperture arrays (known as tiles) distributed over a ~3-km diameter area. Novel hybrid hardware/software correlation and a real-time imaging and calibration systems comprise the MWA signal processing backend. In this paper, the as-built MWA is described both at a system and sub-system level, the expected performance of the array is presented, and the science goals of the instrument are summarised.

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* > 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 α-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 radio pulsar/x-ray binary link

From X-ray Binary to Pulsar Pulsars with millisecond rotational periods are thought to originate from neutron stars in low-mass x-ray binaries that had their spin frequencies increased by long-lasting mass transfer from their companion stars. Using data from a radio pulsar survey, Archibald et al. (p. 1411, published online 21 May; see the Perspective by Kramer) found a neutron star in a low-mass X-ray binary that is in the process of turning into a radio millisecond pulsar. The system, which consists of a solar-like star and a 1.69-millisecond radio pulsar, has gone through a recent accretion phase, characteristic of low-mass X-ray binaries, but it shows no accretion disk anymore, confirming the evolutionary connection between millisecond radio pulsars and low-mass X-ray binaries. Radio observations reveal a system undergoing the transition from a low-mass x-ray binary star to a millisecond radio pulsar. Radio pulsars with millisecond spin periods are thought to have been spun up by the transfer of matter and angular momentum from a low-mass companion star during an x-ray–emitting phase. The spin periods of the neutron stars in several such low-mass x-ray binary (LMXB) systems have been shown to be in the millisecond regime, but no radio pulsations have been detected. Here we report on detection and follow-up observations of a nearby radio millisecond pulsar (MSP) in a circular binary orbit with an optically identified companion star. Optical observations indicate that an accretion disk was present in this system within the past decade. Our optical data show no evidence that one exists today, suggesting that the radio MSP has turned on after a recent LMXB phase.

Compact Stellar X-Ray Sources: Black hole binaries

4.1.1 Scope of this review We focus on 18 black holes with measured masses that are located in X–ray binary systems. These black holes are the most visible representatives of an estimated ∼300 million stellar–mass black holes that are believed to exist in the Galaxy (van den Heuvel 1992; Brown & Bethe 1994; Timmes et al. 1996; Agol et al. 2002). Thus the mass of this particular form of dark matter, assuming ∼10M⊙ per black hole, is ∼4% of the total baryonic mass (i.e., stars plus gas) of the Galaxy (Bahcall 1986; Bronfman et al. 1988). Collectively this vast population of black holes outweighs the Galactic–center black hole, SgrA, by a factor of ∼1000. These stellar–mass black holes are important to astronomy in numerous ways. For example, they are one endpoint of stellar evolution for massive stars, and the collapse of their progenitor stars enriches the universe with heavy elements (Woosley et al. 2002). Also, the measured mass distribution for even the small sample of 18 black holes featured here are used to constrain models of black hole formation and binary evolution (Brown et al. 2000a; Nelemans & van den Heuvel 2001; Fryer & Kalogera 2001). Lastly, some black hole binaries appear to be linked to the hypernovae believed to power gamma–ray bursts (Israelian et al. 1999; Brown et al. 2000b; Orosz et al. 2001). This review is focused on the X–ray timing and spectral properties of these 18 black holes, plus a number of black hole candidates, with an eye to their importance to physics as potential sites for tests of general relativity (GR) in the strongest possible gravitational fields. There are now several current areas of research that probe phenomena in these systems that are believed to occur very near the event horizon. These X–ray phenomena include quasi–periodic oscillations (QPOs) at high frequency (40–450 Hz) observed from seven systems, relativistically broadened iron lines from the inner accretion disk, and thermal disk emission from near the innermost stable circular orbit allowed by GR. We also comment on evidence for the existence of the event horizon, which is based on a comparison of black–hole and neutron–star binaries and on models for advective accretion flows. The black hole binaries featured here are mass–exchange binaries that contain an accreting black hole primary and a nondegenerate secondary star. They comprise about 10% of all bright X–ray binaries. For background on X–ray binaries, see Chapter 1, and references therein. For comprehensive reviews on black hole binaries, see

The Mass of the Black Hole in Cygnus X-1

Cygnus X-1 is a binary star system that is comprised of a black hole and a massive giant companion star in a tight orbit. Building on our accurate distance measurement reported in the preceding paper, we first determine the radius of the companion star, thereby constraining the scale of the binary system. To obtain a full dynamical model of the binary, we use an extensive collection of optical photometric and spectroscopic data taken from the literature. By usingalloftheavailableobservationalconstraints,weshowthattheorbitisslightlyeccentric(boththeradialvelocity and photometric data independently confirm this result) and that the companion star rotates roughly 1.4 times its pseudosynchronous value. We find a black hole mass of M = 14.8 ± 1.0 M� , a companion mass of Mopt = 19.2 ± 1.9 M� , and the angle of inclination of the orbital plane to our line of sight of i = 27.1 ± 0. 8d eg.

A black hole in the superluminal source sax j1819.3-2525 (v4641 sgr)

Spectroscopic observations of the fast X-ray transient and superluminal jet source SAX J1819.3-2525 (V4641 Sgr) reveal a best-fitting period of Pspect = 2.81678 ± 0.00056 days and a semiamplitude of K2 = 211.0 ± 3.1 km s-1. The optical mass function is f(M) = 2.74 ± 0.12 M☉. We find a photometric period of Pphoto = 2.81730 ± 0.00001 days using a light curve measured from photographic plates. The folded light curve resembles an ellipsoidal light curve with two maxima of roughly equal height and two minima of unequal depth per orbital cycle. The secondary star is a late B-type star that has evolved off the main sequence. Using a moderate resolution spectrum (R = 7000) we measure Teff = 10500 ± 200 K, log g = 3.5 ± 0.1, and Vrot sin i = 123 ± 4 km s-1 (1 σ errors). Assuming synchronous rotation, our measured value of the projected rotational velocity implies a mass ratio of Q ≡ M1/M2 = 1.50 ± 0.08 (1 σ). The lack of X-ray eclipses implies an upper limit to the inclination of i ≤ 707. On the other hand, the large amplitude of the folded light curve (≈0.5 mag) implies a large inclination (i 60°). Using the above mass function, mass ratio, and inclination range, the mass of the compact object is in the range 8.73 ≤ M1 ≤ 11.70 M☉ and the mass of the secondary star is in the range 5.49 ≤ M2 ≤ 8.14 M☉ (90% confidence). The mass of the compact object is well above the maximum mass of a stable neutron star, and we conclude that V4641 Sgr contains a black hole. The B-star secondary is by far the most massive, the hottest, and the most luminous secondary of the dynamically confirmed black hole X-ray transients. We find that the α-process elements nitrogen, oxygen, calcium, magnesium, and titanium may be overabundant in the secondary star by factors of 2-10 times with respect to the Sun. Finally, assuming E(B-V) = 0.32 ± 0.10, we find a distance 7.40 ≤ d ≤ 12.31 kpc (90% confidence). This large distance and the high proper motions observed for the radio counterpart make V4641 Sgr possibly the most superluminal galactic source known, with an apparent expansion velocity of 9.5c and a bulk Lorentz factor of Γ 9.5, assuming that the jets were ejected during one of the bright X-ray flares observed with the Rossi X-ray Timing Explorer.

Estimating the spin of stellar-mass black holes by spectral fitting of the X-ray continuum

We fit X-ray spectral data in the thermal-dominant, or high-soft, state of two dynamically confirmed black holes, GRO J1655-40 and 4U 1543-47, and estimate the dimensionless spin parameters a* ≡ a/M of the two holes. For GRO J1655-40, using a spectral hardening factor computed for a non-LTE relativistic accretion disk, we estimate a* ~ 0.75 and a* ~ 0.65-0.75, respectively, from ASCA and RXTE data. For 4U 1543-47, we estimate a* ~ 0.75-0.85 from RXTE data. Thus, neither black hole has a spin approaching the theoretical maximum a* = 1.

Quasi-periodic Oscillations and Spectral States in GRS 1915+105

We present results from the analysis of X-ray energy spectra and quasi-periodic oscillations (QPOs) from a set of observations that samples a broad range of time variability in GRS 1915+105. We first demonstrate that the frequency and integrated amplitude of a 0.5-10 Hz QPO is correlated with the apparent temperature of the accretion disk for the majority of observations. We then show that the behavior of GRS 1915+105 exhibits two distinct modes of accretion. In the first mode, the QPO is present between 0.5 and 10 Hz and variability in the source luminosity is dominated by the power-law component. In the second mode, the QPO is absent and the changes in the luminosity are dominated by thermal emission from the accretion disk. We find that the color radius and temperature of the inner accretion disk are empirically related by Rcol ∝ T + const. We discuss these results in terms of ongoing efforts to explain the origin of both the QPOs and the hard X-ray component in the spectrum of GRS 1915+105.