M. A. P. Torres

THE 2003 OUTBURST OF THE X-RAY TRANSIENT H1743–322: COMPARISONS WITH THE BLACK HOLE MICROQUASAR XTE J1550–564

The bright X-ray transient H1743–322 was observed daily by the Rossi X-ray Timing Explorer during most of its eight-month outburst in 2003. We present a detailed spectral analysis and a supporting timing analysis of all of these data, and we discuss the behavior and evolution of the source in terms of the three principal X-ray states defined by Remillard and McClintock. These X-ray results are complemented by Very Large Array data obtained at six frequencies that provide quite complete coverage of the entire outburst cycle at 4.860 GHz and 8.460 GHz. We also present photometric data and finding charts for the optical counterpart in both outburst and quiescence. We closely compare H1743–322 to the well-studied black hole X-ray transient XTE J1550–564 and find the behaviors of these systems to be very similar. As reported elsewhere, both H1743–322 and XTE J1550–564 are relativistic jet sources and exhibit a pair of high-frequency quasi-periodic oscillations with a 3:2 frequency ratio. The many striking similarities between these two sources argue strongly that H1743–322 is a black hole binary, although presently no dynamical data exist to support this conclusion.

Swift Observations of the Cooling Accretion Disk of XTE J1817–330

The black hole candidate X-ray transient XTE J1817-330 was observed by the Swift satellite over 160 days of its 2006 outburst with the XRT and UVOT instruments. At the start of the observations, the XRT spectra show that the 0.6-10 keV emission is dominated by an optically thick, geometrically thin accretion disk with an inner disk temperature of ~0.8 keV, indicating that the source was in a high/soft state during the initial outburst phase. We tracked the source through its decline into the low/hard state with the accretion disk cooling to ~0.2 keV and the inner disk radius consistent with the innermost stable circular orbit at all times. Furthermore, the X-ray luminosity roughly follows LX T4 during the decline, consistent with a geometrically stable blackbody. These results are the strongest evidence yet obtained that accretion disks do not automatically recede after a state transition, down to accretion rates as low as 0.001LEdd. Meanwhile, the near-UV flux does not track the X-ray disk flux and is well in excess of what is predicted if the near-UV emission is from viscous dissipation in the outer disk. The strong correlation between the hard X-ray flux and the near-UV flux, which scale as L, indicate that reprocessed emission is most likely the dominate contribution to the near-UV flux. We discuss our results in the context of accretion disks and the overall accretion flow geometry in accreting black holes.

SWIFT J1753.5-0127 : THE BLACK HOLE CANDIDATE WITH THE SHORTEST ORBITAL PERIOD

We present time-resolved photometry of the optical counterpart to the black hole candidate Swift J1753.5–0127 which has remained in the low/hard X-ray state and bright at optical/IR wavelengths since its discovery in 2005. At the time of our observations Swift J1753.5–0127 does not show a decay trend but remains stable at R = 16.45 with a night-to-night variability of ~0.05 mag. The R-band light curves, taken from 2007 June 3 to August 31, are not sinusoidal, but exhibit a complex morphology with remarkable changes in shape and amplitude. The best period determination is 3.2443 ± 0.0010 hr. This photometric period is likely a superhump period, slightly larger than the orbital period. Therefore, Swift J1753.5–0127 is the black hole candidate with the shortest orbital period observed to date. Our estimation of the distance is comparable to values previously published and likely places Swift J1753.5–0127 in the Galactic halo.

A Black Hole Nova Obscured by an Inner Disk Torus

A Hidden Black Hole? Black holes with masses comparable to that of the Sun are often associated with variable x-ray sources. Corral-Santana et al. (p. 1048) report optical observations of a faint and variable x-ray source detected in our galaxy with the Swift Burst Alert Telescope. The optical data reveal a black hole with a mass greater than three times that of the Sun in a 2.8-hour period around a low-mass donor star. Unusual for this type of system, the black hole binary is seen at a very high inclination. Optical observations of a variable binary x-ray source reveal a black hole 3.6 times the Suns mass in a short-period orbit. Stellar-mass black holes (BHs) are mostly found in x-ray transients, a subclass of x-ray binaries that exhibit violent outbursts. None of the 50 galactic BHs known show eclipses, which is surprising for a random distribution of inclinations. Swift J1357.2−093313 is a very faint x-ray transient detected in 2011. On the basis of spectroscopic evidence, we show that it contains a BH in a 2.8-hour orbital period. Further, high–time-resolution optical light curves display profound dips without x-ray counterparts. The observed properties are best explained by the presence of an obscuring toroidal structure moving outward in the inner disk, seen at very high inclination. This observational feature should play a key role in models of inner accretion flows and jet collimation mechanisms in stellar-mass BHs.

OBSERVATIONS OF THE 599 Hz ACCRETING X-RAY PULSAR IGR J00291+5934 DURING THE 2004 OUTBURST AND IN QUIESCENCE

We report on optical and near-infrared observations obtained during and after the 2004 December discovery outburst of the X-ray transient and accretion-powered millisecond pulsar IGR J00291+5934. Our observations monitored the evolution of the brightness and the spectral properties of IGR J00291+5934 during the outburst decay toward quiescence. We also present optical, near-infrared, and Chandra observations obtained during true quiescence. Photometry of the field during outburst reveals an optical and near-infrared counterpart that brightened from R similar or equal to 23 to R similar or equal to 17 and from K = 19 to K similar or equal to 16. Spectral analysis of the RIJHK broadband photometry shows excess in the near-infrared bands that may be due to synchrotron emission. The H alpha emission line profile suggests the orbital inclination is similar or equal to 22 degrees-32 degrees. The preferred range for the reddening toward the source is 0.7 <= E(B - V) <= 0: 9, which is equivalent to 4.06 x 10(21) cm(-2) <= N-H <= 5.22 x 10(21) cm(-2). The Chandra observations of the pulsar in its quiescent state gave an unabsorbed 0.5-10 keV flux for the best-fitting power-law model to the source spectrum of (7.0 +/- 0.9)x10(-14) ergs cm(-2) s(-1) (adopting a hydrogen column of 4.6x10(21) cm(-2)). The fit resulted in a power-law photon index of 2.4(-0.4)(+0.5). The (R - K)(0) color observed during quiescence supports an irradiated donor star and accretion disk. We estimate a distance of 2-4 kpc toward IGR J00291+ 5934 by using the outburst X-ray light curve and the estimated critical X-ray luminosity necessary to keep the outer parts of the accretion disk ionized. Using the quiescent X-ray luminosity and the spin period, we constrain the magnetic field of the neutron star to be <3x10(8) G.

He II lambda-4686 in Eta Carinae: Collapse of the Wind-Wind Collision Region During Periastron Passage

The periodic spectroscopic events in ? Carinae are now well established and occur near the periastron passage of two massive stars in a very eccentric orbit. Several mechanisms have been proposed to explain the variations of different spectral features, such as an eclipse by the wind-wind collision (WWC) boundary, a shell ejection from the primary star or accretion of its wind onto the secondary. All of them have problems explaining all the observed phenomena. To better understand the nature of the cyclic events, we performed a dense monitoring of ? Carinae with five Southern telescopes during the 2009 low-excitation event, resulting in a set of data of unprecedented quality and sampling. The intrinsic luminosity of the He II ?4686 emission line (L ~ 310 L ?) just before periastron reveals the presence of a very luminous transient source of extreme UV radiation emitted in the WWC region. Clumps in the primarys wind probably explain the flare-like behavior of both the X-ray and He II ?4686 light curves. After a short-lived minimum, He II ?4686 emission rises again to a new maximum, when X-rays are still absent or very weak. We interpret this as a collapse of the WWC onto the surface of the secondary star, switching off the hard X-ray source and diminishing the WWC shock cone. The recovery from this state is controlled by the momentum balance between the secondarys wind and the clumps in the primarys wind.

Optical and X‐Ray Observations of IGR J00291+5934 in Quiescence

We report on optical and X-ray observations of the accretion-powered millisecond pulsar IGR J00291+5934 in quiescence. Time-resolved I-band photometry has been obtained with the 4.2 m William Herschel Telescope, while a 3 ks Chandra observation provided contemporaneous X-ray coverage. We found an unabsorbed 0.5-10 keV X-ray flux of 1 × 10−13 erg cm−2 s−1, which implies that the source was in quiescence at the time of the optical observations. Nevertheless, the optical I-band light curve of IGR J00291+5934 shows evidence for strong flaring. After removal of the strongest flares, we find evidence for an orbital modulation in the phase-folded I-band light curve. The overall modulation can be described by effects resulting from the presence of a superhump. Comparing our light curve with that reported recently we find evidence for a change in the quiescent base level. Similar changes have now been reported for four soft X-ray transients, implying that they may be a common feature of such systems in quiescence. Furthermore, the maximum in our folded light curve occurs at a different phase than observed before.

Optical spectroscopy of the quiescent counterpart to EXO 0748−676

We present phase resolved optical spectroscopy and X-ray timing of the neutron star X-ray binary EXO 0748-676 after the source returned to quiescence in the autumn of 2008. The X-ray light curve displays eclipses consistent in orbital period, orbital phase and duration with the predictions and measurements before the return to quiescence. Ha and He I emission lines are present in the optical spectra and show the signature of the orbit of the binary companion, placing a lower limit on the radial velocity semi-amplitude of K-2 > 405 km s(-1). Both the flux in the continuum and the emission lines show orbital modulations, indicating that we observe the hemisphere of the binary companion that is being irradiated by the neutron star. Effects due to this irradiation preclude a direct measurement of the radial velocity semi-amplitude of the binary companion; in fact, no stellar absorption lines are seen in the spectrum. Nevertheless, our observations place a stringent lower limit on the neutron star mass of M-1 > 1.27 M-circle dot. For the canonical neutron star mass of M-1 = 1.4 M-circle dot, the mass ratio is constrained to 0.075 < q < 0.105.

Time‐resolved spectroscopy of the pulsating CV GW Lib

We present time-resolved optical spectroscopy of the dwarf nova GW Librae during its rare 2007 April superoutburst and compare these with quiescent epochs. The data provide the first opportunity to track the evolution of the principal spectral features. In the early stages of the outburst, the optically thick disc dominates the optical and the line components show clear orbital radial velocity excursions. In the course of several weeks, optically thin regions become more prominent as strong emission lines replace the broad disc absorption. Post-outburst spectroscopy covering the I band illustrates the advantages of Ca II relative to the commonly used Balmer lines when attempting to constrain binary parameters. Due to the lower ionization energy combined with smaller thermal and shear broadening of these lines, a sharp emission component is seen to be moving in between the accretion disc peaks in the Ca II line. No such component is visible in the Balmer lines. We interpret this as an emission component originating on the hitherto unseen mass donor star. This emission component has a mean velocity of similar to -15 +/- 5 km s(-1) which is associated with the systemic velocity., and a velocity semi-amplitude of K-em = 82.2 +/- 4.9 km s(-1). Doppler tomography reveals an asymmetric accretion disc, with the S-wave mapping to a sharp spot in the tomogram with a velocity consistent to what is obtained with line profile fitting. A centre of symmetry analysis of the disc component suggests a very small value for the WD orbital velocity K-1 as is also inferred from double Gaussian fits to the spectral lines. While our conservative dynamical limits place a hard upper limit on the binary mass ratio of q < 0.23, we favour a significantly lower value near q similar to 0.06. Pulsation modelling suggests a white dwarf mass similar to 1 M-circle dot. This, paired with a low-mass donor, near the empirical sequence of an evolved cataclysmic variable close to the period bounce, appears to be consistent with all the observational constraints to date.

IDENTIFICATION OF GALACTIC BULGE SURVEY X-RAY SOURCES WITH TYCHO-2 STARS

We identify 69 X-ray sources discovered by the Galactic Bulge Survey (GBS) that are coincident with or very close to bright stars in the Tycho-2 catalog. Additionally, two other GBS sources are resolved binary companions to Tycho-2 stars where both components are separately detected in X-rays. Most of these are likely to be real matches, but we identify nine objects with large and significant X-ray-to-optical offsets as either detections of resolved binary companions or chance alignments. We collate known spectral types for these objects, and also examine Two Micron All Sky Survey colors, variability information from the All-Sky Automated Survey, and X-ray hardness ratios for the brightest objects. Nearly a third of the stars are found to be optically variable, divided roughly evenly between irregular variations and periodic modulations. All fall among the softest objects identified by the GBS. The sample forms a very mixed selection, ranging in spectral class from O9 to M3. In some cases, the X-ray emission appears consistent with normal coronal emission from late-type stars, or wind emission from early-types, but the sample also includes one known Algol, one W UMa system, two Be stars, and several X-ray bright objects likely to be coronally active stars or binaries. Surprisingly, a substantial fraction of the spectroscopically classified, non-coincidental sample (12 out of 38 objects) have late B or A type counterparts. Many of these exhibit redder near-IR colors than expected for their spectral type and/or variability, and it is likely that the X-rays originate from a late-type companion star in most or all of these objects.