T. Shahbaz

The Halo Black Hole X-Ray Transient XTE J1118+480*

Optical spectra were obtained of the optical counterpart of the high-latitude (b 62°) soft X-ray transient XTE J1118+480 near its quiescent state (R 18.3) with the new 6.5 m Multiple Mirror Telescope and the 4.2 m William Herschel Telescope. The spectrum exhibits broad, double-peaked emission lines of hydrogen (FWHM 2400 km s-1) arising from an accretion disk superposed with absorption lines of a late-type secondary star. Cross-correlation of the 27 individual spectra with late-type stellar template spectra reveals a sinusoidal variation in radial velocity with amplitude K = 701 ± 10 km s-1 and orbital period P = 0.169930 ± 0.000004 days. The mass function, 6.1 ± 0.3 M☉, is a firm lower limit on the mass of the compact object and strongly implies that it is a black hole. We estimate the spectral type of the secondary to be K7 V-M0 V, and that it contributes 28% ± 2% of the light in the 5800-6400 A region on 2000 November 20, increasing to 36% ± 2% by 2001 January 4 as the disk faded. Photometric observations (R-band) with the Instituto de Astrofisica de Canarias 0.8 m telescope reveal ellipsoidal light variations of full amplitude 0.2 mag. Modeling of the light curve gives a large mass ratio (M1/M2 ~ 20) and a high orbital inclination (i = 81° ± 2°). Our combined fits yield a mass of the black hole in the range M1 = 6.0-7.7 M☉ (90% confidence) for plausible secondary star masses of M2 = 0.09-0.5 M☉. The photometric period measured during the outburst is 0.5% longer than our orbital period and probably reflects superhump modulations, as observed in some other soft X-ray transients. The estimated distance is d = 1.9 ± 0.4 kpc, corresponding to a height of 1.7 ± 0.4 kpc above the Galactic plane. The spectroscopic, photometric, and dynamical results indicate that XTE J1118+480 is the first firmly identified black hole X-ray system in the Galactic halo.

Rapid optical and X-ray timing observations of GX 339−4: multicomponent optical variability in the low/hard state

A rapid timing analysis of Very Large Telescope (VLT)/ULTRACAM (optical) and RXTE (X- ray) observations of the Galactic black hole binary GX 339−4 in the low/hard, post-outburst state of 2007 June is presented. The optical light curves in the r � ,gand ufilters show slow (∼20 s) quasi-periodic variability. Upon this is superposed fast flaring activity on times approaching the best time resolution probed (∼50 ms inrandg � ) and with maximum strengths of more than twice the local mean. Power spectral analysis over ∼0.004-10 Hz is presented, and shows that although the average optical variability amplitude is lower than that in X-rays, the peak variability power emerges at a higher Fourier frequency in the optical. Energetically, we measure a large optical versus X-ray flux ratio, higher than that seen on previous occasions when the source was fully jet dominated. Such a large ratio cannot be easily explained with a disc alone. Studying the optical-X-ray cross-spectrum in Fourier space shows a markedly different behaviour above and below ∼0.2 Hz. The peak of the coherence function above this threshold is associated with a short optical time lag with respect to X-rays, also seen as the dominant feature in the time-domain cross-correlation at ≈150 ms. The rms energy spectrum of these fast variations is best described by distinct physical components over the optical and X-ray regimes, and also suggests a maximal irradiated disc fraction of 20 per cent around 5000 A. If the constant time delay is due to propagation of fluctuations to (or within) the jet, this is the clearest optical evidence to date of the location of this component. The low-frequency quasi-periodic oscillation is seen in the optical but not in X-rays, and is associated with a low coherence. Evidence of reprocessing emerges at the lowest Fourier frequencies, with optical lags at ∼10 s and strong coherence in the blue ufilter. Consistent with this, simultaneous optical spectroscopy also shows the Bowen fluorescence blend, though its emission location is

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.

Evidence for Optical Flares in Quiescent Soft X-Ray Transients

We present the results of high time resolution optical photometry of five quiescent soft X-ray transients (SXTs): V404 Cyg, A0620� 00, J0422+32, GS 2000+25, and Cen X-4. We detect fast optical variations superposed on the secondary star’s double-humped ellipsoidal modulation. The variability resembles typical flare activity and has amplitudes ranging from 0.06 to 0.6 mag. Flares occur on timescales of minutes to a few hours, with no dependency on orbital phase, and contribute � 19%–46% to the total veiling observed in the R band. We find that the observed level of flaring activity is veiled by the light of the companion star, and therefore, systems with cool companions (e.g., J0422+32) exhibit stronger variability. After correcting for this dilution, we do not find any correlation between the flaring activity and fundamental system parameters. We find no underlying coherent periods in the data, only quasi-periodic variations ranging between 30 and 90 minutes for the short-period SXTs and longer than 1 hr for V404 Cyg. The power-law index of the power spectra is consistent with what is observed at X-rays wavelengths, i.e., a 1/f distribution, which is compatible with the cellular automaton model. Our observed R 0 -band luminosities, which are in the range 10 31 –10 33 ergs s � 1 , are too large to be due to chromospheric activity in the rapidly rotating companions. Since the typical timescale of the flares increases with orbital period, they are most likely produced in the accretion disk. The associated dynamical (Keplerian) timescales suggest that flares are produced at � 0.3Rd–0.7Rd. Possible formation mechanisms are magnetic loop reconnection events in the disk or, less likely, optical reprocessing of X-ray flares. In the former scenario, the maximum duration of the flares suggests that the outer disk is responsible for the flare events and so allows us to constrain the sharing timescale to � � 5 6 ðÞ � � 1 K . Subject headings: accretion, accretion disks — binaries: close — stars: individual (V404 Cygni)

Detection of superhumps in XTE J1118+480 approaching quiescence

We present the results of our monitoring of the halo black hole soft X-ray transient (SXT) XTE J1118+480 during its decline to quiescence. The system has decayed 0.5 mag from 2000 December to its present near-quiescent level at R ≃ 18.65 (2001 June). The ellipsoidal light curve is distorted by an additional modulation that we interpret as a superhump of P s h = 0.17049(1)d i.e. 0.3 per cent longer than the orbital period. This implies a disc precession period P p r e c ∼ 52 d. After correcting the average phase-folded light curve for veiling, the amplitude difference between the minima suggests that the binary inclination angle lies in the range i = 71-82°. However, we urge caution in the interpretation of these values because of residual systematic contamination of the ellipsoidal light curve by the complex form of the superhump modulation. The orbital-mean Ha profiles exhibit clear velocity variations with ∼500 km s - 1 amplitude. We interpret this as the first spectroscopic evidence of an eccentric precessing disc.

Correlated X-Ray and Optical Variability in V404 Cygni in Quiescence

We report simultaneous X-ray and optical observations of V404 Cyg in quiescence. The X-ray flux varied dramatically by a factor of 20 during a 60 ks observation. X-ray variations were well correlated with those in Hα, although the latter include an approximately constant component as well. Correlations can also be seen with the optical continuum, although these are less clear. We see no large lag between X-ray and optical line variations; this implies they are causally connected on short timescales. As in previous observations, Hα flares exhibit a double-peaked profile suggesting emission distributed across the accretion disk. The peak separation is consistent with material extending outward to at least the circularization radius. The prompt response in the entire Hα line confirms that the variability is powered by X-ray (and/or EUV) irradiation.

Rapid optical and X-ray timing observations of GX 339–4: flux correlations at the onset of a low/hard state

We present the discovery of optical/X-ray flux correlations on rapid time-scales in the low/hard state of the Galactic black hole GX339-4. The source had recently emerged from outburst and was associated with a relatively faint counterpart with mag V approximate to 17. The optical [ Very Large Telescope (VLT)/ULTRACAM] and X-ray (Rossi X-ray Timing Explorer) data show a clear positive cross-correlation function (CCF) signal, with the optical peak lagging X-rays by similar to 150 ms, preceded by a shallow rise and followed by a steep decline along with broad anticorrelation dips. An examination of the light curves shows that the main CCF features are reproduced in superpositions of flares and dips. The CCF peak is narrow and the X-ray autocorrelation function (ACF) is broader than the optical ACF, arguing against reprocessing as the origin for the rapid optical emission. X-ray flaring is associated with spectral hardening, but no corresponding changes are detected around optical peaks and dips. The variability may be explained in the context of synchrotron emission with interaction between a jet and a corona. The complex CCF structure in GX 339-4 has similarities to that of another remarkable X-ray binary XTE J1118+480, in spite of showing a weaker maximum strength. Such simultaneous multiwavelength, rapid timing studies provide key constraints for modelling the inner regions of accreting stellar sources.

Multiwavelength spectral and high time resolution observations of SWIFT J1753.5−0127: new activity?

We have conducted an extensive observational campaign of SWIFT J1753.5−0127 during 2007 June after its bright outburst episode in 2005. We have performed multiband optical photometry, optical spectroscopy, X-ray spectroscopy and timing and ULTRACAM optical photometry simultaneously in three bands. Both the optical spectrum and the X-ray spectrum along with enhanced brightness in broad-band photometry point to recent increased activity. We analyse the different spectral regions, finding a smooth optical continuum with a remarkable lack of lines and a very blue component modulated with a period of 3.2 h and a hard power-law X-ray spectrum. Both the X-ray and optical power spectra are flat at low frequencies up to the 0.1 Hz (10 s) range, then decreasing roughly as a power law consistent with flickering. Furthermore, the optical data show quasi-periodic oscillations near 0.08 Hz (13 s). Together with a dynamical and autocorrelation analysis of the light curves we attempt to construct a complete physical picture of this intriguing system.

REFINED ORBITAL SOLUTION AND QUIESCENT VARIABILITY IN THE BLACK HOLE TRANSIENT GS 1354-64 (= BW Cir)

In Casares et al. we presented the first radial velocity curve of the companion star to BW Cir which demonstrates the presence of a black hole in this historical X-ray transient. But these data were affected by aliasing and two possible periods at 2.5445 days and 2.5635 days were equally possible. Here we present new spectroscopic data that enable us to break the 1-year aliasing and confirm 2.5445 days as the correct orbital period. We also present R-band photometry over 14 years, which reveals the presence of important flaring activity dominating the light curves.

SWIFT J1753.5-0127 : a surprising optical/X-ray cross-correlation function

We have conducted simultaneous optical and X-ray observations of SWIFT J1753.5-0127 with RXTE and ULTRACAM, while the system persisted in its relatively bright low/hard state. In the cross-correlation function, we find that the optical emission, with a broad negative peak, leads the X-ray emission by a few seconds and has a smaller positive peak at positive lags. This is markedly different from what was seen for the similarly interesting system XTE J1118+480, and it is the first time that such a correlation function has been so clearly measured. We suggest a physical scenario for its origin.