No abstract

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.

We present the results of high time resolution optical photometry of five quiescent soft X-ray transients (SXTs): V404 Cyg, A0620À00, 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 À1 , 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 $0.3R d -0.7R d . 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 ð Þ À1 K .

We present high time-resolution optical and infrared observations of the edge-on black hole X-ray transient Swift J1357.2-0933. Our data taken in 2012 shows the system to be at its pre-outburst magnitude and so the system is in quiescence. In contrast to other X-ray transients, the quiescent light curves of Swift J1357.2-0933 do not show the secondary star's ellipsoidal modulation. The optical and infrared light curves is dominated by variability with an optical fractional rms of about 20 per cent, much larger than what is observed in other systems. The quiescent ultraviolet to mid-IR spectral energy distribution in quiescence is dominated by a nonthermal component with a power-law index of-1.4, (the broad-band rms SED has a similar index) which arises from optically thin synchrotron emission from a jet; the lack of a peak in the spectral energy distribution rules out advection-dominated models (based on [19]).

We present a multiwavelength study of the black hole X‐ray binary V404 Cyg in quiescence, focusing upon the spectral energy distribution (SED). Radio, optical, ultraviolet (UV) and X‐ray coverage is simultaneous. We supplement the SED with additional non‐simultaneous data in the optical through infrared where necessary. The compiled SED is the most complete available for this, the X‐ray and radio brightest quiescent black hole system. We find no need for a substantial contribution from accretion light from the near‐UV to the near‐IR, and in particular the weak UV emission constrains published spectral models for V404 Cyg. We confirm that no plausible companion spectrum and interstellar extinction can fully explain the mid‐IR, however, and an infrared (IR) excess from a jet or cool disc appears to be required. The X‐ray spectrum is consistent with a Γ∼ 2 power law as found by all other studies to date. There is no evidence for any variation in the hardness over a range of a factor of 10 in luminosity. The radio flux is consistent with a flat spectrum (in fν). The break frequency between a flat and optically thin spectrum most likely occurs in the mid or far‐IR, but is not strongly constrained by these data. We find the radio to be substantially variable but with no clear correlation with X‐ray variability.

A B S T R A C TWe present the results of our monitoring of the halo black hole soft X-ray transient (SXT) XTE J1118þ 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 sh ¼ 0:17049ð1Þ d i.e. 0.3 per cent longer than the orbital period. This implies a disc precession period P prec , 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 ¼ 71 -828. 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 21 amplitude. We interpret this as the first spectroscopic evidence of an eccentric precessing disc.

We report simultaneous X-ray and optical observations of V404 Cyg in quiescence. The X-ray flux varied dramatically by a factor of տ20 during a 60 ks observation. X-ray variations were well correlated with those in Ha, 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, Ha 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 Ha line confirms that the variability is powered by X-ray (and/or EUV) irradiation.

We present optical photometry and spectroscopy of the X‐ray transient XTE J1859+226, obtained during outburst and its subsequent decay to quiescence. Both the X‐ray and optical properties are very similar to those of well‐studied black hole soft X‐ray transients. We have detected three mini‐outbursts, when XTE J1859+226 was approaching quiescence, as has been previously detected in the soft X‐ray transients GRO J0422+32 and GRS 1009–45. By 2000 August 24 the system had reached quiescence with R= 22.48 ± 0.07. The estimated distance to the source is ∼11 kpc. Photometry taken during quiescence shows a sinusoidal modulation with a peak to peak amplitude of about 0.4 mag. A period analysis suggests that periods from 0.28 to 0.47 d are equally possible at the 68 per cent confidence level. The amplitude of the quiescent light curve and the relatively low ratio of X‐ray to optical flux indicate that the binary inclination should be high. The measured colours during the outburst allow us to obtain the basic properties of the disc, which agrees well with irradiated disc model predictions.