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We present a study of the X-ray flaring activity of Sgr A* during all the 150XMM-Newton and Chandra observations pointed at the Milky Way center over thelast 15 years. This includes the latest XMM-Newton and Chandra campaignsdevoted to monitoring the closest approach of the very red Br-Gamma emittingobject called G2. The entire dataset analysed extends from September 1999through November 2014. We employed a Bayesian block analysis to investigate anypossible variations in the characteristics (frequency, energetics, peakintensity, duration) of the flaring events that Sgr A* has exhibited sincetheir discovery in 2001. We observe that the total bright-or-very bright flareluminosity of Sgr A* increased between 2013-2014 by a factor of 2-3 (~3.5 sigmasignificance). We also observe an increase (~99.9% significance) from0.27+-0.04 to 2.5+-1.0 day^-1 of the bright-or-very bright flaring rate of SgrA*, starting in late summer 2014, which happens to be about six months afterG2's peri-center passage. This might indicate that clustering is a generalproperty of bright flares and that it is associated with a stationary noiseprocess producing flares not uniformly distributed in time (similar to what isobserved in other quiescent black holes). If so, the variation in flaringproperties would be revealed only now because of the increased monitoringfrequency. Alternatively, this may be the first sign of an excess accretionactivity induced by the close passage of G2. More observations are necessary todistinguish between these two hypotheses.

AX J1745.6-2901 is a high-inclination (eclipsing) neutron star Low Mass X-rayBinary (LMXB) located less than ~1.5 arcmin from Sgr A*. Ongoing monitoringcampaigns have targeted Sgr A* frequently and these observations also cover AXJ1745.6-2901. We present here an X-ray analysis of AX J1745.6-2901 using alarge dataset of 38 XMM-Newton observations, including eleven which caught AXJ1745.6-2901 in outburst. Fe K absorption is clearly seen when AX J1745.6-2901is in the soft state, but disappears during the hard state. The variability ofthese absorption features does not appear to be due to changes in the ionizingcontinuum. The small Kalpha/Kbeta ratio of the equivalent widths of the Fe xxvand Fe xxvi lines suggests that the column densities and turbulent velocitiesof the absorbing ionised plasma are in excess of N_H ~ 10^23 cm^-2 and v_turb>~ 500 km s^-1. These findings strongly support a connection between the wind(Fe K absorber) and the accretion state of the binary. These results revealstrong similarities between AX J1745.6-2901 and the eclipsing neutron starLMXB, EXO 0748-676, as well as with high-inclination black hole binaries, wherewinds (traced by the same Fe K absorption features) are observed only duringthe accretion-disc-dominated soft states, and disappear during the hard statescharacterised by jet emission.

We present Very Large Telescope spectroscopy, high-resolution imaging, and time-resolved photometry of KY TrA, the optical counterpart to the X-ray binary A 1524-61. We perform a refined astrometry of the field, yielding improved coordinates for KY TrA and the field star interloper of similar optical brightness that we locate 0.64 ± 0.04 arcsec SE. From the spectroscopy, we refine the radial velocity semi-amplitude of the donor star to K 2 = 501 ± 52 km s−1 by employing the correlation between this parameter and the full width at half-maximum of the H α emission line. The r -band light curve shows an ellipsoidal-like modulation with a likely orbital period of 0.26 ± 0.01 d (6.24 ± 0.24 h). These numbers imply a mass function f (M 1) = 3.2 ± 1.0 M⊙. The KY TrA de-reddened quiescent colour (r − i)0 = 0.27 ± 0.08 is consistent with a donor star of spectral type K2 or later, in case of significant accretion disc light contribution to the optical continuum. The colour allows us to place a very conservative upper limit on the companion star mass, M 2 ≤ 0.94 M⊙, and, in turn, on the binary mass ratio, q  = M 2/ M 1 ≤ 0.31. By exploiting the correlation between the binary inclination and the depth of the H α line trough, we establish i  = 57 ± 13 deg. All these values lead to a compact object and donor mass of |$M_1 = 5.8^{+3.0}_{-2.4}$| |$\, M_\odot$| and M 2 = 0.5 ± 0.3 M⊙, thus confirming the black hole nature of the accreting object. In addition, we estimate a distance towards the system of 8.0 ± 0.9 kpc. [ABSTRACT FROM AUTHOR]

We present 20 epochs of optical spectroscopy obtained with the GTC-10.4m telescope across the bright discovery outburst of the black hole candidate Swift J1727.8−162. The spectra cover the main accretion states and are characterised by the presence of hydrogen and helium emission lines, commonly observed in these objects. They show complex profiles, including double peaks, but also blue-shifted absorptions (with blue-edge velocities of 1150 km s−1), broad emission wings, and flat-top profiles, which are the usual signatures of accretion disc winds. Moreover, red-shifted absorptions accompanied by blue emission excesses suggest the presence of inflows in at least two epochs, although a disc origin cannot be ruled out. Using pre-outburst imaging from Pan-STARRS, we identify a candidate quiescent optical counterpart with a magnitude of g ∼ 20.8. This implies an outburst optical amplitude of ΔV ∼ 7.7, supporting an estimated orbital period of ∼7.6 h, which favours an early K-type companion star. Employing various empirical methods, we derive a distance to the source of d = 2.7 ± 0.3 kpc, corresponding to a Galactic plane elevation of z = 0.48 ± 0.05 kpc. Based on these findings, we propose that Swift J1727.8−162 is a nearby black hole X-ray transient that exhibited complex signatures of optical inflows and outflows throughout its discovery outburst. [ABSTRACT FROM AUTHOR]

Swift J1357.2-0933 is a black hole transient that is of particular interest due to the optical recurrent dips found during its first two outbursts (in 2011 and 2017), which lack an obvious X-ray equivalent. We present a study based on fast optical photometry during its two most recent outbursts, in 2019 and 2021. Our observations reveal that the optical dips were present in every observed outburst of the source, although they were shallower and showed longer recurrence periods in the two most recent and fainter events. We performed a global study of the dips properties in the four outbursts and found that they do not follow a common temporal evolution. In addition, we discovered a correlation with the X-ray and optical fluxes, with the dips being more profound and showing shorter recurrence periods for brighter stages. This trend seems to extend even to the faintest, quiescent states of the source. We discuss these results in the context of the possible connection between optical dips and outflows found in previous works. [ABSTRACT FROM AUTHOR]

We present evidence for a 0.278(8) d ( =6.7 h) orbital period in the X-ray transient GRS 1716-249 (=N Oph 93), based on a superhump modulation detected during the 1995 mini-outburst plus ellipsoidal variability in quiescence. With a quiescent magnitude of r  = 23.19 ± 0.15 N Oph 93 is too faint to warrant a full dynamical study through dedicated time-resolved spectroscopy. Instead, we apply the FWHM– K 2 correlation to the disc H α emission line detected in Gran Telescopio Canarias spectra and obtain K 2 = 521 ± 52 km s−1. This leads to a mass function f (M) = 4.1 ± 1.2 M⊙, thus indicating the presence of a black hole in this historic X-ray transient. Furthermore, from the depth of the H α trough and the quiescent light curve we constrain the binary inclination to i  = 61 ± 15°, while the detection of superhumps sets an upper limit to the donor to compact star mass ratio q  = M 2/ M 1 ≲ 0.25. Our de-reddened (r − i) colour is consistent with a ≈ K6 main sequence star that fills its Roche lobe in a 0.278 d orbit. Using all this information we derive a compact object mass |$M_{1}=6.4^{+3.2}_{-2.0}$| M⊙ at 68 per cent confidence. We also constrain the distance to GRS 1716-249 to 6.9 ± 1.1 kpc, placing the binary ∼0.8 kpc above the Galactic Plane, in support of a large natal kick. [ABSTRACT FROM AUTHOR]

We present two contiguous nights of simultaneous time-resolved Gran Telescopio Canarias spectroscopy and William Herschel Telescope photometry of the black hole X-ray transient XTE J1859+226, obtained in 2017 July during quiescence. Cross-correlation of the individual spectra against a late K-type spectral template enabled us to constrain the orbital period to 0.276 ± 0.003 d and the radial velocity semi-amplitude of the donor star to K 2 = 550 ± 59 km s−1. An ellipsoidal modulation is detected in the photometric r - and i -band light curves, although it is strongly contaminated by flickering activity. By exploiting correlations between the properties of the double-peaked H α emission-line profile and the binary parameters, we derived an orbital inclination of 66.6 ± 4.3 deg, a refined K 2 = 562 ± 40 km s−1 and mass ratio q  = M 2/ M 1 = 0.07 ± 0.01. From these values, we obtained an updated black hole mass of M 1 = 7.8 ± 1.9 M⊙. An independent mass estimate based on X-ray timing agrees well with our value, which gives further support for the outburst quasi-periodic oscillation triplet being explained by the relativistic precession model. We also obtained a companion star mass M 2 = 0.55 ± 0.16 M⊙, which is consistent with its K5-K7 V spectral type. [ABSTRACT FROM AUTHOR]

We present a new method to derive binary inclinations in quiescent black hole (BH) X-ray transients (XRTs), based on the depth of the trough (T) from double-peaked H α emission profiles arising in accretion discs. We find that the inclination angle (i) is linearly correlated with T in phase-averaged spectra with sufficient orbital coverage (≳50 per cent) and spectral resolution, following i (deg) = 93.5 × T + 23.7. The correlation is caused by a combination of line opacity and local broadening, where a leading (excess broadening) component scales with the deprojected velocity of the outer disc. Interestingly, such scaling allows to estimate the fundamental ratio M 1/ P orb by simply resolving the intrinsic width of the double-peak profile. We apply the T – i correlation to derive binary inclinations for GRO J0422+32 and Swift J1357−0933, two BH XRTs where strong flickering activity has hindered determining their values through ellipsoidal fits to photometric light curves. Remarkably, the inclination derived for GRO J0422+32 (i = 55.6 ± 4.1○) implies a BH mass of |$2.7^{+0.7}_{-0.5}$| M⊙ thus placing it within the gap that separates BHs from neutron stars. This result proves that low-mass BHs exist in nature and strongly suggests that the so-called 'mass gap' is mainly produced by low number statistics and possibly observational biases. On the other hand, we find that Swift J1357−0933 contains a |$10.9^{+1.7}_{-1.6}$| M⊙ BH, seen nearly edge on (⁠|$i=87.4^{+2.6}_{-5.6}$| deg). Such extreme inclination, however, should be treated with caution since it relies on extrapolating the T – i correlation beyond i ≳ 75○, where it has not yet been tested. [ABSTRACT FROM AUTHOR]

IGR J17285−2922 is a known X-ray binary with a low peak 2–10 keV X-ray luminosity of ∼ 1036 erg s−1 during outburst. IGR J17285−2922 exhibited two outbursts in 2003 and 2010 and went into outburst again in 2019. We have monitored this ∼ 4-month long 2019 outburst with Swift in X-ray and the Very Large Array in radio. We have also obtained four optical spectra with the Gran Telescopio Canarias and Southern Astrophysical Research Telescope, three optical photometry measurements with the Las Cumbres Observatory, and one near-infrared spectrum with the Gemini South telescope. The ratio between its X-ray and radio luminosity is consistent with both samples of neutron star and black hole (BH) X-ray binaries, while the ratio between the X-ray and optical luminosity is consistent with BH X-ray binaries. Studying the evolution of its X-ray power-law index throughout the outburst, we find additional evidence for a BH as compact object. The four optical spectra show no H α emission and the nIR spectrum shows no Brγ emission, suggesting that the donor star could be hydrogen-poor and hence that IGR J17285−2922 might have an ultracompact binary orbit. The shape of the X-ray light curve is well described by an exponential, followed by a linear decay, from which we obtain a relation between the orbital period P orb and the binary mass ratio. We discuss how this relation is consistent with theoretical predictions and known ultracompact X-ray binaries. Lastly, we discuss how the observed properties are reminiscent of short- P orb BH X-ray binaries. [ABSTRACT FROM AUTHOR]

We present the discovery of eclipses in the X-ray light curves of the X-ray binary Swift J1858.6–0814. From these, we find an orbital period of |$P=76841.3_{-1.4}^{+1.3}$| s (≈21.3 h) and an eclipse duration of |$t_{\rm ec}=4098_{-18}^{+17}$| s (≈1.14 h). We also find several absorption dips during the pre-eclipse phase. From the eclipse duration to orbital period ratio, the inclination of the binary orbit is constrained to i > 70°. The most likely range for the companion mass suggests that the inclination is likely to be closer to this value than 90. The eclipses are also consistent with earlier data, in which strong variability ('flares') and the long orbital period prevent clear detection of the period or eclipses. We also find that the bright flares occurred preferentially in the post-eclipse phase of the orbit, likely due to increased thickness at the disc-accretion stream interface preventing flares being visible during the pre-eclipse phase. This supports the notion that variable obscuration is responsible for the unusually strong variability in Swift J1858.6–0814. [ABSTRACT FROM AUTHOR]

We present optical spectroscopy obtained with the GTC, VLT, and SALT telescopes during the decline of the 2016–2017 outburst of the black hole candidate GRS 1716−249 (Nova Oph 1993). Our 18-epoch data set spans 6 months and reveals that the observational properties of the main emission lines are very variable, even on time-scales of a few hours. Several epochs are characterized by P-Cyg (as well as flat-top and asymmetric) profiles in the  Hα,  Hβ, and He  ii (λ4686) emission lines, implying the presence of an accretion disc wind, which is likely hot and dense. The wind's terminal velocity (∼2000 km s−1) is similar to that observed in other black hole X-ray transients. These lines also show transient and sharp red-shifted absorptions, taking the form of inverted P-Cyg profiles. We argue that these profiles can be explained by the presence of infalling material at ∼1300 km s−1. We propose a failed wind scenario to explain this inflow and discuss other alternatives, such as obscuration produced by an accretion-related structure (e.g. the gas stream) in a high inclination system. [ABSTRACT FROM AUTHOR]

We find soft X-ray emission lines from the X-ray binary Swift J1858.6–0814 in data from XMM–Newton Reflection Grating Spectrometer (RGS): N  vii , O  vii , and O  viii , as well as notable residuals short of a detection at Ne  ix and other higher ionization transitions. These could be associated with the disc atmosphere, as in accretion disc corona sources, or with a wind, as has been detected in Swift J1858.6–0814 in emission lines at optical wavelengths. Indeed, the N  vii line is redshifted, consistent with being the emitting component of a P-Cygni profile. We find that the emitting plasma has an ionization parameter log (ξ) = 1.35 ± 0.2 and a density n > 1.5 × 1011 cm−3. From this, we infer that the emitting plasma must be within 1013 cm of the ionizing source, ∼5 × 107 rg for a 1.4 M⊙ neutron star, and from the line width that it is at least 104 rg away [2 × 109(M /1.4 M⊙) cm]. We compare this with known classes of emission-line regions in other X-ray binaries and active galactic nuclei. [ABSTRACT FROM AUTHOR]

Swift J1858.6-0814 is a transient neutron star X-ray binary discovered in 2018 October. Multiwavelength follow-up observations across the electromagnetic spectrum revealed many interesting properties, such as erratic flaring on minute time-scales and evidence for wind outflows at both X-ray and optical wavelengths, strong and variable local absorption, and an anomalously hard X-ray spectrum. Here, we report on a detailed radio observing campaign consisting of one observation at 5.5/9 GHz with the Australia Telescope Compact Array, and nine observations at 4.5/7.5 GHz with the Karl G. Jansky Very Large Array. A radio counterpart with a flat to inverted radio spectrum is detected in all observations, consistent with a compact jet being launched from the system. Swift J1858.6-0814 is highly variable at radio wavelengths in most observations, showing significant variability when imaged on 3-to-5-min time-scales and changing up to factors of 8 within 20 min. The periods of brightest radio emission are not associated with steep radio spectra, implying they do not originate from the launching of discrete ejecta. We find that the radio variability is similarly unlikely to have a geometric origin, be due to scintillation, or be causally related to the observed X-ray flaring. Instead, we find that it is consistent with being driven by variations in the accretion flow propagating down the compact jet. We compare the radio properties of Swift J1858.6-0814 with those of Eddington-limited X-ray binaries with similar X-ray and optical characteristics, but fail to find a match in radio variability, spectrum, and luminosity. [ABSTRACT FROM AUTHOR]

The black hole candidate and X-ray binary MAXI J1535−571 was discovered in 2017 September. During the decay of its discovery outburst, and before returning to quiescence, the source underwent at least four reflaring events, with peak luminosities of ∼1035–36 erg s−1 (d /4.1 kpc)2. To investigate the nature of these flares, we analysed a sample of NICER (Neutron star Interior Composition Explorer) observations taken with almost daily cadence. In this work, we present the detailed spectral and timing analysis of the evolution of the four reflares. The higher sensitivity of NICER at lower energies, in comparison with other X-ray detectors, allowed us to constrain the disc component of the spectrum at ∼0.5 keV. We found that during each reflare the source appears to trace out a q-shaped track in the hardness–intensity diagram similar to those observed in black hole binaries during full outbursts. MAXI J1535−571 transits between the hard state (valleys) and softer states (peaks) during these flares. Moreover, the Comptonized component is undetected at the peak of the first reflare, while the disc component is undetected during the valleys. Assuming the most likely distance of 4.1 kpc, we find that the hard-to-soft transitions take place at the lowest luminosities ever observed in a black hole transient, while the soft-to-hard transitions occur at some of the lowest luminosities ever reported for such systems. [ABSTRACT FROM AUTHOR]

We present high time resolution optical spectroscopy and imaging of the black hole transient Swift  J1357.2−0933 during its 2017 outburst. The light curves show recurrent dips resembling those discovered during the 2011 outburst. The dip properties (e.g. duration and depth) as well as the evolution of their recurrence time are similar to those seen in 2011. Spectra obtained during the dips are characterized by broad and blueshifted absorptions in Balmer and He  ii. The absorptions show core velocities of ∼−800 km s−1 and terminal velocities approaching ∼3000 km s−1 i.e. in the upper end of wind velocities measured in other black hole transients (both at optical and X-ray wavelengths). Our observations suggest that the dips are formed in a dense and clumpy outflow, produced near the disc equatorial plane and seen at high inclination. We also study the colour evolution and observe that, as it has been previously reported, the source turns bluer during dips. We show that this is due to a gradual change in the slope of the optical continuum and discuss possible implications of this behaviour. [ABSTRACT FROM AUTHOR]

We study the optical evolution of the 2015 outburst in V404 Cyg, with emphasis on the peculiar nebular phase and subsequent decay to quiescence. From the decay time-scale of the Balmer emission associated with the nebula, we measure an outflow mass M wind ≃ 4 × 10−6 M⊙. Remarkably, this is ∼100 times larger than the accreted mass and ∼10 per cent of the total mass stored in the disc. The wind efficiency must therefore be significantly larger than previous estimates for black hole transients, suggesting that radiation pressure (in addition to other mechanisms such as Compton-heating) plays a key role in V404 Cyg. In addition, we compare the evolution of the 2015 and 1989 outbursts and find not only clear similarities (namely a large luminosity drop ∼10 d after the X-ray trigger, followed by a brief nebular phase) but also remarkable differences in decay time-scales and long-term evolution of the H α profile. In particular, we see evidence for a rapid disc contraction in 2015, consistent with a burst of mass transfer. This could be driven by the response of the companion to hard X-ray illumination, most notably during the last gigantic (super-Eddington) flare on 2015 June 25. We argue that irradiation and consequential disc wind are key factors to understand the different outburst histories in 1989 and 2015. In the latter case, radiation pressure may be responsible for the abrupt end of the outburst through depleting inner parts of the disc, thus quenching accretion and X-ray irradiation. We also present a refined orbital period and updated ephemeris. [ABSTRACT FROM AUTHOR]

MAXI J1813-095 is an X-ray transient discovered during an outburst in 2018. We report on X-ray and optical observations obtained during this event, which indicate that the source is a new low-mass X-ray binary. The outburst lasted ∼70 d and peaked at L X(0.5–10 keV) ∼ 7.6 × 1036 erg s−1, assuming a distance of 8 kpc. Swift /XRT follow-up covering the whole activity period shows that the X-ray emission was always dominated by a hard power-law component with a photon index in the range of 1.4–1.7. These values are consistent with MAXI J1813-095 being in the hard state, in agreement with the ∼30 per cent fractional root-mean-square amplitude of the fast variability (0.1–50 Hz) inferred from the only XMM–Newton observation available. The X-ray spectra are well described by a Comptonization emission component plus a soft, thermal component (kT ∼ 0.2 keV), which barely contributes to the total flux (≲8 per cent). The Comptonization y-parameter (∼1.5), together with the low temperature and small contribution of the soft component supports a black hole accretor. We also performed optical spectroscopy using the Very Large Telescope and Gran Telescopio Canarias telescopes during outburst and quiescence, respectively. In both cases, the spectrum lacks emission lines typical of X-ray binaries in outburst. Instead, we detect the Ca  ii triplet and H α in absorption. The absence of velocity shifts between the two epochs, as well as the evolution of the H α equivalent width, strongly suggest that the optical emission is dominated by an interloper, likely a G–K star. This favours a distance ≳3 kpc for the X-ray transient. [ABSTRACT FROM AUTHOR]

MAXI J1807+132 is an X-ray transient discovered during the decay of an outburst in 2017. We present optical and X-ray monitoring of the source over more than 125 d, from outburst to quiescence. The outburst decay is characterized by the presence of several reflares with a quasi-periodic recurrence time of ∼6.5 d. We detect broad H and He emission lines during outburst, characteristic of transient low-mass X-ray binaries. These emission lines show strong variability from epoch to epoch and, in particular, during the early stages are found embedded into deep and very broad absorption features. The quiescent spectrum shows Hα in emission and no obvious signatures of the donor star. XMM–Newton and Swift spectra can be fitted with standard X-ray models for accreting black holes and neutron stars, although the obtained spectral parameters favour the latter scenario. Conversely, other observables such as the optical/X-ray flux ratio, the likely systemic velocity (γ ∼ −150 km s−1), and the reflares recurrence time suggest a black hole nature. We discuss all the above possibilities with emphasis on the strong similarities of MAXI J1807+132 with short orbital period systems. [ABSTRACT FROM AUTHOR]

The new recurrent X-ray transient MAXI J1957+032 has had four X-ray outbursts within 16 months, all very briefly detected (lasting <5 d). During the most recent event (2016 September/October), we obtained with the Southern African Large Telescope the first optical spectrum of the transient counterpart, showing the classic blue continuum of an X-ray irradiated disc in an LMXB and no other features. At high Galactic latitude below the plane (-13°) reddening is low but there is no quiescent counterpart visible on any of the existing sky surveys, nor any other known X-ray source in the region. Swift monitoring of three of the four events is presented, showing rapidly fading X-ray outbursts together with significant UVOT detections in the UV (W1,M2,W2), U and B bands. The optical properties are most like those of the short-period LMXBs, which, combined with the softening witnessed during the decay to quiescence would place the system at d < 13 kpc. The short duration and short recurrence time of the outbursts are reminiscent of the accreting millisecond X-ray pulsars, which exhibit peak luminosities of ~1 per cent LEdd. Assuming this peak luminosity would place MAXI J1957+032 at a distance of d ~ 5-6 kpc. [ABSTRACT FROM AUTHOR]

The low-mass X-ray binary Aquila X-1 is one of the most active neutron star X-ray transients. Despite its relatively bright quiescent optical counterpart, the detection of its companion has been hampered by the presence of a nearby interloper star. Using the Spectrograph for INtegral Field Observations in the Near Infrared (SINFONI) on the Very Large Telescope-8.2m telescope, we unambiguously single out Aquila X-1 from the interloper. Phase-resolved near-infrared spectroscopy reveals absorption features from a K4 ± 2 companion star moving at a projected velocity of K2 = 136 ± 4km s-1. We here present the first dynamical solution and associated fundamental parameters of Aquila X-1, imposing new constraints on the orbital inclination (36° < i < 47°) and the distance (d = 6 ± 2 kpc) to this prototypical neutron star transient. [ABSTRACT FROM AUTHOR]

We study in detail the evolution of the 2015 outburst of GS 1354-64 (BW Cir) at optical, UV and X-ray wavelengths using Faulkes Telescope South/Las Cumbres Observatory Global Telescope Network, Small & Moderate Aperture Research Telescope System and Swift. The outburst was found to stay in the hard X-ray state, albeit being anomalously luminous with a peak luminosity of LX > 0.15 LEdd, which could be the most luminous hard state observed in a black hole X-ray binary. We found that the optical/UV emission is tightly correlated with the X-ray emission, consistent with accretion disc irradiation and/or a jet producing the optical emission. The X-ray spectra can be fitted well with a Comptonization model, and show softening towards the end of the outburst. In addition, we detect a QPO in the X-ray light curves with increasing centroid frequency during the peak and decay periods of the outburst. The long-term optical light curves during quiescence show a statistically significant, slow rise of the source brightness over the 7 years prior to the 2015 outburst. This behaviour as well as the outburst evolution at all wavelengths studied can be explained by the disc instability model with irradiation and disc evaporation/condensation. [ABSTRACT FROM AUTHOR]

MAXI J1659-152 is a bright X-ray transient black-hole candidate binary system discovered in September 2010. We report here on MAXI, RXTE, Swift, and XMM-Newton observations during its 2010/2011 outburst. We find that during the first one and a half week of the outburst the X-ray light curves display drops in intensity at regular intervals, which we interpret as absorption dips. About three weeks into the outbursts, again drops in intensity are seen. These dips have, however, a spectral behaviour opposite to that of the absorption dips, and are related to fast spectral state changes (hence referred to as transition dips). The absorption dips recur with a period of 2.414 ± 0.005 h, which we interpret as the orbital period of the system. This implies that MAXI J1659-152 is the shortest period black-hole candidate binary known to date. The inclination of the accretion disk with respect to the line of sight is estimated to be 65-80. We propose the companion to the black-hole candidate to be close to an M5 dwarf star, with a mass and radius of about 0.15-0.25 M☉ and 0.2-0.25 R☉, respectively. We derive that the companion had an initial mass of about 1.5 M☉, which evolved to its current mass in about 5-6 billion years. The system is rather compact (orbital separation of ⋀1.33 R☉), and is located at a distance of 8.6 ± 3.7 kpc, with a height above the Galactic plane of 2.4± 1.0 kpc. The characteristics of short orbital period and high Galactic scale height are shared with two other transient black-hole candidate X-ray binaries, i.e., XTE J1118+480 and Swift J1735.5-0127. We suggest that all three are kicked out of the Galactic plane into the halo, rather than being formed in a globular cluster. [ABSTRACT FROM AUTHOR]

We present two observational campaigns performed with the RXTE satellite on the black hole transient H322. The source was observed in outburst on two separate occasions between 2008 October-November and 2009 May-July. We have carried out timing and spectral analysis of the data set, obtaining a complete state classification of all the observations. We find that all the observations are well described by using a spectral model consisting of a disc blackbody, a power law + reflection + absorption and a Gaussian emission component. During the 2009 outburst, the system followed the canonical evolution through all the states seen in black hole transients. In the 2008 outburst only the hard states were reached. The early evolution of the spectral parameters is consistent between the two epochs, and it does not provide clues about the subsequent behaviour of the source. The variation of the flux associated to the two main spectral components (i.e. disc and power law) allows us to set a lower limit to the orbital inclination of the system of ≥43°. [ABSTRACT FROM AUTHOR]

We present a 2-day long Rossi X-ray Timing Explorer observation and simultaneous Swift data of the bright X-ray transient XTE J 223. Spectral and timing properties were stable during the observation. The energy spectrum is well described by a broken power law with a high-energy cut-off. A cold disc (∼0.3 keV) is observed when Swift/X-Ray Telescope data are considered. The fractional root mean square amplitude of the aperiodic variability (0.002–128 Hz) is per cent, and it is not energy dependent. The continuum of the power density spectrum can be fitted by using four broad-band Lorentzians. A high-frequency (∼21 Hz) component and two weak quasi-periodic oscillation-like features are also present. Time lags between soft and hard X-rays roughly follow the relation , with delays dropping from ∼0.5 (0.003 Hz) to ∼0.0015 (≥10 Hz) s. Our results are consistent with XTE J 223 being a black hole candidate, with all timing and spectral components very similar to those of Cyg X-1 during its canonical hard state. [ABSTRACT FROM AUTHOR]

We analyse V-band photometry of the aperiodic variability in T CrB. By applying a simple idea of angular momentum transport in the accretion disc, we have developed a method to simulate the statistical distribution of flare durations with the assumption that the aperiodic variability is produced by turbulent elements in the disc. Both cumulative histograms with Kolmogorov–Smirnov tests, and power density spectra are used to compare the observed data and simulations. The input parameters of the model Rin and α are correlated on a certain interval and the most probable values are an inner disc radius of and a viscosity of . The disc is then weakly truncated. We find that the majority of turbulent events producing flickering activity are concentrated in the inner parts of the accretion disc. [ABSTRACT FROM AUTHOR]

We apply the K-correction to the black hole low-mass X-ray binary (LMXB) GX 339-4 which implies by only assuming that the companion is more massive than , the lower limit allowed by applying a ‘stripped-giant’ model. This evolutionary model successfully reproduces the observed properties of the system. We obtain a maximum mass for the companion of and an upper limit to the mass ratio of . The high X-ray activity displayed by the source suggests a relatively large mass transfer rate which, according to the model, results in and . We have also applied this scenario to the black hole binary XTE J1550-564, which has a similar orbital period but the donor is detected spectroscopically. The model successfully reproduces the observed stellar parameters. [ABSTRACT FROM AUTHOR]

We present simultaneous X-ray ( RXTE) and optical (ULTRACAM) narrow-band (Bowen blend/He ii and nearby continuum) observations of Sco X-1 at 2–10 Hz time resolution. We find that the Bowen/He ii emission lags the X-ray light curves with a light traveltime of s which is consistent with reprocessing in the companion star. The echo from the donor is detected at orbital phase ∼0.5 when Sco X-1 is at the top of the flaring branch (FB). Evidence of echoes is also seen at the bottom of the FB but with time-lags of 5–10 s which are consistent with reprocessing in an accretion disc with a radial temperature profile. We discuss the implication of our results for the orbital parameters of Sco X-1. [ABSTRACT FROM AUTHOR]

We model the K-correction for emission lines formed on the irradiated face of companion stars in compact binaries. We compute this K-correction in a general approach as function of the mass ratio and the disc flaring angle. Our results, combined with the detection of high excitation emission lines arising from the donor star, can be used to set constraints to the masses of neutron stars and black holes in persistent Low Mass X-ray Binaries (LMXBs).The application of our model to X1822-371 (Muñoz-Darias, Casares & Martínez-Pais 2005) lends strong support to the presence of a massive neutron star in this LMXB (i.e. MNS > 1.6M⊙). Here, we also present the K-correction for the Black Hole binary GX339-4, where we obtain a solid lower limit to the black hole mass of MBH > 5.8M⊙. [ABSTRACT FROM PUBLISHER]

Sixteen years of observations of black hole transients with the Rossi X-ray Timing Explorer, complemented by other X-ray observatories and ground-based optical/infrared/radio telescopes have given us a clear view of the complex phenomenology associated with their bright outbursts. This has led to the definition of a small number of spectral/timing states which are separated by marked transitions in observables. The association of these states and their transitions to changes in the radio emission from relativistic radio jets completes the picture and have led to the study of the connection between accretion and ejection. A good number of fundamental questions are still unanswered, but the existing picture provides a good framework on which to base theoretical studies. We discuss the current observational standpoint, with emphasis onto the spectral and timing evolution during outbursts, as well as the prospects for future missions such as ASTROSAT (2012) and LOPT (>2020 if selected). [ABSTRACT FROM AUTHOR]