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.
All disc-accreting astrophysical objects produce powerful disc winds. In compact binaries containing neutron stars or black holes, accretion often takes place during violent outbursts. The main disc ...wind signatures during these eruptions are blue-shifted X-ray absorption lines, which are preferentially seen in disc-dominated 'soft states'
. By contrast, optical wind-formed lines have recently been detected in 'hard states', when a hot corona dominates the luminosity
. The relationship between these signatures is unknown, and no erupting system has as yet revealed wind-formed lines between the X-ray and optical bands, despite the many strong resonance transitions in this ultraviolet (UV) region
. Here we report that the transient neutron star binary Swift J1858.6-0814 exhibits wind-formed, blue-shifted absorption lines associated with C IV, N V and He II in time-resolved UV spectroscopy during a luminous hard state, which we interpret as a warm, moderately ionized outflow component in this state. Simultaneously observed optical lines also display transient blue-shifted absorption. Decomposing the UV data into constant and variable components, the blue-shifted absorption is associated with the former. This implies that the outflow is not associated with the luminous flares in the data. The joint presence of UV and optical wind features reveals a multi-phase and/or spatially stratified evaporative outflow from the outer disc
. This type of persistent mass loss across all accretion states has been predicted by radiation-hydrodynamic simulations
and helps to explain the shorter-than-expected duration of outbursts
.
We studied the outburst evolution and timing properties of the recently discovered X-ray transient MAXI J1348−630 as observed with NICER. We produced the fundamental diagrams commonly used to trace ...the spectral evolution, and power density spectra to study the fast X-ray variability. The main outburst evolution of MAXI J1348−630 is similar to that commonly observed in blackhole transients. The source evolved from the hard state (HS), through hard- and soft-intermediate states, into the soft state in the outburst rise, and back to the HS in reverse during the outburst decay. At the end of the outburst, MAXI J1348−630 underwent two reflares with peak fluxes approximately one and two orders of magnitude fainter than the main outburst, respectively. During the reflares, the source remained in the HS only, without undergoing any state transitions, which is similar to the so-called ‘failed outbursts’. Different types of quasi-periodic oscillations (QPOs) are observed at different phases of the outburst. Based on our spectral-timing results, we conclude that MAXI J1348−630 is a black hole candidate.
ABSTRACT
Black hole low-mass X-ray binaries (BH LMXBs) evolve in a similar way during outburst. Based on the X-ray spectrum and variability, this evolution can be divided into three canonical states: ...low/hard, intermediate, and high/soft state. BH LMXBs evolve from the low/hard to the high/soft state through the intermediate state in some outbursts (here called ‘full outbursts’). However, in other cases, BH LMXBs undergo outbursts in which the source never reaches the high/soft state, here called ‘failed-transition outbursts’ (FT outbursts). From a sample of 56 BH LMXBs undergoing 128 outbursts, we find that 36 per cent of these BH LMXBs experienced at least one FT outburst, and that FT outbursts represent ∼33 per cent of the outbursts of the sample, showing that these are common events. We compare all the available X-ray data of full and FT outbursts of BH LMXBs from RXTE/PCA, Swift/BAT, and MAXI, and find that FT and full outbursts cannot be distinguished from their X-ray light curves, hardness–intensity diagrams, or X-ray variability during the initial 10–60 d after the outburst onset. This suggests that both types of outbursts are driven by the same physical process. We also compare the optical and infrared (O/IR) data of FT and full outbursts of GX 339−4. We found that this system is generally brighter in O/IR bands before an FT outburst, suggesting that the O/IR flux points to the physical process that later leads to a full or an FT outburst. We discuss our results in the context of models that describe the onset and evolution of outbursts in accreting X-ray binaries.
Accretion disks around compact objects are expected to enter an unstable phase at high luminosity
. One instability may occur when the radiation pressure generated by accretion modifies the disk ...viscosity, resulting in the cyclic depletion and refilling of the inner disk on short timescales
. Such a scenario, however, has only been quantitatively verified for a single stellar-mass black hole
. Although there are hints of these cycles in a few isolated cases
, their apparent absence in the variable emission of most bright accreting neutron stars and black holes has been a continuing puzzle
. Here we report the presence of the same multiwavelength instability around an accreting neutron star. Moreover, we show that the variability across the electromagnetic spectrum-from radio to X-ray-of both black holes and neutron stars at high accretion rates can be explained consistently if the accretion disks are unstable, producing relativistic ejections during transitions that deplete or refill the inner disk. Such a new association allows us to identify the main physical components responsible for the fast multiwavelength variability of highly accreting compact objects.
Ultracompact X-ray binaries (UCXBs) are a distinctive but elusive family of low-mass X-ray binaries (LMXBs) characterised by their tight orbits and degenerate donor stars. Here we present ...UltraCompCAT, the first online and comprehensive catalogue of UCXBs. The initial version of UltraCompCAT comprises 49 sources, including 20 confirmed UCXBs (those with a measured orbital period shorter than 80 min) and 25 systems that we label ‘candidates’ based on their multi-wavelength phenomenology. For completeness, we also include four LMXBs with orbital periods in the range of 80 to 120 min, since they might be related to (e.g. close progenitors) or even part of the UCXB population that evolved towards longer periods. We discuss the orbital period and Galactic distribution of the catalogue sample. We provide evidence for the presence of at least two separate groups of UCXBs, one formed by persistent systems with orbital periods shorter than 30 min and a second group of transient objects (70 %) with periods in the range of 40 to 60 min. We show that the former group is dominated by sources formed in globular clusters, while the latter accounts for the (known) UCXB population in the Galactic field. We discuss the possible evolutionary channels for the two groups.
ABSTRACT
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 M1/Porb 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
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–K2 correlation to the disc H α emission line detected in Gran Telescopio Canarias spectra and obtain K2 = 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 = M2/M1 ≲ 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
We report on the first simultaneous high-time resolution X-ray and infrared (IR) observations of a neutron star low mass X-ray binary in its hard state. We performed $\approx 2\,$ h of ...simultaneous observations of 4U 1728−34 using HAWK-I@VLT, XMM–Newton, and NuSTAR. The source displayed significant X-ray and IR variability down to sub-second time-scales. By measuring the cross-correlation function between the IR and X-ray lightcurves, we discovered a significant correlation with an IR lead of $\approx 30 \!-\! 40\,$ ms with respect to the X-rays. We analysed the X-ray energy dependence of the lag, finding a marginal increase towards higher energies. Given the sign of the lag, we interpret this as possible evidence of Comptonization from external seed photons. We discuss the origin of the IR seed photons in terms of cyclo-synchrotron radiation from an extended hot flow. Finally, we also observed the IR counterpart of a type-I X-ray burst, with a delay of $\approx 7.2\,$ s. Although some additional effects may be at play, by assuming that this lag is due to light travel time between the central object and the companion star, we find that 4U 1728−34 must have an orbital period longer than $3\,$ h and an inclination higher than 8○.
Abstract
We present 12 epochs of optical spectroscopy taken across the discovery outburst of the black hole (BH) candidate MAXI J1803−298 with the Gran Telescopio Canarias and Very Large Telescope. ...The source followed a standard outburst evolution with hard and soft states. The system displays a triangular shape in the hardness intensity diagram, consistent with that seen in high-inclination BH transients and the previously reported detection of X-ray dips. The two epochs observed during the initial hard state exhibited asymmetric emission-line profiles, including a P-Cygni profile simultaneously detected in H
α
and He
i
6678, which indicates the presence of an optical wind in the system. The remaining spectra, obtained during the transition to the soft state and the subsequent decay, are instead characterized by narrower, double-peaked emission lines embedded into broad absorption components. One epoch (intermediate state) also includes near-infrared (NIR) coverage, revealing complex line profiles in the Paschen and Bracket series, which suggests that the outflow is still present during the outburst decay through the soft state. The growing list of low-mass X-ray binaries with optical and NIR outflow signatures indicates that these are common features. Furthermore, the lowest luminosity spectrum exhibits an H
α
FWHM of 1570 ± 100 km s
−1
. This, together with previous constraints on the binary parameters, allows us to favor a compact object mass of ∼3–10
M
⊙
, further supporting its BH nature.
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