Outburst Decay Research Articles

The extreme coronal line emitter AT 2022fpx: varying optical polarization properties and late-time X-ray flare

ABSTRACT Supermassive black holes disrupt passing stars, producing outbursts called tidal disruption events (TDEs). TDEs have recently gained attention due to their unique dynamics and emission processes, which are still not fully understood. Especially, the so-called optical TDEs are of interest as they often exhibit delayed or obscured X-ray emission from the accretion disc, making the origin of the prompt emission unclear. In this paper, we present multiband optical polarization observations and optical spectrometry of a recent TDE candidate AT 2022fpx, alongside monitoring observations in optical, ultraviolet, and X-rays. The optical spectra of AT 2022fpx show Bowen fluorescence as well as highly ionized iron emission lines, which are characteristic of extreme coronal line emitters. Additionally, the source exhibits variable but low-polarized continuum emission at the outburst peak, with a clear rotation of the polarization angle. X-ray emission observed approximately 250 d after the outburst peak in the decay appear flare-like but is consistent with constant temperature blackbody emission. The overall outburst decay is slower than for typical TDEs, and resembles more the ones seen from Bowen fluorescence flares. These observations suggest that AT 2022fpx could be a key source in linking different long-lived TDE scenarios. Its unique characteristics, such as extreme coronal line emission, variable polarization, and delayed X-ray flare, can be attributed to the outer shock scenario or a clumpy torus surrounding the supermassive black hole. Further studies, especially in the context of multiwavelength observations, are crucial to fully understand the dynamics and emission mechanisms of these intriguing astrophysical events.

The bright black hole X-ray binary 4U 1543-47 during 2021 outburst. A clear state transition from super-Eddington to sub-Eddington accretion revealed by Insight-HXMT

ABSTRACT We present a detailed analysis of the observations with the Hard X-ray Modulation Telescope of the black hole X-ray transient 4U 1543-47 during its outburst in 2021. We find a clear state transition during the outburst decay of the source. Using previous measurements of the black hole mass and distance to the source, the source luminosity during this transition is close to the Eddington limit. The light curves before and after the transition can be fitted by two exponential functions with short (∼16 d) and long (∼130 d) decay time-scales, respectively. We detect strong reflection features in all observations that can be described with either the relxillns or reflionx_bb reflection models, both of which have a black-body incident spectrum. In the super-Eddington state, we observe a Comptonized component characterized by a low electron temperature of approximately 2.0 keV. We suggest that this component appears exclusively within the inner radiation-pressure-dominated region of the supercritical disc as a part of the intrinsic spectrum of the accretion disc itself. This feature vanishes as the source transitions into the sub-Eddington state. The emissivity index of the accretion disc in the reflection component is significantly different before and after the transition, ∼3.0–5.0 and ∼7.0–9.0 in the super- and sub-Eddington states, respectively. Based on the reflection geometry of returning disc radiation, the geometrically thicker the accretion disc, the smaller the emissivity index. Therefore, we propose that the transition is primarily driven by the change of the accretion flow from a supercritical to a thin disc configuration.

VLA monitoring of LS V +44 17 reveals scatter in the X-ray–radio correlation of Be/X-ray binaries

ABSTRACT LS V +44 17 is a persistent Be/X-ray binary (BeXRB) that displayed a bright, double-peaked period of X-ray activity in late 2022/early 2023. We present a radio monitoring campaign of this outburst using the Very Large Array. Radio emission was detected, but only during the second, X-ray brightest, peak, where the radio emission followed the rise and decay of the X-ray outburst. LS V +44 17 is therefore the third neutron star BeXRB with a radio counterpart. Similar to the other two systems (Swift J0243.6+6124 and 1A 0535+262), its X-ray and radio luminosity are correlated: we measure a power-law slope $\beta = 1.25^{+0.64}_{-0.30}$ and a radio luminosity of LR = (1.6 ± 0.2) × 1026 erg s−1 at a 0.5–10 keV X-ray luminosity of 2 × 1036 erg s−1 (i.e. $\sim 1~{{\ \rm per\ cent}}$LEdd). This correlation index is slightly steeper than measured for the other two sources, while its radio luminosity is higher. We discuss the origin of the radio emission, specifically in the context of jet launching. The enhanced radio brightness compared to the other two BeXRBs is the first evidence of scatter in the giant BeXRB outburst X-ray–radio correlation, similar to the scatter observed in subclasses of low-mass X-ray binaries. While a universal explanation for such scatter is not known, we explore several options: we conclude that the three sources do not follow proposed scalings between jet power and neutron star spin or magnetic field, and instead briefly explore the effects that ambient stellar wind density may have on BeXRB jet luminosity.

Tracing the Accretion Geometry of H1743-322 with Type C Quasiperiodic Oscillations in Multiple Outbursts

We present a systematic analysis of type C quasiperiodic oscillation (QPO) observations of H1743-322 throughout the Rossi X-ray Timing Explorer era. We find that, while different outbursts have significant flux differences, they show consistent positive correlations between the QPO fractional rms amplitude and nonthermal fraction of the emission, which indicate an independence of the intrinsic QPO rms on individual outburst brightnesses in H1743-322. However, the dependence of the QPO rms on frequency is different between the outburst rise and decay phases, where the QPO fractional rms of the decay phase is significantly lower than that of the rise phase at low frequencies. The spectral analysis also reveals different ranges of coronal temperature between the two outburst stages. A semiquantitative analysis shows that the Lense–Thirring precession model could be responsible for the QPO rms differences, requiring a variable coronal geometric shape. However, the variable-Comptonization model could also account for the findings. The fact that the rms differences and the hysteresis traces in the hardness–intensity diagram accompany each other indicates a connection between the two phenomena. By correlating the findings with QPO phase lags and the quasi-simultaneous radio flux previously published, we propose there could be corona-jet transitions in H1743-322 similar to those that have been recently reported in GRS 1915+105.

The unaltered pulsar: GRO J1750-27, a supercritical X-ray neutron star that does not blink an eye

When accreting X-ray pulsars (XRPs) undergo bright X-ray outbursts, their luminosity-dependent spectral and timing features can be analyzed in detail. The XRP GRO J1750-27 recently underwent one such episode, during which it was observed withNuSTARand monitored with NICER. Such a data set is rarely available, as it samples the outburst over more than 1 month at a luminosity that is always exceeding ∼5 × 1037 erg s−1. This value is larger than the typical critical luminosity value, where a radiative shock is formed above the surface of the neutron star. Our data analysis of the joint spectra returns a highly (NH ∼ (5 − 8) × 1022 cm−2) absorbed spectrum showing a Kαiron line, a soft blackbody component likely originating from the inner edge of the accretion disk, and confirms the discovery of one of the deepest cyclotron lines ever observed, at a centroid energy of ∼44 keV corresponding to a magnetic field strength of 4.7 × 1012 G. This value is independently supported by the best-fit physical model for spectral formation in accreting XRPs which, in agreement with recent findings, favors a distance of 14 kpc and also reflects a bulk-Comptonization-dominated accretion flow. Contrary to theoretical expectations and observational evidence from other similar sources, the pulse profiles as observed by NICER remain remarkably steady through the outburst rise, peak and decay. The NICER spectrum, including the iron Kαline best-fit parameters, also remain almost unchanged at all probed outburst stages, similar to the pulsed fraction behavior. We argue that all these phenomena are linked and interpret them as resulting from a saturation effect of the emission from the accretion column, which occurs in the high-luminosity regime.

Multiwavelength observations of Swift J0243.6+6124 from 2017 to 2022

Context.Swift J0243.6+6124 is a high-mass X-ray binary that went into a giant X-ray outburst in 2017. During this event, the X-ray luminosity reached the highest value ever measured in a galactic Be/X-ray binary. Aims. Our aim is to study the long-term variability of Swift J0243.6+6124 after the 2017 major X-ray outburst. Methods. We have obtained optical spectroscopy and photometry data during four years after the event. The long-term photometric light curve and the equivalent widths of the Hα and He I λ6678 lines were used to monitor the state of the Be star’s circumstellar disk. The Hα line profiles show evidence for V/R variability that was accounted for by fitting the Hα spectral line profile with two Gaussian functions. We divided our data into three phases according to the intensity of the X-ray, optical, and infrared emission. Results. Phase I covers the rise and decay of the giant X-ray outburst that took place in October–November 2017. We interpret phase II as the dissipation of the Be star’s equatorial disk and phase III as its recovery. The timescale of a complete formation and dissipation process is about 1250 days. The epoch when the dissipation process stopped and the reformation period began is estimated to be around MJD 58530. We find a delay of ∼100–200 days between the minimum of the optical or infrared intensity and the strength of the Hα line after the X-ray outburst, which may indicate that the dissipation of the disk begins from the inner parts. The motion of the density perturbation inside the disk is prograde, with a V/R quasi-period of about four years. The source shows a positive correlation in the (B − V) color index versus V-band magnitude diagram, which implies that the system is seen at a small or moderate inclination angle. Conclusions. Despite the super-Eddington X-ray luminosity during the outburst, the subsequent pattern of long-term optical and IR variability of Swift J0243.6+6124 is typical of Be/X-ray binaries.

Outflows and spectral evolution in the eclipsing AMXP SWIFT J1749.4–2807 with NICER, XMM-Newton, and NuSTAR

ABSTRACT The neutron star low-mass X-ray binary SWIFT J1749.4–2807 is the only known eclipsing accreting millisecond X-ray pulsar. In this manuscript, we perform a spectral characterization of the system throughout its 2021, 2-week-long outburst, analysing 11 NICER observations and quasi-simultaneous XMM-Newton and NuSTAR single observations at the outburst peak. The broad-band spectrum is well-modelled with a blackbody component with a temperature of ∼0.6 keV, most likely consistent with a hotspot on the neutron star surface, and a Comptonization spectrum with power-law index Γ ∼ 1.9, arising from a hot corona at ∼12 keV. No direct emission from the disc was found, possibly due to it being too cool. A high truncation radius for the disc, i.e. at ∼20–30 RG, was obtained from the analysis of the broadened profile of the Fe line in the reflection component. The significant detection of a blue-shifted Fe XXVI absorption line at ∼7 keV indicates weakly relativistic X-ray disc winds, which are typically absent in the hard state of X-ray binaries. By comparing the low flux observed during the outburst and the one expected in a conservative mass-transfer, we conclude that mass-transfer in the system is highly non-conservative, as also suggested by the wind detection. Finally, using the NICER spectra alone, we followed the system while it was fading to quiescence. During the outburst decay, as the spectral shape hardened, the hotspot on the neutron star surface cooled down and shrank, a trend which could be consistent with the pure power-law spectrum observed during quiescence.

Multiwavelength observations of MAXI J1820+070 during its outburst decay and subsequent mini-outburst

ABSTRACT We present results from quasi-simultaneous multiwavelength observations of the Galactic black hole X-ray transient MAXI J1820+070 during the decay of the 2018 outburst and its entire subsequent mini-outburst in March 2019. We fit the X-ray spectra with phenomenological and Comptonizaton models and discuss the X-ray spectral evolution in comparison with the multiwavelength behaviour of the system. The system showed a rebrightening in UV/Optical/NIR bands 7 d after the soft-to-hard transition during the main outburst decay while it was fading in X-rays and radio. In contrast, the mini-outburst occurred 165 d after the hard state transition of the initial outburst decay and was detected in all wavelengths. For both events, the measured time-scales are consistent with those observed in other black hole systems. Contemporaneous hard X-ray/soft γ-ray observations indicate a non-thermal electron energy distribution at the beginning of the UV/Optical/NIR rebrightening, whereas a thermal distribution can fit the data during the hard mini-outburst activity. The broad-band spectral energy distributions until the rebrightening are consistent with the irradiated outer accretion disc model. However, both the SEDs produced for the peak of rebrightening and close to the peak of mini-outburst provided good fits only with an additional power law component in the UV/Optical/NIR frequency ranges, which is often interpreted with a jet origin.

Investigating the nature and properties of MAXI J1810−222 with radio and X-ray observations

ABSTRACT We present results from radio and X-ray observations of the X-ray transient MAXI J1810−222. The nature of the accretor in this source has not been identified. In this paper, we show results from a quasi-simultaneous radio and X-ray monitoring campaign taken with the Australia Telescope Compact Array, the Neil Gehrels Swift Observatory X-ray Telescope (XRT), and the Swift Burst Alert Telescope. We also analyse the X-ray temporal behaviour using observations from the Neutron star Interior Composition Explorer. Results show a seemingly peculiar X-ray spectral evolution of MAXI J1810−222 during this outburst, where the source was initially only detected in the soft X-ray band for the early part of the outburst. Then, ∼200 d after MAXI J1810−222 was first detected the hard X-ray emission increased and the source transitioned to a long-lived (∼1.5 yr) bright, harder X-ray state. After this hard state, MAXI J1810−222 returned back to a softer state, before fading and transitioning again to a harder state and then appearing to follow a more typical outburst decay. From the X-ray spectral and timing properties, and the source’s radio behaviour, we argue that the results from this study are most consistent with MAXI J1810−222 being a relatively distant (≳6 kpc) black hole X-ray binary. A sufficiently large distance to source can simply explain the seemingly odd outburst evolution that was observed, where only the brightest portion of the outburst was detectable by the all-sky XRTs.

Hard-state Optical Wind during the Discovery Outburst of the Black Hole X-Ray Dipper MAXI J1803–298

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 Hα 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 Hα 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.

Pulse Peak Migration during the Outburst Decay of the Magnetar SGR 1830-0645: Crustal Motion and Magnetospheric Untwisting

Abstract Magnetars, isolated neutron stars with magnetic-field strengths typically ≳1014 G, exhibit distinctive months-long outburst epochs during which strong evolution of soft X-ray pulse profiles, along with nonthermal magnetospheric emission components, is often observed. Using near-daily NICER observations of the magnetar SGR 1830-0645 during the first 37 days of a recent outburst decay, a pulse peak migration in phase is clearly observed, transforming the pulse shape from an initially triple-peaked to a single-peaked profile. Such peak merging has not been seen before for a magnetar. Our high-resolution phase-resolved spectroscopic analysis reveals no significant evolution of temperature despite the complex initial pulse shape, yet the inferred surface hot spots shrink during peak migration and outburst decay. We suggest two possible origins for this evolution. For internal heating of the surface, tectonic motion of the crust may be its underlying cause. The inferred speed of this crustal motion is ≲100 m day−1, constraining the density of the driving region to ρ ∼ 1010 g cm−3, at a depth of ∼200 m. Alternatively, the hot spots could be heated by particle bombardment from a twisted magnetosphere possessing flux tubes or ropes, somewhat resembling solar coronal loops, that untwist and dissipate on the 30–40 day timescale. The peak migration may then be due to a combination of field-line footpoint motion (necessarily driven by crustal motion) and evolving surface radiation beaming. This novel data set paints a vivid picture of the dynamics associated with magnetar outbursts, yet it also highlights the need for a more generic theoretical picture where magnetosphere and crust are considered in tandem.

Physical modelling of viscous disc evolution around magnetized neutron star. Aql X-1 2013 outburst decay

ABSTRACT We present a model of a viscously evolving accretion disc around a magnetized neutron star. The model features the varying outer radius of the hot ionized part of the disc due to cooling and the varying inner radius of the disc due to interaction with the magnetosphere. It also includes hindering of accretion on the neutron star because of the centrifugal barrier and irradiation of the outer disc and companion star by X-rays from the neutron star and disc. When setting inner boundary conditions, we take into account that processes at the inner disc occur on a time-scale much less than the viscous time-scale of the whole disc. We consider three types of outflow from the disc inner edge: zero outflow, one based on MHD calculations, and a very efficient propeller mechanism. The light curves of an X-ray transient after the outburst peak can be calculated by a corresponding, publicly available code. We compare observed light curves of the 2013 burst of Aql X-1 in X-ray and optical bands with modelled ones. We find that the fast drop of the 0.3–10 keV flux can be solely explained by a radial shrinking of the hot disc. At the same time, models with the neutron star magnetic field >108 G have better fits because the accretion efficiency behaviour emphasizes the ‘knee’ on the light curve. We also find that a plato emission can be produced by a `disc-reservoir' with stalled accretion.

ALMA/NICER observations of GRS 1915+105 indicate a return to a hard state

Context. GRS 1915+105 is a transient black hole X-ray binary consistently emitting 10–100% of the Eddington luminosity in the X-ray band over the last three decades until mid-2018 when the source luminosity suddenly decreased by an order of magnitude. This phase was followed by a change to a state with even lower average X-ray fluxes never seen before during the outburst but presenting renewed flaring activity at different wavelengths, albeit with mean fluxes still in decline. Aims. GRS 1915+105 has the longest orbital period known among low-mass X-ray binaries, the largest accretion disk size, and therefore the largest mass supply for accretion. The high inclination of the disk allows the study of geometrical effects of the accretion flow such as changes in the height-to-radius ratio or the effect of accretion disk winds on the intrinsic emission that is expected during the outburst decay. In addition, the transient jet is expected to change to a compact, self-absorbed, steady jet. Methods. We conducted two full polarization Atacama Large Millimeter Array observations to study the jet properties during the outburst decay by analyzing the spectral, polarization, and intra-epoch variability for both observation epochs. In addition, we analyzed almost daily Neutron Star Interior Composition Explorer pointing observations, modeling X-ray power spectral densities, spectral energy distributions, and light curves with a physically motivated model to follow the changing accretion disk properties throughout the outburst decay and relating them to the jet emission. Results. We show that the X-ray and millimeter (mm) spectral, timing, and polarization properties are consistent with those of a typical decaying X-ray binary outburst and that GRS 1915+105 has descended into the low-luminosity hard X-ray state. The jet emission in the mm is consistent with a compact, steady jet with ∼1% linear polarization, and the magnetic field is likely aligned with the jet position angle. Relating the mm emission to the X-ray emission reveals that the source has changed from a higher radio/X-ray correlation index to a lower one; Lradio ∝ LX0.6.

Probing Jet Launching in Neutron Star X-Ray Binaries: The Variable and Polarized Jet of SAX J1808.4–3658

Abstract We report on an optical photometric and polarimetric campaign on the accreting millisecond X-ray pulsar (AMXP) SAX J1808.4–3658 during its 2019 outburst. The emergence of a low-frequency excess in the spectral energy distribution in the form of a red excess above the disk spectrum (seen most prominently in the z, i, and R bands) is observed as the outburst evolves. This is indicative of optically thin synchrotron emission due to a jet, as seen previously in this source and in other AMXPs during outburst. At the end of the outburst decay, the source enters a reflaring state. The low-frequency excess is still observed during the reflares. Our optical (BVRI) polarimetric campaign shows variable linear polarization (LP) throughout the outburst. We show that this is intrinsic to the source, with low-level but significant detections (0.2%–2%) in all bands. The LP spectrum is red during both the main outburst and the reflaring state, favoring a jet origin for this variable polarization over other interpretations, such as Thomson scattering with free electrons from the disk or the propelled matter. During the reflaring state, a few episodes with stronger LP levels (1%–2%) are observed. The low-level, variable LP is suggestive of strongly tangled magnetic fields near the base of the jet. These results clearly demonstrate how polarimetry is a powerful tool for probing the magnetic field structure in X-ray binary jets, as for active galactic nuclei jets.

Models of ultraluminous X-ray transient sources

Context. It is now widely accepted that most ultraluminous X-ray sources (ULXs) are binary systems whose large (above 1039 erg s−1) apparent luminosities are explained by super-Eddington accretion onto a stellar-mass compact object. Many of the ULXs, especially those containing magnetized neutron stars, are highly variable; some exhibit transient behaviour. Large luminosities might imply large accretion discs that could be therefore prone to the thermal–viscous instability known to drive outbursts of dwarf novae and low-mass X-ray binary transient sources. Aims. The aim of this paper is to extend and generalize the X-ray transient disc-instability model to the case of large (outer radius larger than 1012 cm) accretion discs and apply it to the description of systems with super-Eddington accretion rates at outburst and, in some cases, super-Eddington mass transfer rates. Methods. We have used our disc-instability-model code to calculate the time evolution of the accretion disc and the outburst properties. Results. We show that, provided that self-irradiation of the accretion disc is efficient even when the accretion rate exceeds the Eddington value, possibly due to scattering back of the X-ray flux emitted by the central parts of the disc on the outer portions of the disc, heating fronts can reach the disc’s outer edge generating high accretion rates. We also provide analytical approximations for the observable properties of the outbursts. We have successfully reproduced the observed properties of galactic transients with large discs, such as V404 Cyg, as well as some ULXs such as M51 XT-1. Our model can reproduce the peak luminosity and decay time of ESO 243-39 HLX-1 outbursts if the accretor is a neutron star. Conclusions. Observational tests of our predicted relations between the outburst duration and decay time with peak luminosity would be most welcome.

NICER observations reveal that the X-ray transient MAXI J1348−630 is a black hole X-ray binary

ABSTRACT 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 black hole 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.

The X-Ray Outburst of the Galactic Center Magnetar over Six Years of Chandra Observations

Abstract The magnetar SGR J1745−2900, discovered at a distance of parsecs from the Milky Way central black hole, Sagittarius A⋆, represents the closest pulsar to a supermassive black hole ever detected. Furthermore, its intriguing radio emission has been used to study the environment of the black hole, as well as to derive a precise position and proper motion for this object. The discovery of SGR J1745−2900 has led to interesting debates about the number, age, and nature of pulsars expected in the Galactic center region. In this work, we present extensive X-ray monitoring of the outburst of SGR J1745−2900 using the Chandra X-ray Observatory, the only instrument with the spatial resolution to distinguish the magnetar from the supermassive black hole (2.″4 angular distance). It was monitored from its outburst onset in 2013 April until 2019 August, collecting more than 50 Chandra observations for a total of more than 2.3 Ms of data. Soon after the outburst onset, the magnetar emission settled onto a purely thermal emission state that cooled from a temperature of about 0.9–0.6 keV over 6 yr. The pulsar timing properties showed at least two changes in the period derivative, increasing by a factor of about 4 during the outburst decay. We find that the long-term properties of this outburst challenge current models for the magnetar outbursts.

Runaway Coalescence of Pre-common-envelope Stellar Binaries

Abstract We study the process of runaway, unstable Roche lobe overflow in coalescing binary systems and its dependence on the properties of the binary involved. We create three-dimensional hydrodynamic models of binary coalescences and follow them through a phase of increasing Roche lobe overflow until the accretor is engulfed by the donor at the onset of a common-envelope phase. In these models, we vary binary properties of mass ratio, donor structure and spin, and equation of state through the gas adiabatic index. We compare the numerical results to semianalytic models of binary orbit evolution based on mass and angular momentum exchange between two point masses. Using our hydrodynamic simulations, we measure the key parameters: the donor mass-loss rate and the angular momentum exchanged per unit mass loss from the donor. Using these calibrations, the semianalytic model closely reproduces the escalating mass loss and runaway orbital decay observed in the hydrodynamic models. The semianalytic model accurately reproduces the major differences in orbit evolution that arise with varying mass ratio and donor structure. We encapsulate the semianalytic model in a publicly released Python package, RLOF. We apply this model to the observed period decay and subsequent merger of the binary V1309 Sco and find that it can simultaneously reproduce the observed orbital decay and time of outburst. We further demonstrate that there is a relationship between the period derivative and second derivative that can be a useful metric for evaluating candidate merging binaries.

A strongly changing accretion morphology during the outburst decay of the neutron star X-ray binary 4U 1608−52

ABSTRACT It is commonly assumed that the properties and geometry of the accretion flow in transient low-mass X-ray binaries (LMXBs) significantly change when the X-ray luminosity decays below ∼10−2 of the Eddington limit (LEdd). However, there are few observational cases where the evolution of the accretion flow is tracked in a single X-ray binary over a wide dynamic range. In this work, we use NuSTAR and NICER observations obtained during the 2018 accretion outburst of the neutron star LMXB 4U 1608−52, to study changes in the reflection spectrum. We find that the broad Fe–Kα line and Compton hump, clearly seen during the peak of the outburst when the X-ray luminosity is ∼1037 erg s−1 (∼0.05 LEdd), disappear during the decay of the outburst when the source luminosity drops to ∼4.5 × 1035 erg s−1 (∼0.002 LEdd). We show that this non-detection of the reflection features cannot be explained by the lower signal-to-noise ratio at lower flux, but is instead caused by physical changes in the accretion flow. Simulating synthetic NuSTAR observations on a grid of inner disc radius, disc ionization, and reflection fraction, we find that the disappearance of the reflection features can be explained by either increased disc ionization (log ξ ≳ 4.1) or a much decreased reflection fraction. A changing disc truncation alone, however, cannot account for the lack of reprocessed Fe–Kα emission. The required increase in ionization parameter could occur if the inner accretion flow evaporates from a thin disc into a geometrically thicker flow, such as the commonly assumed formation of a radiatively inefficient accretion flow at lower mass accretion rates.

Physical Constraints from Near-infrared Fast Photometry of the Black Hole Transient GX 339–4

Abstract We present results from the first multi-epoch X-ray/IR fast-photometry campaign on the black hole transient GX 339–4, during its 2015 outburst decay. We studied the evolution of the power spectral densities finding strong differences between the two bands. The X-ray power spectral density follows standard patterns of evolution, plausibly reflecting changes in the accretion flow. The IR power spectral density instead evolves very slowly, with a high-frequency break consistent with remaining constant at 0.63 ± 0.03 Hz throughout the campaign. We discuss this result in the context of the currently available models for the IR emission in black hole transients. While all models will need to be tested quantitatively against this unexpected constraint, we show that an IR-emitting relativistic jet that filters out the short-timescale fluctuations injected from the accretion inflow appears as the most plausible scenario.