1RXS J180408.9-342058: An ultra compact X-ray binary candidate with a transient jet

Low-mass X-ray transients hosting black hole candidates display on average a factor of ~100 larger swing in the minimum (quiescent) to maximum (outburst) X-ray luminosity than neutron star systems do, despite the fact that the swing in the mass inflow rate is likely in the same range. Advection-dominated accretion flows, ADAFs, were proposed to interpret such a difference, because the advected energy disappears beyond the event horizon in black hole candidates but must be radiated away in neutron star systems. The residual optical/UV emission of quiescent low-mass X-ray transients, after subtraction of the companion star spectrum, was originally ascribed to optically thick emission from the outer accretion disk regions, where matter accumulates. Difficulties with this interpretation led to a revised ADAF model in which the bulk of the residual optical/UV emission in quiescence does not originate in the outermost disk regions but is instead produced by synchrotron radiation in the ADAF, and therefore is part of the ADAF's luminosity budget. We demonstrate that, once the residual optical/UV emission is taken into account, the bolometric luminosity swing of black hole candidates is consistent with that of neutron star systems. Therefore, ascribing the bulk of the residual optical/UV flux to the ADAF removes much of the evidence on which ADAF models for low-mass X-ray transients were originally developed, namely, the higher luminosity swing in black holes than in neutron stars. We also find that, for the neutron star spin periods (a few milliseconds) and magnetic fields (~108-109 G) inferred from some low-mass X-ray transients, the mass-to-radiation conversion efficiency of recently proposed ADAF/propeller models is considerably higher than would be required in order to match the observations, once the contribution from accretion onto the magnetospheric boundary is taken into account. Motivated by these findings, we explore here an alternative scenario to ADAFs in which very little mass accretion onto the collapsed star (if any) takes place in the quiescence intervals, whereas a sizeable fraction of the mass being transferred from the companion star (if not all) accumulates in an outer disk region. As in some pre-ADAF models, the residual optical/UV emissions of black hole candidate systems are expected to derive from the gravitational energy released by the matter transferred from the companion star at radii comparable to the circularization radius. The quiescent X-ray luminosity originates from accretion onto the black hole candidates at very low rates and/or from coronal activity in the companion star or in the outer disk. For comparably small mass inflow rates, it can be concluded that the neutron stars in these systems are likely in the radio pulsar regime. In the interaction of the radio pulsar relativistic wind with matter transferred from the companion star, a shock forms, the power law-like emission of which powers both the harder X-ray emission component and most of the residual optical/UV observed in quiescence. The soft, thermal-like X-ray component may arise from the cooling of the neutron star surface in between outbursts or, perhaps, heating of the magnetic polar caps by relativistic particles in the radio pulsar magnetosphere. This scenario matches well both the X-ray and bolometric luminosity swing of black hole candidate as well as neutron star systems, for comparable swings of mass inflow rates toward the collapsed object.

High angular resolution radio observations of relativistic jets are necessary to understand the causal connection between accretion and jet ejection in low-mass X-ray binaries. Images from these observations can be difficult to reconstruct due to the rapid intra-observational motion and variability of transient jets. We have developed a time-dependent visibility model fitting and self-calibration procedure and applied it to a single 4 hr VLBA observation of the low-mass X-ray binary Swift J1727.8−1613 during the bright flaring period of its 2023 outburst. This allowed us to detect and model a slightly resolved self-absorbed compact core, as well as three downstream transient jet knots. We were able to precisely measure the proper motion and flux density variability of these three jet knots, as well as (for the first time) their intra-observational expansion. Using simultaneous multifrequency data, we were also able to measure the spectral index of the furthest downstream jet knot, and the core, as well as the frequency-dependent core shift between 2.3 and 8.3 GHz. Using these measurements, we inferred the ejection dates of the three jet knots, including one to within ±40 minutes, which is one of the most precise ever measured. The ejection of the transient jet knots coincided with a bright X-ray flare and a drastic change in the X-ray spectral and timing properties as seen by HXMT, which is the clearest association ever seen between the launching of transient relativistic jets in an X-ray binary and a sudden change in the X-ray properties of the accretion inflow.

MXB 1659–298 is a transient neutron-star low-mass X-ray binary system that shows eclipses with a periodicity of 7.1 h. MXB 1659–298 went to outburst in 2015 August, after 14 years of quiescence. We investigate the orbital properties of this source with a baseline of 40 years, obtained by combining the eight eclipse arrival times present in the literature with 51 eclipse arrival times collected during the last two outbursts. A quadratic ephemeris does not fit the delays associated with the eclipse arrival times and the addition of a sinusoidal term with a period of 2.31 ± 0.02 yr is required. We infer a binary orbital period of P = 7.1161099(3) h and an orbital period derivative of |$\dot{P}=-8.5(1.2) \times 10^{-12}$| s s−1. We show that the large orbital period derivative can be explained with a highly non-conservative mass-transfer scenario, in which more than 98 per cent of the mass provided by the companion star leaves the binary system. We predict an orbital period derivative value of |$\dot{P}=-6(3) \times 10^{-12}$| s s−1 and constrain the companion-star mass between 0.3 and 1.2 M⊙. Assuming that the companion star is in thermal equilibrium, the periodic modulation can be due to either a gravitational quadrupole coupling arising from variations of the oblateness of the companion star or the presence of a third body of mass M3 > 21 Jovian masses.

This paper will review a new technique of detecting companion stars in LMXBs and X-ray transients in outburst using the Bowen fluorescence NIII lines at 4634-4640. These lines are very efficiently reprocessed in the atmospheres of the companion stars and, thereby, provide estimates of the K2 velocities and mass functions. The method has been applied to Sco X-1, X1822-371 and GX339-4 which, in the latter case, provides dynamical evidence for the presence of an accreting black hole. Preliminary results from a VLT campaign on V801 Ara, V926 Sco and XTE J1814-338 are also presented.

We present time-resolved Gran Telescopio Canarias optical spectroscopy and William Herschel Telescope i-band photometry of the X-ray transient SWIFT J1753.5–0127 in quiescence. The i-band light curve is dominated by flickering with an amplitude of ∼0.5 mag and shows no evidence of the ellipsoidal modulation of the companion star. The telluric-corrected average spectrum, on the other hand, reveals the presence of weak (strongly veiled) TiO bands at 7055 Å and 7589 Å. We used them for a spectral classification, finding an M4-5 V companion star. However, as velocity shifts are not clearly detected in the individual spectra, we turned the analysis to the double-peaked Hα emission line from the accretion disc. By exploiting the empirical correlations established for quiescent X-ray transients between the line morphology and fundamental binary parameters, we estimated the radial velocity semi-amplitude of the companion K2 = 820 ± 36 km s−1, a mass ratio q = 0.023 ± 0.006 and an inclination i = 79 ± 5 deg. Moreover, an orbital period of 3.26 ± 0.02 h was measured from the modulation of the centroid velocities and the double-peak trough depth of the Hα profile. These quantities yielded a mass function f(M1) = 7.8 ± 1.0 M⊙ and black hole and companion star masses of M1 = 8.8 ± 1.3 M⊙ and M2 = 0.20 ± 0.06 M⊙, respectively. The companion star mass is in line with the spectral classification obtained from the relative depth of the TiO bands. Based on the mean quiescent magnitude (i = 21.4 ± 0.1), orbital period, and interstellar extinction, we estimate the distance to the source to be 3.9 ± 0.7 kpc and a Galactic plane elevation of 0.8 ± 0.2 kpc, supporting the case for a large natal kick.

This work is focused on the study of millisecond pulsars in globular cluster by using multi-wavelength observations. Radio observations have been used to search for and timing the pulsars. An alternative method to search for very faint pulsars is first presented and then successfully applied to observations of stellar system Terzan 5, leading to the discovery of three new pulsars. The update of the timing solutions of 9 pulsars in M28 is then presented. The timing solutions now cover a data span of 10 years and provide spin, astrometric and orbital properties for all the systems. For the case of the eccentric binaries M28C and M28D, post-Keplerian corrections to the orbit have been measured in order to derive the masses of the binary components. The measurement of the pulsar proper motions allowed to constrain the cluster motion as a whole and its orbit around the Galaxy. Finally, the pulsar spin and orbital period derivatives have been used to measure their accelerations induced by the cluster potential field. Optical observations have been used to search for millisecond pulsar optical counterparts. Six new companion stars have been discovered. In particular, four companions turned out to be He white dwarfs. One companion turned out to be a faint and non-degenerate object, strongly affected by heating of the stellar side exposed to the pulsar flux. Finally, one companion is a main-sequence star which shows Hα emission likely due to a low-level mass transfer. Furthermore, we identified the companion star to a transient low-mass X-ray binary. This companion turned out to be a sub-giant branch star and therefore this system is likely in a very early phase of the mass accretion stage. Conclusion are drawn in the final chapter, where the evolution of millisecond pulsars is discussed and possible future developments are suggested.

We find that the mass ratio q in quiescent black hole (BH) X-ray transients is tightly correlated with the ratio of the double-peak separation (DP) to the full width half maximum (FWHM) of the H α emission line, log q = − 6.88 − 23.2 log ( DP / FWHM ) . This correlation is explained through the efficient truncation of the outer disk radius by the 3:1 resonance with the companion star. This is the dominant tidal interaction for extreme mass ratios q = M 2 / M 1 ≲ 0.25 , the realm of BH (and some neutron star) X-ray transients. Mass ratios can thus be estimated with a typical uncertainty of ≈32%, provided that the H α profile used to measure DP/FWHM is an orbital phase average. We apply the DP/FWHM–q relation to the three faint BH transients XTE J1650–500, XTE J1859+226, and Swift J1357–0933 and predict q = 0.026 − 0.007 + 0.038 , 0.049 − 0.012 + 0.023 and 0.040 − 0.005 + 0.003 , respectively. This new relation, together with the FWHM – K 2 correlation presented in Paper I, allows the extraction of fundamental parameters from very faint targets and, therefore, the extension of dynamical BH studies to much deeper limits than was previously possible. As an example, we combine our mass ratio determination for Swift J1357–0933 with previous reported values to yield a BH mass of 12.4 ± 3.6 M ⊙. This confirms Swift J1357–0933 as one of the most massive BH low-mass X-ray binaries in the Galaxy.

Millisecond radio pulsars are neutron stars that have been spun up by the transfer of angular momentum during the low-mass X-ray binary phase. The transition from an accretion-powered pulsar to a rotation-powered pulsar takes place on evolutionary timescales at the end of the accretion process; however, it may also occur sporadically in systems undergoing transient X-ray activity. We have obtained the first optical spectrum of the low-mass transient X-ray pulsar SAX J1808.4-3658 in quiescence. Similar to the black widow millisecond pulsar B1957+20, this X-ray pulsar shows a large optical modulation at the orbital period due to an irradiated companion star. Using the brightness of the companion star as a bolometer, we conclude that a very high irradiating luminosity, a factor of ~100 larger than directly observed, must be present in the system. This most likely derives from a rotation-powered neutron star that resumes activity during quiescence.

We present near-infrared (NIR) imaging observations of three transient neutron star X-ray binaries, SAX J1753.5-2349, SAX J1806.5-2215 and AX J1754.2-2754. All three sources are members of the class of `very faint' X-ray transients which exhibit X-ray luminosities $L_X\lesssim10^{36}$ erg s$^{-1}$. The nature of this class of sources is still poorly understood. We detect NIR counterparts for all three systems and perform multi-band photometry for both SAX J1753.5-2349 and SAX J1806.5-2215, including narrow-band Br$_{\gamma}$ photometry for SAX J1806.5-2215. We find that SAX J1753.5-2349 is significantly redder than the field population, indicating that there may be absorption intrinsic to the system, or perhaps a jet is contributing to the infrared emission. SAX J1806.5-2215 appears to exhibit absorption in Br$_{\gamma}$, providing evidence for hydrogen in the system. Our observations of AX J1754.2--2754 represent the first detection of a NIR counterpart for this system. We find that none of the measured magnitudes are consistent with the expected quiescent magnitudes of these systems. Assuming that the infrared radiation is dominated by either the disc or the companion star, the observed magnitudes argue against an ultracompact nature for all three systems.

We present multiwavelength characterization of 65 high-mass X-ray binary (HMXB) candidates in M33. We use the Chandra ACIS survey of M33 (ChASeM33) catalog to select hard X-ray point sources that are spatially coincident with UV-bright point-source optical counterparts in the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region catalog, which covers the inner disk of M33 at near-IR, optical, and near-UV wavelengths. We perform spectral energy distribution fitting on multiband photometry for each point-source optical counterpart to measure its physical properties including mass, temperature, luminosity, and radius. We find that the majority of the HMXB companion star candidates are likely B-type main-sequence stars, suggesting that the HMXB population of M33 is dominated by Be X-ray binaries (Be-XRBs), as is seen in other Local Group galaxies. We use spatially resolved recent star formation history maps of M33 to measure the age distribution of the HMXB candidate sample and the HMXB production rate for M33. We find a bimodal distribution for the HMXB production rate over the last 80 Myr, with a peak at ∼10 and ∼40 Myr, which match theoretical formation timescales for the most massive HMXBs and Be-XRBs, respectively. We measure an HMXB production rate of 107–136 HMXBs/(M ⊙ yr−1) over the last 50 Myr and 150–199 HMXBs/(M ⊙ yr−1) over the last 80 Myr. For sources with compact object classifications from overlapping NuSTAR observations, we find a preference for giant/supergiant companion stars in black hole HMXBs and main-sequence companion stars in neutron star HMXBs.

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.

We have observed the X-ray transient XTE J0421+56 in quiescence with XMM-Newton. The observed spectrum is highly unusual being dominated by an emission feature at ~6.5 keV. The spectrum can be fit using a partially covered power-law and Gaussian line model, in which the emission is almost completely covered (covering fraction of 0.98_{-0.06}^{+0.02}) by neutral material and is strongly absorbed with an N_H of (5_{-2}^{+3}) x 10^{23} atom cm^{-2}. This absorption is local and not interstellar. The Gaussian has a centroid energy of 6.4 +/- 0.1 keV, a width < 0.28 keV and an equivalent width of 940 ^{+650}_{-460} eV. It can be interpreted as fluorescent emission line from iron. Using this model and assuming XTE J0421+56 is at a distance of 5 kpc, its 0.5-10 keV luminosity is 3.5 x 10^{33} erg s^{-1}. The Optical Monitor onboard XMM-Newton indicates a V magnitude of 11.86 +/- 0.03. The spectra of X-ray transients in quiescence are normally modeled using advection dominated accretion flows, power-laws, or by the thermal emission from a neutron star surface. The strongly locally absorbed X-ray emission from XTE J0421+56 is therefore highly unusual and could result from the compact object being embedded within a dense circumstellar wind emitted from the supergiant B[e] companion star. The uncovered and unabsorbed component observed below 5 keV could be due either to X-ray emission from the supergiant B[e] star itself, or to the scattering of high-energy X-ray photons in a wind or ionized corona, such as observed in some low-mass X-ray binary systems.

Black widows (BWs) are a type of eclipsing millisecond pulsars (MSPs) with companion masses $M_2\lesssim 0.05\, \rm M_\odot$, which can be used to study the accretion history and the radiation of pulsars, as well as the origin of isolated MSPs. Recent observations indicate that there are two subtypes of BWs. One is the BWs with $M_2 \sim 0.01\!-\!0.05\, \rm M_\odot$, whereas another with $M_2 \lesssim 0.01\, \rm M_\odot$. However, the origin of the latter is still highly uncertain. In this paper, we investigated the formation of BWs with $M_2 \lesssim 0.01\, \rm M_\odot$ through ultracompact X-ray binaries (UCXBs) with He star companions, in which a neutron star (NS) accretes material from a He star through Roche lobe overflow. By considering different He star masses and evaporation efficiencies with the stellar evolution code Modules for Experiments in Stellar Astrophysics (mesa ), we evolved a series of NS+He star systems that can undergo UCXB stage. This channel can explain the formation of the BWs with $M_2 \lesssim 0.01\, \rm M_\odot$ within the Hubble time, especially three widely studied BWs, i.e. PSRs J1719−1438, J2322−2650, and J1311−3430. We found that X-ray irradiation feedback does not affect the evolutionary tracks of evaporation process. The simulations indicate that the UCXBs with He star companions are the potential progenitors of isolated MSPs, and that the origin of BWs with $M_2 \lesssim 0.01\, \rm M_\odot$ is different with another subtype of BWs. In addition, this work suggests that the BWs with $M_2 \lesssim 0.01\, \rm M_\odot$ may not be produced by redback systems.

Swift J1753.5-0127 (J1753) is a candidate black hole low-mass X-ray binary (BH-LMXB) that was discovered in outburst in May 2005. It remained in outburst for $\sim12$ years, exhibiting a wide range of variability on various timescales, before entering quiescence after two short-lived, low-luminosity "mini-outbursts" in April 2017. The unusually long outburst duration in such a short-period ($P_{\rm orb}\approx3.24$ hrs) source, and complex variability observed during this outburst period, challenges the predictions of the widely accepted disc-instability model, which has been shown to broadly reproduce the behaviour of LMXB systems well. The long-term behaviour observed in J1753 is reminiscent of the Z Cam class of dwarf novae, whereby variable mass transfer from the companion star drives unusual outbursts, characterized by stalled decays and abrupt changes in luminosity. Using sophisticated modelling of the multi-wavelength light curves and spectra of J1753, during the $\sim12$ years the source was active, we investigate the hypothesis that periods of enhanced mass transfer from the companion star may have driven this unusually long outburst. Our modelling suggests that J1753 is in fact a BH-LMXB analogue to Z Cam systems, where the variable mass transfer from the companion star is driven by the changing irradiation properties of the system, affecting both the disc and companion star.

Recent data on low-mass X-ray binaries (LMXBs) and msec pulsars (MSPs) pose a challenge to evolutionary theories which neglect the effects of disk and companion irradiation. Here we discuss the main features of a radiation-driven (RD) evolutionary model that may be applicable to several LMXBs. According to this model, radiation from the accreting compact star in LMXBs ‘vaporizes’ the accretion disk and the companion star by driving a self-sustained mass loss until a sudden accretion-turn off occurs. The main characteristics of the RD-evolution are: (1) the lifetime of RD-LMXB’s is of order 107 years or less; (2) both the orbital period gap and the X-ray luminosity may be consequences of RD-evolution of LMXB’s containing lower main sequence and degenerate companion stars; (3) the companion star may transfer mass to the primary even if it underfills its Roche lobe; (4) the recycled msec pulsar can continue to vaporize the low-mass companion star even after the accretion turn-off produced by a strong pulsar wind; (5) the RD-evolutionary model resolves the apparent statistical discrepancy between the number of MSP’s and their LMXB progenitors.