Searcharxiv⌕ Search

arXiv subjects

J. Casares

Publications and source records attributed to J. Casares.

At least 37 records · Page 2Linked to original sources
arXiv2023

Fast infrared winds during the radio-loud and X-ray obscured stages of the black hole transient GRS 1915+105

The black hole transient GRS 1915+105 entered a new phase of activity in 2018, generally characterised by low X-ray and radio fluxes. This phase has been only interrupted by episodes of strong and variable radio emission, during which high levels of X-ray absorption local to the source were measured. We present 18 epochs of near-infrared spectroscopy (2018-2023) obtained with GTC/EMIR and VLT/X-shooter, spanning both radio-loud and radio-quiet periods. We demonstrate that radio-loud phases are characterised by strong P-Cygni line profiles, indicative of accretion disc winds with velocities of up to $\mathrm{\sim 3000~km~s^{-1}}$. This velocity is consistent with those measured in other black hole transients. It is also comparable to the velocity of the X-ray winds detected during the peak outburst phases in GRS 1915+105, reinforcing the idea that massive, multi-phase outflows are characteristic features of the largest and most powerful black hole accretion discs. Conversely, the evolution of the Br$γ$ line profile during the radio-quiet phases follows the expected trend for accretion disc lines in a system that is gradually decreasing its intrinsic luminosity, exhibiting weaker intensities and more pronounced double-peaks.

astro-ph.HE↗
arXiv2023

PSR J0210+5845; An ultra wide binary pulsar with a B6V main-sequence star companion

We report on radio timing observations of PSR J0210+5845 which reveal large deviations from typical pulsar spin-down behaviour. We interpret these deviations as being due to binary motion around the $V=13.5$ star 2MASS J02105640$+$5845176, which is coincident in celestial position and distance with the pulsar. Archival observations and new optical spectroscopy identify this star as a B6V star with a temperature of $T_\mathrm{eff}\approx 14\,000$K and a mass of $M_\mathrm{c}= 3.5$ to $3.8$M$_\odot$, making it the lowest mass main-sequence star known orbiting a non-recycled pulsar. We found that the timing observations constrain the binary orbit to be wide and moderately eccentric, with an orbital period of $P_\mathrm{b}=47^{+40}_{-14}$yr and eccentricity $e=0.46^{+0.10}_{-0.07}$. We predict that the next periastron passage will occur between 2030 and 2034. Due to the low companion mass, we find that the probability for a system with the properties of PSR J0210+5845 and its binary companion to survive the supernova is low. We show that a low velocity and fortuitously directed natal kick is required for the binary to remain bound during the supernova explosion, and argue that an electron-capture supernova is a plausible formation scenario for the pulsar.

astro-ph.HE↗
arXiv2023

Evidence for a black hole in the historical X-ray transient A 1524-61 (=KY TrA)

We present VLT 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 $0.64 \pm 0.04$ arcsec SE. From the spectroscopy, we refine the radial velocity semi-amplitude of the donor star to $K_2 = 501 \pm 52$ 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 $0.26 \pm 0.01$ d ($6.24 \pm 0.24$ h). These numbers imply a mass function $f(M_1) = 3.2 \pm 1.0$ M$_\odot$. The KY TrA de-reddened quiescent colour $(r-i)_0 = 0.27 \pm 0.08$ 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, $M_2 \leq 0.94$ M$_\odot$, and, in turn, on the binary mass ratio, $q = M_2/M_1 \leq 0.31$. By exploiting the correlation between the binary inclination and the depth of the H$α$ line trough, we establish $i = 57 \pm 13$ deg. All these values lead to a compact object and donor mass of $M_1 = 5.8^{+3.0}_{-2.4}$ M$_\odot$ and $M_2 = 0.5 \pm 0.3$ M$_\odot$, respectively, thus confirming the black hole nature of the accreting object. In addition, we estimate a distance toward the system of $8.0 \pm 0.9$ kpc.

astro-ph.HE↗
arXiv2023

The omnipresent flux-dependent optical dips of the black hole transient Swift J1357.2-0933

Swift J1357.2-0933 is a black hole transient of particular interest due to the optical, recurrent dips found during its first two outbursts (in 2011 and 2017), with no obvious X-ray equivalent. We present 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 perform a global study of the dips properties in the four outbursts, and find that they do not follow a common temporal evolution. In addition, we discover a correlation with the X-ray and optical fluxes, with 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. Finally, we discuss these results in the context of the possible connection between optical dips and outflows found in previous works.

astro-ph.HE↗
arXiv2023

An infrared FWHM-$K_2$ correlation to uncover highly reddened quiescent black holes

Among the sample of Galactic transient X-ray binaries (SXTs) discovered to date, about 70 have been proposed as likely candidates to host a black hole. Yet, only 19 have been dynamically confirmed. Such a reliable confirmation requires phase-resolved spectroscopy of their companion stars, which is generally feasible when the system is in a quiescent state. However, since most of the SXT population lies in the galactic plane, which is strongly affected by interstellar extinction, their optical brightness during quiescence usually falls beyond the capabilities of the current instrumentation ($R\gtrsim22$). To overcome these limitations and thereby increase the number of confirmed Galactic black holes, a correlation between the full-width at half maximum (FWHM) of the H$α$ line and the semi-amplitude of the donor's radial velocity curve ($K_2$) was presented in the past. Here, we extend the FWHM-$K_2$ correlation to the near-infrared (NIR), exploiting disc lines such as He I $λ$10830, Pa$γ$, and Br$γ$, in a sample of dynamically confirmed black-hole SXTs. We obtain $K_2 = 0.22(3) ~\textrm{FWHM}$, in good agreement with the optical correlation derived using H$α$. The similarity of the two correlations seems to imply that the widths of H$α$ and the NIR lines are consistent in quiescence. When combined with information on orbital periods, the NIR correlation allows us to constrain the mass of the compact object of systems in quiescence by using single-epoch spectroscopy. We anticipate that this new correlation will give access to highly reddened black-hole SXTs, which cannot be otherwise studied at optical wavelengths.

astro-ph.HE↗
arXiv2023

H$β$ spectroscopy of the high-inclination black hole transient Swift J1357.2-0933 during quiescence

Swift J1357.2-0933 is a transient low-mass X-ray binary hosting a stellar-mass black hole. The source exhibits optical dips and very broad emission lines during both outburst and quiescence, which are thought to be the result of a high orbital inclination. We present phase-resolved spectroscopy obtained with the 10.4m Gran Telescopio Canarias (GTC). The spectra focus on the $\rm{H}β$ spectral region during X-ray quiescence. The emission line is exceptionally broad (full width at half maximum, FWHM > 4000 Å), in agreement with previous studies focused on $\rm{H}α$. A two-Gaussian fit to the prominent double-peaked profile reveals a periodic variability in the centroid position of the line. We also produced a diagnostic diagram aimed at constraining additional orbital parameters. Together, they allow us to independently confirm the orbital period of the system using a new dataset obtained five years after the previous outburst. However, our estimates for both the systemic velocity and the radial velocity semi-amplitude of the black hole reveal larger values than those found in previous studies. We argue that this could be explained by the precession of the disc and the presence of a hotspot. We found evidence of a narrow inner core in the double-peaked H$β$ emission profile. We studied its evolution across the orbit, finding that it is likely to result from the occultation of inner material by the outer rim bulge, further supporting the high orbital inclination hypothesis.

astro-ph.HE↗
arXiv2023

Unveiling optical signatures of outflows in accreting white dwarfs

Accreting white dwarfs are known to show signatures of wind-type outflows in the ultraviolet. At optical wavelengths, however, wind detections have only been reported for a few sources. We present GTC-10.4m optical spectroscopy of four accreting white dwarfs (BZ Cam, V751 Cyg, MV Lyr, and V425 Cas) observed during luminous epochs, when their optical emission is expected to be dominated by the accretion disc. We focused the analysis on four emission lines: H$α$ and He I $λ$5876, $λ$6678, $λ$7065. Line profiles are complex and variable on short (minutes) and long (days to weeks) time scales, with transient absorption and emission components. Among them, we detect strong blue-shifted absorptions at $\gtrsim 1000$ km s$^{-1}$. These high-velocity components, present only in the blue wing of the emission lines, are observed in all four sources and could be associated with accretion disc winds. For MV Lyr and V425 Cas, these would represent the first detection of optical outflows in these objects, while in the case of BZ Cam and V751 Cyg, the presence of outflows has been previously reported. This study suggests that, in addition to ultraviolet winds, optical outflows might be also common in accreting white dwarfs. We discuss the observational properties of these winds and their possible similarity to those detected in accreting black holes and neutrons stars.

astro-ph.SR↗
arXiv2023

The orbital period, black hole mass and distance to the X-ray transient GRS 1716-249 (=N Oph 93)

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 Halpha emission line detected in Gran Telescopio Canarias spectra and obtain K2=521+-52 km/s. This leads to a mass function f(M)=4.1+-1.2 Msun, thus indicating the presence of a black hole in this historic X-ray transient. Furthermore, from the depth of the Halpha trough and the quiescent light curve we constrain the binary inclination to i=61+-15 deg, 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 M1=6.4+3.2-2.0 Msun 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.

astro-ph.HE↗
arXiv2023

Precision Ephemerides for Gravitational-wave Searches -- IV: Corrected and refined ephemeris for Scorpius X-1

Low-mass X-ray binaries have long been theorised as potential sources of continuous gravitational-wave radiation, yet there is no observational evidence from recent LIGO/Virgo observing runs. Even for the theoretically 'loudest' source, Sco X-1, the upper limit on gravitational-wave strain has been pushed ever lower. Such searches require precise measurements of the source properties for sufficient sensitivity and computational feasibility. Collating over 20 years of high-quality spectroscopic observations of the system, we present a precise and comprehensive ephemeris for Sco X-1 through radial velocity measurements, performing a full homogeneous reanalysis of all relevant datasets and correcting previous analyses. Our Bayesian approach accounts for observational systematics and maximises not only precision, but also the fidelity of uncertainty estimates - crucial for informing principled continuous-wave searches. Our extensive dataset and analysis also enables us to construct the highest signal-to-noise, highest resolution phase-averaged spectrum of a low-mass X-ray binary to date. Doppler tomography reveals intriguing transient structures present in the accretion disk and flow driven by modulation of the accretion rate, necessitating further characterisation of the system at high temporal and spectral resolution. Our ephemeris corrects and supersedes previous ephemerides, and provides a factor three reduction in the number of templates in the search space, facilitating precision searches for continuous gravitational-wave emission from Sco X-1 throughout the upcoming LIGO/Virgo/KAGRA O4 observing run and beyond.

astro-ph.HE↗
arXiv2023

Sub-second infrared variability from the archetypal accreting neutron star 4U~1728-34

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 timescales. By measuring the cross-correlation function between the infrared and X-ray lightcurves, we discovered a significant correlation with an infrared 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 $\approx7.2\,$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$^\circ$.

astro-ph.HE↗
arXiv2023

UltraCompCAT: a comprehensive Catalogue of Ultra-Compact and Short Orbital Period X-ray Binaries

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 as 'candidate' 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 (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's 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 per cent) 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 both groups.

astro-ph.HE↗
arXiv2023

Ask The Machine: Systematic detection of wind-type outflows in low-mass X-ray binaries

The systematic discovery of outflows in the optical spectra of low-mass X-ray binaries opened a new avenue for the study of the outburst evolution in these extreme systems. However, the efficient detection of such features in a continuously growing database requires the development of new analysis techniques with a particular focus on scalability, adaptability, and automatization. In this pilot study, we explore the use of machine learning algorithms to perform the identification of outflows in spectral line profiles observed in the optical range. We train and test the classifier on a simulated database, constructed through a combination of disc emission line profiles and outflow signatures, emulating typical observations of low-mass X-ray binaries. The final, trained classifier is applied to two sets of spectra taken during two bright outbursts that were particularly well covered, those of V404 Cyg (2015) and MAXI J1820+070 (2018). The resulting classification gained by this novel approach is overall consistent with that obtained through traditional techniques, while it simultaneously provides a number of key advantages over the latter, including the access to low velocity outflows. This study sets the foundations for future studies on large samples of spectra from low-mass X-ray binaries and other compact binaries.

astro-ph.HE↗
arXiv2023

Optical and near-infrared spectroscopy of the black hole transient 4U 1543-47 during its 2021 ultra-luminous state

We present simultaneous optical and near-infrared spectra obtained during the 2021 outburst of the black hole transient 4U 1543-47. The X-ray hardness-intensity diagram and the comparison with similar systems reveal a luminous outburst, probably reaching the Eddington luminosity, as well as a long-lasting excursion to the so-called ultra-luminous state. VLT/X-shooter spectra were taken in two epochs 14 days apart during the early and brightest part of the outburst, while the source was in this ultra-luminous accretion state. The data show strong H and HeI emission lines, as well as high-excitation HeII and OIII transitions. Most lines are single-peaked in both spectra, except for the OIII lines that exhibit evident double-peaked profiles during the second epoch. The Balmer lines are embedded in broad absorption wings that we believe are mainly produced by the contribution of the A2V donor to the optical flux, which we estimate to be in the range of 11 to 14 per cent in the $r$ band during our observations. Although no conspicuous outflow features are found, we observe some wind-related line profiles, particularly in the near-infrared. Such lines include broad emission line wings and skewed red profiles, suggesting the presence of a cold (i.e. low ionisation) outflow with similar observational properties to those found in other low-inclination black hole transients.

astro-ph.HE↗
arXiv2022

A refined dynamical mass for the black hole in the X-ray transient XTE J1859+226

We present two contiguous nights of simultaneous time-resolved GTC spectroscopy and WHT 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 $0.276 \pm 0.003$ d and the radial velocity semi-amplitude of the donor star to $K_2 = 550 \pm 59$ 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 $66.6 \pm 4.3$ deg, a refined $K_2 = 562 \pm 40$ km s$^{-1}$ and mass ratio $q = M_2/M_1 = 0.07 \pm 0.01$. From these values we obtained an updated black hole mass of $M_1 = 7.8 \pm 1.9$ M$_\odot$. An independent mass estimate based on X-ray timing agrees well with our value, which gives further support for the outburst QPO triplet being explained by the relativistic precession model. We also obtained a companion star mass $M_2 = 0.55 \pm 0.16$ M$_\odot$, which is consistent with its K5-K7 V spectral type.

astro-ph.HE↗
arXiv2022

A correlation between Ha trough depth and inclination in quiescent X-ray transients: evidence for a low-mass black hole in GRO J0422+32

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 Ha 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 x 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 de-projected 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 Msun 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 Msun 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 deg, where it has not yet been tested.

astro-ph.HE↗
arXiv2022

Discovery of optical and infrared accretion disc wind signatures in the black hole candidate MAXI J1348-630

MAXI J1348-630 is a low mass X-ray binary discovered in 2019 during a bright outburst. During this event, the system sampled both hard and soft states following the standard evolution. We present multi-epoch optical and near-infrared spectroscopy obtained with X-shooter at the Very Large Telescope. Our dataset includes spectra taken during the brightest phases of the outburst as well as the decay towards quiescence. We study the evolution of the main emission lines, paying special attention to the presence of features commonly associated with accretion disc winds, such as blue-shifted absorptions, broad emission line wings and flat-top profiles. We find broad emission line wings in H-alpha during the hard-to-soft transition and blue-shifted absorption troughs at ~-500 km/s in H-beta, HeI-5876, H-alpha and Pa-beta during the bright soft-intermediate state. In addition, flat-top profiles are seen throughout the outburst. We interpret these observables as signatures of a cold (i.e. optical to infrared) accretion disc wind present in the system. We discuss the properties of the wind and compare them with those seen in other X-ray transients. In particular, the wind velocity that we observe is low when compared to those of other systems, which might be a direct consequence of the relatively low binary inclination, as suggested by several observables. This study strengthen the hypothesis that cold winds are a common feature in low mass X-ray binaries and that they can also be detected in low inclination objects via high-quality optical and infrared spectroscopy.

astro-ph.HE↗
arXiv2022

A tentative 114-minute orbital period challenges the ultra-compact nature of the X-ray binary 4U 1812-12

We present a detailed time-resolved photometric study of the ultra-compact X-ray binary candidate 4U 1812-12. The multicolor light curves obtained with HiPERCAM on the 10.4-m Gran Telescopio Canarias show an aprox 114 min modulation similar to a superhump. Under this interpretation, this period should lie very close to the orbital period of the system. Contrary to what its other observational properties suggest (namely, persistent dim luminosity, low optical-to-X-ray flux ratio and lack of hydrogen features in the optical spectrum), this implies that 4U1812-12 is most likely not an ultra-compact X-ray binary, which are usually defined as systems with orbital periods lower than 80 min. We discuss the nature of the system, showing that a scenario in which 4U 1812-12 is the progenitor of an ultra-compact X-ray binary may reconcile all the observables.

astro-ph.HE↗
arXiv2022

A persistent ultraviolet outflow from an accreting neutron star binary transient

All disc-accreting astrophysical objects produce powerful outflows. In binaries containing neutron stars (NS) 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 revealed wind-formed lines between the X-ray and optical bands yet, despite the many strong resonance transitions in this ultraviolet (UV) region. Here, we show that the transient NS binary Swift J1858.6-0814 exhibits wind-formed, blue-shifted absorption associated with C IV, N V and He II in time-resolved UV spectroscopy during a luminous hard state. This represents the first evidence for 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 connect to the luminous flares in the data. The joint presence of UV and optical wind features reveals a multi-phase and/or stratified 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 outbursts duration.

astro-ph.HE↗