Hβ\beta 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 $\rm{H}\beta$ spectral region during X-ray quiescence. The emission line is exceptionally broad (full width at half maximum, FWHM > 4000 \AA), in agreement with previous studies focused on $\rm{H}\alpha$. 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$\beta$ 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.Comment: 6 pages, 4 figure

A revised view of the Canis Major stellar overdensity with DECam and Gaia: new evidence of a stellar warp of blue stars

We present DECam imaging combined with Gaia DR2 data to study the Canis Major overdensity. The presence of the so-called Blue Plume stars in a low-pollution area of the color-magnitude diagram allows us to derive the distance and proper motions of this stellar feature along the line of sight of its hypothetical core. The stellar overdensity extends on a large area of the sky at low Galactic latitudes, below the plane, and between 230$^\circ < \ell < 255^\circ$. According to the orbit derived for Canis Major, it presents an on-plane rotation around the Milky Way. Moreover, additional overdensities of Blue Plume stars are found around the plane and across the Galaxy, proving that these objects are not only associated with that structure. The spatial distribution of these stars, derived using Gaia astrometric data, confirms that the detection of the Canis Major overdensity results more from the warped structure of the Milky Way disk than from the accretion of a dwarf galaxy.Comment: 11 pages, 9 figures, accepted for publication in MNRA

Evidence for a black-hole in the X-ray transient XTE J1859+226

We present the results of time-resolved optical photometry and spectroscopy of the X-ray transient XTE J1859+226 (V406 Vul). Photometric observations taken during 2000 and 2008 reveals the presence of the secondary star's ellipsoidal modulation. Further photometry obtained in 2010 shows the system ~1 mag brighter than its quiescence level and the ellipsoidal modulation diluted by strong flaring activity. Spectroscopic data obtained with the 10.4-m GTC in 2010 reveals radial velocity variations of ~500 km/s over 3 h. A simultaneous fit to the photometry and spectroscopy using sinusoids to represent the secondary star's ellipsoidal and radial velocity variations, yields an orbital period of 6.58+-0.05 h and a secondary star's radial velocity semi-amplitude of K_2= 541+-70 km/s. The implied mass function is f(M)=4.5+-0.6 Msun, significantly lower than previously reported but consistent with the presence of a black hole in XTE J1859+226. The lack of eclipses sets an upper limit to the inclination of 70 degrees which yields a lower limit to the black hole mass of 5.42 Msun.Comment: Accepted for publication in MNRAS. Contains 5 pages and 4 figure

The dynamical mass of the white dwarf in XY Ari questions intermediate polar X-ray spectral models

We present a dynamical study of the eclipsing intermediate polar XY Ari based on time-resolved near-infrared spectroscopy obtained with the EMIR spectrograph on the 10.4-m Gran Telescopio Canarias. Using main sequence template spectra taken with the same instrument setup as the target spectra, we measure a radial velocity amplitude of the late K-type donor star $K_2=256 \pm 2$ km s$^{-1}$. We also obtain the rotational broadening of its photospheric lines $v_\mathrm{rot} \sin i = 141 \pm 3$ km s$^{-1}$. From these and the eclipse geometry, we derive a donor-to-white dwarf mass ratio $q = M_2/M_1 = 0.62 \pm 0.02$, an orbital inclination $i = 80.8^{\circ} \pm 0.5^{\circ}$ and dynamical masses $M_{1} = 1.21 \pm 0.04 \, \mathrm{M}_{\odot}$ and $M_2 = 0.75 \pm 0.04 \, \mathrm{M}_{\odot}$ ($1 \sigma$). This result places the white dwarf in XY Ari as one of the three most massive known in a cataclysmic variable. Comparison with a number of white dwarf mass estimates from X-ray spectral modelling indicates the necessity of a major revision of the cooling models currently assumed for magnetic accretion in intermediate polars, as most of the X-ray white dwarf masses lie significantly below the dynamical mass value.Comment: 9 pages, 4 figures, 5 tables, submitted for publication in MNRA

REFINED ORBITAL SOLUTION AND QUIESCENT VARIABILITY IN THE BLACK HOLE TRANSIENT GS 1354-64 (= BW Cir)

In Casares et al. we presented the first radial velocity curve of the companion star to BW Cir which demonstrates the presence of a black hole in this historical X-ray transient. But these data were affected by aliasing and two possible periods at 2.5445 days and 2.5635 days were equally possible. Here we present new spectroscopic data that enable us to break the 1-year aliasing and confirm 2.5445 days as the correct orbital period. We also present R-band photometry over 14 years, which reveals the presence of important flaring activity dominating the light curves.Spain. Ministerio de Ciencia y Tecnología (Spanish MCYT grant AYA2002-0036)Spain. Ministerio de Ciencia y Tecnología (programme Ramon y Cajal)Chandra X-ray Center (U.S.) (NASA Contract NAS8-03060

XTE J1752-223 in outburst: a persistent radio jet, dramatic flaring, multiple ejections and linear polarisation

The black hole candidate, XTE J1752-223, was discovered in 2009 October when it entered an outburst. We obtained radio data from the Australia Telescope Compact Array for the duration of the ~9 month event. The lightcurves show that the radio emission from the compact jet persisted for the duration of an extended hard state and through the transition to the intermediate state. The flux then rose rapidly by a factor of 10 and the radio source entered a series of at least 7 maxima, the first of which was likely to be emission associated with the compact jet. The subsequent 6 flares were accompanied by variable behaviour in terms of radio spectrum, degree of linear polarisation, morphology and associated X-ray behaviour. They were, however, remarkably similar in terms of the estimated minimum power required to launch such an ejection event. We compare the timing of radio peaks with the location of the ejecta, imaged by contemporaneous VLBI experiments. We then discuss the mechanism behind the events, in terms of whether discrete ejections is the most likely description of the behaviour. One ejection, at least, appears to be travelling with apparent superluminal motion. The range of properties, however, suggests that mutiple mechanisms may be relevant and that at least some of the emission is coming from shocked interactions amongst the ejecta and between the ejecta and the interstellar medium. We also compare the radio flux density with the X-ray source during the hard state and conclude that XTE J1752-223 is a radio-weak/X-ray-bright outlier on the universal correlation for black hole transient sources.Comment: 14 pages; Accepted for publication in MNRA

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.Comment: Accepted for publication in A&