arXiv subjects

J. Casares

Publications and source records attributed to J. Casares.

At least 55 records · Page 3Linked to original sources

arXiv2022

Hard-state optical wind during the discovery outburst of the black-hole X-ray dipper MAXI J1803-298

We present twelve epochs of optical spectroscopy taken across the discovery outburst of the black hole candidate MAXI J1803-298 with the GTC and VLT telescopes. 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 black hole transients and the previously reported detection of X-ray dips. The two epochs observed during the initial hard state exhibited asymmetric emission line profiles, including a P-Cygni profile simultaneously detected in H-alpha 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 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 near-infrared outflow signatures indicates that these are common features. Furthermore, the lowest luminosity spectrum exhibits an H-alpha full-width-at-half-maximum of 1570 +- 100 km/s. This, together with previous constraints on the binary parameters, allows us to favor a compact object mass of ~ 3-10 Msun, further supporting its black hole nature.

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arXiv2021

HiPERCAM: a quintuple-beam, high-speed optical imager on the 10.4-m Gran Telescopio Canarias

HiPERCAM is a portable, quintuple-beam optical imager that saw first light on the 10.4-m Gran Telescopio Canarias (GTC) in 2018. The instrument uses re-imaging optics and 4 dichroic beamsplitters to record $u_s g_s r_s i_s z_s$ ($320-1060$ nm) images simultaneously on its five CCD cameras, each of 3.1 arcmin (diagonal) field of view. The detectors in HiPERCAM are frame-transfer devices cooled thermo-electrically to 183 K, thereby allowing both long-exposure, deep imaging of faint targets, as well as high-speed (over 1000 windowed frames per second) imaging of rapidly varying targets. A comparison-star pick-off system in the telescope focal plane increases the effective field of view to 6.7 arcmin for differential photometry. Combining HiPERCAM with the world's largest optical telescope enables the detection of astronomical sources to $g_s \sim 23$ in 1 s and $g_s \sim 28$ in 1 h. In this paper we describe the scientific motivation behind HiPERCAM, present its design, report on its measured performance, and outline some planned enhancements.

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arXiv2021

Optical nebular emission following the most luminous outburst of Aquila X-1

Aquila X-1 is a prototypical neutron star low mass X-ray binary and one of the most studied X-ray transients. We present optical spectroscopy obtained with the Gran Telescopio Canarias (10.4 m) during the 2016 outburst, the brightest in recent times, which showed a standard evolution with hard and soft accretion states. Our data set includes a dense coverage of the brightest phases of the event, as well as the decay towards quiescence. We searched for optical winds by studying the profiles and evolution of the main emission lines and found no indisputable wind signatures, such as P-Cyg profiles. Nonetheless, our detailed analysis of the particularly strong and broad Halpha emission line, detected at the end of the outburst, is consistent with the presence of a nebular phase produced by optically thin ejecta at ~800 km/s or, alternatively, an extended disc atmosphere. We discuss these possibilities as well as the similarities with the phenomenology observed in other black hole and neutron star systems. Our study suggests that optical nebular phases might be a relatively common observational feature during the late stages of low mass X-ray binaries outbursts, enabling to probe the presence of outflows at low-to-intermediate orbital inclinations.

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arXiv2021

Hubble spectroscopy of LB-1: comparison with B+black-hole and Be+stripped-star models

LB-1 has variously been proposed as either an X-ray dim B-type star plus black hole (B+BH) binary, or a Be star plus an inflated stripped star (Be+Bstr) binary. The Space Telescope Imaging Spectrograph (STIS) on board HST was used to obtain a flux-calibrated spectrum that is compared with non-LTE spectral energy distributions (SED) and line profiles for the proposed models. The Hubble data, together with the Gaia EDR3 parallax, provide tight constraints on the properties and stellar luminosities of the system. In the case of the Be+Bstr model we adopt the published flux ratio for the Be and Bstr stars, re-determine the T$_{eff}$ of the Bstr using the silicon ionization balance, and infer Teff for the Be star from the fit to the SED. We derive stellar parameters consistent with previous results, but with greater precision enabled by the Hubble SED. While the Be+Bstr model is a better fit to the HeI lines and cores of the Balmer lines in the optical, the B+BH model provides a better fit to the Si iv resonance lines in the UV. The analysis also implies that the Bstr star has roughly twice solar silicon abundance, difficult to reconcile with a stripped star origin. The Be star on the other hand has a rather low luminosity, and a spectroscopic mass inconsistent with its possible dynamical mass. The fit to the UV can be significantly improved by reducing the T$_{eff}$ and radius of the Be star, though at the expense of leading to a different mass ratio. In the B+BH model, the single B-type spectrum is a good match to the UV spectrum. Adopting a mass ratio of 5.1$\pm$0.1 (Liu et al. 2020) implies a BH mass of $\sim$21$^{+9}_{-8}M_{\odot}$.

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arXiv2020

Delimiting the black hole mass in the X-ray transient MAXI J1659-152 with H$α$ spectroscopy

MAXI J1659-152 is a 2.4 h orbital period X-ray dipping transient black hole candidate. We present spectroscopy of its $I\approx23$ quiescent counterpart where we detect H$α$ emission with full-width at half-maximum (FWHM) of $3200 \pm 300$ km s$^{-1}$. Applying the correlation between the H$α$ FWHM and radial velocity semi-amplitude of the donor star for quiescent X-ray transients, we derive $K_2 = 750 \pm 80$ km s$^{-1}$. The orbital period and $K_2$ lead to a mass function of $4.4\pm1.4~M{_\odot}$ (1$σ$). The donor to compact object mass ratio and binary inclination are likely in the range $q={M_2}/{M_1} = 0.02-0.07$ and $i=70{^\circ}-80{^\circ}$. These constraints imply a 68% confidence level interval for the compact object mass of $3.3 \lesssim M_1(M_\odot) \lesssim 7.5$, confirming its black hole nature. These quasi-dynamical limits are compared to mass estimates from modelling of X-ray data and any discrepancies are discussed. We review the properties of optical spectroscopy and time-series photometry collected during the 2010-2011 outburst. We interpret the apparent modulations found soon after the onset of high-accretion activity and during the 2011 rebrightening event as originating in the accretion disc. These have signatures consistent with superhumps, with the 2011 modulation having a fractional period excess $< 0.6\%$ (3$σ$). We propose that direct irradiation of the donor by the central X-ray source was not possible due to its occultation by the disc outer regions. We argue that disc shielding significantly weakens the donor star contribution to the optical variability in systems with $q\lesssim0.07$, including neutron star ultra-compact X-ray binaries.

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arXiv2020

Using radial velocities to reveal black holes in binaries: a test case

Aims. Large radial velocity variations in the LAMOST spectra of giant stars have been used to infer the presence of unseen companions. Some of them have been proposed as possible black hole candidates. We test this selection by investigating the classification of the one candidate having a known X-ray counterpart (UCAC4 721-037069). Methods. We obtained time-resolved spectra from the Liverpool Telescope and a 5ks observation from the Chandra observatory to fully constrain the orbital parameters and the X-ray emission of this system. Results. We find the source to be an eclipsing stellar binary that can be classified as a RS CVn. The giant star fills its Roche Lobe and the binary mass ratio is greater than one. The system may be an example of stable mass transfer from an intermediate-mass star with a convective envelope. Conclusions. Using only radial velocity to identify black hole candidates can lead to many false positives. The presence of an optical orbital modulation, such as observed for all LAMOST candidates, will in most cases indicate that this is a stellar binary.

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arXiv2020

Discovery of optical outflows and inflows in the black hole candidate GRS 1716-249

We present optical spectroscopy obtained with the GTC, VLT and SALT telescopes during the decline of the 2016-2017 outburst of the black hole candidate GRS 1716-249 (Nova Oph 1993). Our 18-epoch data set spans 6 months and reveals that the observational properties of the main emission lines are very variable, even on time scales of a few hours. Several epochs are characterised by P-Cyg (as well as flat-top and asymmetric) profiles in the H$α$, H$β$ and He II ($λ$4686) emission lines, implying the presence of an accretion disc wind, which is likely hot and dense. The wind's terminal velocity ($\sim$2000 km s$^{-1}$) is similar to that observed in other black hole X-ray transients. These lines also show transient and sharp red-shifted absorptions, taking the form of inverted P-Cyg profiles. We argue that these profiles can be explained by the presence of infalling material at $\sim$1300 km s$^{-1}$. We propose a failed wind scenario to explain this inflow and discuss other alternatives, such as obscuration produced by an accretion-related structure (e.g. the gas stream) in a high inclination system.

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arXiv2020

Optical spectroscopy of 4U 1812-12: an ultra-compact X-ray binary seen through an H II region

The persistent, low-luminosity neutron star X-ray binary 4U 1812-12 is a potential member of the scarce family of ultra-compact systems. We performed deep photometric and spectroscopic optical observations with the 10.4 m Gran Telescopio Canarias in order to investigate the chemical composition of the accreted plasma, which is a proxy for the donor star class. We detect a faint optical counterpart (g~25, r~23) that is located in the background of the outskirts of the Sharpless 54 H II region, whose characteristic nebular lines superimpose on the X-ray binary spectrum. Once this is corrected for, the actual source spectrum lacks hydrogen spectral features. In particular, the Halpha emission line is not detected, with an upper limit (3 sigma) on the equivalent width of <1.3 A. Helium (He I) lines are neither observed, albeit our constraints are not restrictive enough to properly test the presence of this element. We also provide stringent upper limits on the presence of emission lines from other elements, such as C and O, which are typically found in ultra-compact systems with C-O white dwarfs donors. The absence of hydrogen features, the persistent nature of the source at low luminosity, as well as the low optical to X-ray flux ratio confirm 4U 1812-12 as a compelling ultra-compact X-ray binary candidate, for which we tentatively propose a He-rich donor based on the optical spectrum and the detection of short thermonuclear X-ray bursts. In this framework, we discuss the possible orbital period of the system according to disc instability and evolutionary models.

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arXiv2020

A DESGW Search for the Electromagnetic Counterpart to the LIGO/Virgo Gravitational Wave Binary Neutron Star Merger Candidate S190510g

We present the results from a search for the electromagnetic counterpart of the LIGO/Virgo event S190510g using the Dark Energy Camera (DECam). S190510g is a binary neutron star (BNS) merger candidate of moderate significance detected at a distance of 227$\pm$92 Mpc and localized within an area of 31 (1166) square degrees at 50\% (90\%) confidence. While this event was later classified as likely non-astrophysical in nature within 30 hours of the event, our short latency search and discovery pipeline identified 11 counterpart candidates, all of which appear consistent with supernovae following offline analysis and spectroscopy by other instruments. Later reprocessing of the images enabled the recovery of 6 more candidates. Additionally, we implement our candidate selection procedure on simulated kilonovae and supernovae under DECam observing conditions (e.g., seeing, exposure time) with the intent of quantifying our search efficiency and making informed decisions on observing strategy for future similar events. This is the first BNS counterpart search to employ a comprehensive simulation-based efficiency study. We find that using the current follow-up strategy, there would need to be 19 events similar to S190510g for us to have a 99\% chance of detecting an optical counterpart, assuming a GW170817-like kilonova. We further conclude that optimization of observing plans, which should include preference for deeper images over multiple color information, could result in up to a factor of 1.5 reduction in the total number of followups needed for discovery.

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arXiv2020

The binary mass ratio in the black hole transient MAXI J1820+070

We present intermediate resolution spectroscopy of the optical counterpart to the black hole X-ray transient MAXI J1820+070 (=ASASSN-18ey) obtained with the OSIRIS spectrograph on the 10.4-m Gran Telescopio Canarias. The observations were performed with the source close to the quiescent state and before the onset of renewed activity in August 2019. We make use of these data and K-type dwarf templates taken with the same instrumental configuration to measure the projected rotational velocity of the donor star. We find $v_{rot} \sin i = 84 \pm 5$ km s$^{-1}$ ($1\!-\!σ$), which implies a donor to black-hole mass ratio $q = {M_2}/{M_1} = 0.072 \pm 0.012$ for the case of a tidally locked and Roche-lobe filling donor star. The derived dynamical masses for the stellar components are $M_1 = (5.95 \pm 0.22)\sin ^{-3}i$ $M_\odot$ and $M_2 = (0.43 \pm 0.08) \sin^{-3}i$ $M_\odot$. The use of $q$, combined with estimates of the accretion disk size at the time of the optical spectroscopy, allows us to revise our previous orbital inclination constraints to $66^{\circ} < i < 81^{\circ}$. These values lead to 95% confidence level limits on the masses of $5.73 <M_1(M_\odot) < 8.34$ and $0.28 < M_2(M_\odot) < 0.77$. Adopting instead the $63 \pm 3^{\circ}$ orientation angle of the radio jet as the binary inclination leads to $M_1 = 8.48^{+0.79}_{-0.72} M_\odot$ and $M_2 = 0.61^{+0.13}_{-0.12} M_\odot$ ($1\!-\!σ$).

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arXiv2020

The Changing-look Optical Wind of the Flaring X-ray Transient Swift J1858.6-0814

We present the discovery of an optical accretion disk wind in the X-ray transient Swift J1858.6-0814. Our 90-spectrum data set, taken with the 10.4m GTC telescope over 8 different epochs and across five months, reveals the presence of conspicuous P-Cyg profiles in He I at 5876 Angs and Halpha. These features are detected throughout the entire campaign, albeit their intensity and main observational properties are observed to vary on time-scales as short as five minutes. In particular, we observe significant variations in the wind velocity, between a few hundreds and ~ 2400 km/s. In agreement with previous reports, our observations are characterised by the presence of frequent flares, although the relation between the continuum flux variability and the presence/absence of wind features is not evident. The reported high activity of the system at radio waves indicates that the optical wind of Swift J1858.6-0814 is contemporaneous with the radio-jet, as is the case for the handful of X-ray binary transients that have shown so far optical P-Cyg profiles. Finally, we compare our results with those of other sources showing optical accretion disk winds, with emphasis on V404 Cyg and V4641 Sgr, since they also display strong and variable optical wind features as well as similar flaring behaviour.

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arXiv2020

IGAPS: the merged IPHAS and UVEX optical surveys of theNorthern Galactic Plane

The INT Galactic Plane Survey (IGAPS) is the merger of the optical photometric surveys, IPHAS and UVEX, based on data from the Isaac Newton Telescope (INT) obtained between 2003 and 2018. Here, we present the IGAPS point source catalogue. It contains 295.4 million rows providing photometry in the filters, i, r, narrow-band Halpha, g and U_RGO. The IGAPS footprint fills the Galactic coordinate range, |b| < 5deg and 30deg < l < 215deg. A uniform calibration, referred to the Pan-STARRS system, is applied to g, r and i, while the Halpha calibration is linked to r and then is reconciled via field overlaps. The astrometry in all 5 bands has been recalculated on the Gaia DR2 frame. Down to i ~ 20 mag (Vega system), most stars are also detected in g, r and Halpha. As exposures in the r band were obtained within the IPHAS and UVEX surveys a few years apart, typically, the catalogue includes two distinct r measures, r_I and r_U. The r 10sigma limiting magnitude is ~21, with median seeing 1.1 arcsec. Between ~13th and ~19th magnitudes in all bands, the photometry is internally reproducible to within 0.02 magnitudes. Stars brighter than r=19.5 have been tested for narrow-band Halpha excess signalling line emission, and for variation exceeding |r_I-r_U| = 0.2 mag. We find and flag 8292 candidate emission line stars and over 53000 variables (both at >5sigma confidence). The 174-column catalogue will be available via CDS Strasbourg.

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arXiv2020

The Formation of a 70 Msun Black Hole at High Metallicity

A 70Msun BH was discovered in Milky Way disk in a long period and almost circular detached binary system (LB-1) with a high metallicity 8Msun B star companion. Current consensus on the formation of BHs from high metallicity stars limits the black hole mass to be below 20Msun. Using simple evolutionary model, we show that the formation of a 70Msun BH in high metallicity environment is possible if stellar wind mass loss rates are reduced by factor of 5. As observations indicate, a fraction of massive stars have surface magnetic fields which may quench the wind mass-loss, independently of stellar mass and metallicity. We also computed detailed stellar evolution models and we confirm such a scenario. A non-rotating 85Msun model at Z=0.014 with decreased winds ends up as a 71Msun star prior core-collapse with a 32Msun helium core and a 28Msun CO core. Such star avoids pair-instability pulsation supernova mass loss and may form a 70Msun BH in the direct collapse. Stars that can form such BHs expand to significant size with radius of R>600Rsun, exceeding the size of LB-1 orbit. Therefore, we can explain the formation of BHs upto 70Msun at high metallicity and this result is independent from LB-1. However, if LB-1 hosts a massive BH we are unable to explain how such a binary star system could have formed without invoking some exotic scenarios.

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arXiv2019

Dynamical confirmation of a black hole in MAXI J1820+070

We present time-resolved 10.4-m GTC and 4.2-m WHT intermediate resolution spectroscopy of the X-ray transient MAXI J1820+070 (=ASASSN-18ey) obtained during its decline to the quiescent state. Cross-correlation of the 21 individual spectra against late-type templates reveals a sinusoidal velocity modulation with a period of 0.68549 +/- 0.00001 d and semi-amplitude of 417.7 +/- 3.9 km/s. We derive a mass function f(M) = 5.18 +/- 0.15 Msun, dynamically confirming the black hole nature of the compact object. Our analysis of the stellar absorption features supports a K3-5 spectral classification for the donor star, which contributes 20% of the total flux at 5200-6800 Angs. The photometric 0.703 +/- 0.003 d periodicity observed during outburst is 2.6% longer than the orbital period supporting the presence of a superhump modulation in the outburst lightcurves. In line with this interpretation, we constrain the binary mass ratio to be q=0.12. In addition, we observe a sharp increase in the Halpha emission line equivalent width during inferior conjunction of the donor star that we interpret as a grazing eclipse of the accretion disc and allows us to constrain the binary inclination to > 69 deg. On the other hand, the absence of X-ray eclipses during outburst imply i < 77 deg. These inclination limits, together with our dynamical solution, lead to a black hole mass in the range 7-8 Msun. We also measure a systemic velocity = -21.6 +/- 2.3 km/s which, combined with the Gaia DR2 proper motion and parallax, implies a large peculiar velocity of 100 km/s.

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arXiv2019

An equatorial outflow in the black hole optical dipper Swift J1357.2-0933

We present high-time resolution optical spectroscopy and imaging of the black hole transient Swift J1357.2-0933 during its 2017 outburst. The light curves show recurrent dips resembling those discovered during the 2011 outburst. The dip properties (e.g duration and depth) as well as the evolution of their recurrence time are similar to those seen in 2011. Spectra obtained during the dips are characterised by broad and blue-shifted absorptions in Balmer and He ii. The absorptions show core velocities of $\sim$ -800 km/s and terminal velocities approaching $\sim$ 3000 km/s i.e. in the upper-end of wind velocities measured in other black hole transients (both at optical and X-ray wavelengths). Our observations suggest that the dips are formed in a dense and clumpy outflow, produced near the disc equatorial plane and seen at high inclination. We also study the colour evolution and observe that, as it has been previously reported, the source turns bluer during dips. We show that this is due to a gradual change in the slope of the optical continuum and discuss possible implications of this behaviour.

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arXiv2019

Accretion and Outflow in V404 Cyg

We study the optical evolution of the 2015 outburst in V404 Cyg, with emphasis on the peculiar nebular phase and subsequent decay to quiescence. From the decay timescale of the Balmer emission associated with the nebula we measure an outflow mass M_wind~4x10^{-6} Msun. Remarkably, this is ~100 times larger than the accreted mass and ~10% of the total mass stored in the disc. The wind efficiency must therefore be significantly larger than previous estimates for black hole transients, suggesting that radiation pressure (in addition to other mechanisms such as Compton-heating) plays a key role in V404 Cyg. In addition, we compare the evolution of the 2015 and 1989 outbursts and find clear similarities (namely a large luminosity drop ~10 d after the X-ray trigger, followed by a brief nebular phase) but also remarkable differences in decay timescales and long-term evolution of the Halpha profile. In particular, we see evidence for a rapid disc contraction in 2015, consistent with a burst of mass transfer. This could be driven by the response of the companion to hard X-ray illumination, most notably during the last gigantic (super-Eddington) flare on 25 June 2015. We argue that irradiation and consequential disc wind are key factors to understand the different outburst histories in 1989 and 2015. In the latter case, radiation pressure may be responsible for the abrupt end of the outburst through depleting inner parts of the disc, thus quenching accretion and X-ray irradiation. We also present a refined orbital period and updated ephemeris.

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arXiv2019

Hard-state accretion disk winds from black holes: the revealing case of MAXI J1820+070

We report on a detailed optical spectroscopic follow-up of the black hole transient MAXI J1820+070 (ASASSN-18ey). The observations cover the main part of the X-ray binary outburst, when the source alternated between hard and soft states following the classical pattern widely seen in other systems. We focus the analysis on the He I emission lines at 5876 and 6678 Angs, as well as on Halpha. We detect clear accretion disk wind features (P-Cyg profiles and broad emission line wings) in the hard state, both during outburst rise and decay. These are not witnessed during the several months long soft state. However, our data suggest that the visibility of the outflow might be significantly affected by the ionisation state of the accretion disk. The terminal velocity of the wind is above ~ 1200 km/s, which is similar to outflow velocities derived from (hard-state) optical winds and (soft-state) X-ray winds in other systems. The wind signatures, in particular the P-Cyg profiles, are very shallow, and their detection has only been possible thanks to a combination of source brightness and intense monitoring at very high signal-to-noise. This study indicates that cold, optical winds are most likely a common feature of black hole accretion, and therefore, that wind-like outflows are a general mechanism of mass and angular momentum removal operating throughout the entire X-ray binary outburst.

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arXiv2019

The binary millisecond pulsar PSR J1023+0038 -- II. Optical spectroscopy

We present time-resolved optical spectroscopy of the `redback' binary millisecond pulsar system PSR J1023+0038 during both its radio pulsar (2009) and accretion disc states (2014 and 2016). We provide observational evidence for the companion star being heated during the disc-state. We observe a spectral type change along the orbit, from G5 to F6 at the secondary star's superior and inferior conjunction, respectively, and find that the corresponding irradiating luminosity can be powered by the high energy accretion luminosity or the spin-down luminosity of the neutron star. We determine the secondary star's radial velocity semi-amplitude from the metallic (primarily Fe and Ca) and Halpha absorption lines during these different states. The metallic and Halpha radial velocity semi-amplitude determined from the 2009 pulsar-state observations allows us to constrain the secondary star's true radial velocity K_2=276.3+/-5.6 km/s and the binary mass ratio q=0.137+/-0.003. By comparing the observed metallic and Halpha absorption-line radial velocity semi-amplitudes with model predictions, we can explain the observed semi-amplitude changes during the pulsar-state and during the pulsar/disc-state transition as being due to different amounts of heating and the presence of an accretion disc, respectively.

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