arXiv subjects

J. V. Hernandez Santisteban

Publications and source records attributed to J. V. Hernandez Santisteban.

12 recordsLinked to original sources

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.

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arXiv2021

On the multi-wavelength variability of Mrk 110: Two components acting at different timescales

We present the first intensive continuum reverberation mapping study of the high accretion rate Seyfert galaxy Mrk 110. The source was monitored almost daily for more than 200 days with the Swift X-ray and UV/optical telescopes, supported by ground-based observations from Las Cumbres Observatory, the Liverpool Telescope, and the Zowada Observatory, thus extending the wavelength coverage to 9100 Å. Mrk 110 was found to be significantly variable at all wavebands. Analysis of the intraband lags reveals two different behaviours, depending on the timescale. On timescales shorter than 10 days the lags, relative to the shortest UV waveband ($\sim1928$ Å), increase with increasing wavelength up to a maximum of $\sim2$days lag for the longest waveband ($\sim9100$ Å), consistent with the expectation from disc reverberation. On longer timescales, however, the g-band lags the Swift BAT hard X-rays by $\sim10$ days, with the z-band lagging the g-band by a similar amount, which cannot be explained in terms of simple reprocessing from the accretion disc. We interpret this result as an interplay between the emission from the accretion disc and diffuse continuum radiation from the broad line region.

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arXiv2021

UV and X-ray observations of the neutron star LMXB EXO 0748-676 in its quiescent state

The accretion behaviour in low-mass X-ray binaries (LMXBs) at low luminosities, especially at 3500 km/s), which could indicate that it results from an outflow such as a pulsar wind. By studying several epochs of X-ray and near-UV data obtained with XMM-Newton, we find no clear indication that the emission in the two wavebands is connected. Moreover, the luminosity ratio of Lx/Luv >100 is much higher than that observed from neutron star LMXBs that exhibit low-level accretion in quiescence. Taken together, this suggests that the UV and X-ray emission of EXO 0748-676 may have different origins, and that thermal emission from crust-cooling of the neutron star, rather than ongoing low-level accretion, may be dominating the observed quiescent X-ray flux evolution of this LMXB.

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arXiv2020

The Sloan Digital Sky Survey Reverberation Mapping Project: How Broad Emission Line Widths Change When Luminosity Changes

Quasar broad emission lines are largely powered by photoionization from the accretion continuum. Increased central luminosity will enhance line emissivity in more distant clouds, leading to increased average distance of the broad-line-emitting clouds and decreased averaged line width, known as the broad-line region (BLR) "breathing". However, different lines breathe differently, and some high-ionization lines, such as C IV, can even show "anti-breathing" where the line broadens when luminosity increases. Using multi-year photometric and spectroscopic monitoring data from the Sloan Digital Sky Survey Reverberation Mapping project, we quantify the breathing effect ($Δ$log W=$αΔ$log L) of broad H$α$, H$β$, Mg II, C IV, and C III] for statistical quasar samples over $z\approx 0.1-2.5$. We found that H$β$ displays the most consistent normal breathing expected from the virial relation ($α\sim-0.25$), Mg II and H$α$ on average show no breathing ($α\sim 0$), and C IV (and similarly C III] and Si IV mostly shows anti-breathing ($α>0$). The anti-breathing of C IV can be well understood by the presence of a non-varying core component in addition to a reverberating broad-base component, consistent with earlier findings. The deviation from canonical breathing introduces extra scatter (a luminosity-dependent bias) in single-epoch virial BH mass estimates due to intrinsic quasar variability, which underlies the long argued caveats of C IV single-epoch masses. Using the line dispersion instead of FWHM leads to less, albeit still substantial, deviations from canonical breathing in most cases. Our results strengthen the need for reverberation mapping to provide reliable quasar BH masses, and quantify the level of variability-induced bias in single-epoch BH masses based on various lines.

astro-ph.GA↗

arXiv2020

Bow-shocks, nova shells, disc winds and tilted discs: The Nova-Like V341 Ara Has It All

V341 Ara was recently recognised as one of the closest (d ~ 150 pc) and brightest (V~ 10) nova-like cataclysmic variables. This unique system is surrounded by a bright emission nebula, likely to be the remnant of a recent nova eruption. Embedded within this nebula is a prominent bow-shock, where the system's accretion disc wind runs into its own nova shell. In order to establish its fundamental properties, we present the first comprehensive multi-wavelength study of the system. Long-term photometry reveals quasi-periodic, super-orbital variations with a characteristic time-scale of 10-16 days and typical amplitude of ~1 mag. High-cadence photometry from TESS reveals for the first time both the orbital period and a "negative superhump" period. The latter is usually interpreted as the signature of a tilted accretion disc. We propose a recently developed disc instability model as a plausible explanation for the photometric behaviour. In our spectroscopic data, we clearly detect anti-phased absorption and emission line components. Their radial velocities suggest a high mass ratio, which in turn implies an unusually low white dwarf mass. We also constrain the wind mass-loss rate of the system from the spatially resolved [O iii] emission produced in the bow-shock; this can be used to test and calibrate accretion disc wind models. We suggest a possible association between V341 Ara and a "guest star" mentioned in Chinese historical records in AD1240. If this marks the date of the system's nova eruption, V341 Ara would be the oldest recovered nova of its class and an excellent laboratory for testing nova theory.

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arXiv2020

The Sloan Digital Sky Survey Reverberation Mapping Project: The M_BH-Host Relations at 0.2<z<0.6 from Reverberation Mapping and Hubble Space Telescope Imaging

We present the results of a pilot Hubble Space Telescope (HST) imaging study of the host galaxies of ten quasars from the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project. Probing more than an order of magnitude in BH and stellar masses, our sample is the first statistical sample to study the BH-host correlations beyond z>0.3 with reliable BH masses from reverberation mapping rather than from single-epoch spectroscopy. We perform image decomposition in two HST bands (UVIS-F606W and IR-F110W) to measure host colors and estimate stellar masses using empirical relations between broad-band colors and the mass-to-light ratio. The stellar masses of our targets are mostly dominated by a bulge component. The BH masses and stellar masses of our sample broadly follow the same correlations found for local RM AGN and quiescent bulge-dominant galaxies, with no strong evidence of evolution in the M_BH-M_*,bulge relation to z~0.6. We further compare the host light fraction from HST imaging decomposition to that estimated from spectral decomposition. We found a good correlation between the host fractions derived with both methods. However, the host fraction derived from spectral decomposition is systematically smaller than that from imaging decomposition by ~30%, indicating different systematics in both approaches. This study paves the way for upcoming more ambitious host galaxy studies of quasars with direct RM-based BH masses at high redshift.

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arXiv2019

Multiwavelength characterisation of the accreting millisecond X-ray pulsar and ultra-compact binary IGR J17062-6143

IGR J17062-6143 is an ultra-compact X-ray binary (UCXB) with an orbital period of 37.96 min. It harbours a millisecond X-ray pulsar that is spinning at 163 Hz and and has continuously been accreting from its companion star since 2006. Determining the composition of the accreted matter in UCXBs is of high interest for studies of binary evolution and thermonuclear burning on the surface of neutron stars. Here, we present a multi-wavelength study of IGR J17062-6143 aimed to determine the detailed properties of its accretion disc and companion star. The multi-epoch photometric UV to near-infrared spectral energy distribution (SED) is consistent with an accretion disc $F_ν\proptoν^{1/3}$. The SED modelling of the accretion disc allowed us to estimate an outer disc radius of $R_{out}=2.2^{+0.9}_{-0.4} \times 10^{10}$ cm and a mass-transfer rate of $\dot{m}=1.8^{+1.8}_{-0.5}\times10^{-10}$ M$_{\odot}$ yr$^{-1}$. Comparing this with the estimated mass-accretion rate inferred from its X-ray emission suggests that $\gtrsim$90% of the transferred mass is lost from the system. Moreover, our SED modelling shows that the thermal emission component seen in the X-ray spectrum is highly unlikely from the accretion disc and must therefore represent emission from the surface of the neutron star. Our low-resolution optical spectrum revealed a blue continuum and no emission lines, i.e. lacking H and He features. Based on the current data we cannot conclusively identify the nature of the companion star, but we make recommendations for future study that can distinguish between the different possible evolution histories of this X-ray binary. Finally, we demonstrate how multiwavelength observations can be effectively used to find more UCXBs among the LMXBs.

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arXiv2018

Quest for the Donor Star in the Magnetic Precataclysmic Variable V1082 Sgr

We obtained high-resolution spectra and multicolor photometry of V1082 Sgr to study the donor star in this 20.8 hr orbital period binary, which is assumed to be a detached system. We measured the rotational velocity (v sin i = 26.5+/-2.0 km/s), which, coupled with the constraints on the white dwarf mass from the X-ray spectroscopy, leads to the conclusion that the donor star barely fills 70% of its corresponding Roche lobe radius. It appears to be a slightly evolved K2-type star. This conclusion was further supported by a recently published distance to the binary system measured by the Gaia mission. At the same time, it becomes difficult to explain a very high (> 10e-9 Msun/yr) mass transfer and mass accretion rate in a detached binary via stellar wind and magnetic coupling.

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arXiv2018

Mid-UV Studies of the Transitional Millisecond Pulsars XSS J12270-4859 and PSR J1023+0038 During Their Radio Pulsar States

We report mid-UV (MUV) observations taken with HST/WFC3, Swift/UVOT and GALEX/NUV of the transitional millisecond pulsars XSS J12270-4859 and PSR J1023+0038 during their radio pulsar states. Both systems were detected in our images and showed MUV variability. At similar orbital phases, the MUV luminosities of both pulsars are comparable. This suggests that the emission processes involved in both objects are similar. We estimated limits on the mass ratio, companion's temperature, inclination, and distance to XSS J12270-4859 by using a Markov Chain Monte Carlo algorithm to fit published folded optical light curves. Using the resulting parameters, we modeled MUV light curves in our HST filters. The resulting models failed to fit our MUV observations. Fixing the mass ratio of XSS J12270-4859 to the value reported in other studies, we obtained a distance of ~3.2 kpc. This is larger than the one derived from radio measurements (~1.4 kpc). Assuming a uniform prior for the mass ratio, the distance is similar to that from radio measurements. However, it requires an undermassive companion (~0.01 M_sun). We conclude that a direct heating model alone can not fully explain the observations in optical and MUV. Therefore, an additional radiation source is needed. The source could be an intrabinary shock which contributes to the MUV flux and likely to the optical one as well. During the radio pulsar state, the MUV orbital variations of PSR J1023+0038 detected with GALEX, suggests the presence of an asymmetric intrabinary shock.

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arXiv2017

The very-faint X-ray binary IGR J17062-6143: a truncated disk, no pulsations and a possible outflow

We present a comprehensive X-ray study of the neutron star low-mass X-ray binary IGR J17062-6143, which has been accreting at low luminosities since its discovery in $2006$. Analysing NuSTAR, XMM-Newton and Swift observations, we investigate the very faint nature of this source through three approaches: modelling the relativistic reflection spectrum to constrain the accretion geometry, performing high-resolution X-ray spectroscopy to search for an outflow, and searching for the recently reported millisecond X-ray pulsations. We find a strongly truncated accretion disk at $77^{+22}_{-18}$ gravitational radii ($\sim 164$ km) assuming a high inclination, although a low inclination and a disk extending to the neutron star cannot be excluded. The high-resolution spectroscopy reveals evidence for oxygen-rich circumbinary material, possibly resulting from a blueshifted, collisionally-ionised outflow. Finally, we do not detect any pulsations. We discuss these results in the broader context of possible explanations for the persistent faint nature of weakly accreting neutron stars. The results are consistent with both an ultra-compact binary orbit and a magnetically truncated accretion flow, although both cannot be unambigiously inferred. We also discuss the nature of the donor star and conclude that it is likely a CO or O-Ne-Mg white dwarf, consistent with recent multi-wavelength modelling.

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arXiv2017

The Space Density of Post-Period Minimum Cataclysmic Variables

Binary evolution theory predicts that accreting white dwarfs with sub-stellar companions dominate the Galactic population of cataclysmic variables (CVs). In order to test these predictions, it is necessary to identify these systems, which may be difficult if the signatures of accretion become too weak to be detected. The only chance to identify such "dead" CVs is by exploiting their close binary nature. We have therefore searched the Sloan Digital Sky Survey (SDSS) Stripe 82 area for apparently isolated white dwarfs that undergo eclipses by a dark companion. We found no such eclipses in either the SDSS or Palomar Transient Factory data sets among our sample of 2264 photometrically selected white dwarf candidates within Stripe 82. This null result allows us to set a firm upper limit on the space density, $ρ_0$, of dead CVs. In order to determine this limit, we have used Monte-Carlo simulations to fold our selection criteria through a simple model of the Galactic CV distribution. Assuming a $T_{WD}=7,500$ K, the resulting 2$σ$ limit on the space density of dead CVs is $ρ_0 \lesssim 2 \times 10^{-5}$ pc$^{-3}$, where $T_{WD}$ is the typical effective temperature of the white dwarf in such systems.

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arXiv2011

Exploring the variable stars in the globular cluster NGC5024 (M53): New RR Lyrae and SX Phoenicis stars

We report CCD V and I time series photometry of the globular cluster NGC 5024 (M53). The technique of difference image analysis has been used which enables photometric precisions better than 10 mmag for stars brighter than V 18.5mag even in the crowded central regions of the cluster. The high photometric precision has resulted in the discovery of two new RR1 stars and thirteen SX Phe stars. A detailed identification chart is given for all the variable stars in the field of our images of the cluster. Periodicities were calculated for all RR Lyraes and SX Phe stars and a critical comparison is made with previous determinations. Out of four probable SX Phe variables reported by Dekany & Kovacs (2009), the SX Phe nature is confirmed only for V80, V81 is an unlikely case while V82 and V83 remain as dubious cases. Previous misidentifications of three variables are corrected. Astrometric positions with an uncertainty of ~ 0.3 arcsec are provided for all variables. The light curve Fourier decomposition of RR0 and RR1 is discussed, we find a mean metallicity of [Fe/H]=-1.92+-0.06 in the scale of Zinn & West from 19 RR0 stars. The true distance moduli 16.36+-0.05 and 16.28+-0.07 and the corresponding distances 18.7+-0.4 and 18.0+-0.5 kpc are found from the RR0 and RR1 stars respectively. These values are in agreement with the theoretical period-luminosity relations for RR Lyrae stars in the I band and with recent luminosity determinations for the RR Lyrae stars in the LMC. The age of 13.25+-0.50 Gyr (Dotter et al. 2010), for NGC 5024, E(B-V)=0.02 and the above physical parameters of the cluster, as indicated from the RR0 stars, produce a good isochrone fitting to the observed CMD. The PL relation for SX Phe stars in NGC 5024 in the V and I bands is discussed in the light of the 13 newly found SX Phe stars and their pulsation mode is identified in most cases.

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