arXiv subjects

M. Armas Padilla

Publications and source records attributed to M. Armas Padilla.

At least 55 records · Page 3Linked to original sources

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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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

On the ultra-compact nature of the neutron star system 1RXS J170854.4-321857: insights from X-ray spectroscopy

The relatively small family of ultra-compact X-ray binary systems is of great interest for many areas of astrophysics. We report on a detailed X-ray spectral study of the persistent neutron star low mass X-ray binary 1RXS J170854.4-321857. We analysed two XMM-Newton observations obtained in late 2004 and early 2005 when, in agreement with previous studies, the system displayed an X-ray luminosity (0.5-10 keV) of ~1 x 10^36 erg s-1. The spectrum can be described by a Comptonized emission component with Gamma~1.9 and a distribution of seed photons with a temperature of ~ 0.23 keV. A prominent residual feature is present at soft energies, which is reproduced by the absorption model if over-abundances of Ne and Fe are allowed. We discuss how similar observables, that might be attributed to the peculiar (non-solar) composition of the plasma donated by the companion star, are a common feature in confirmed and candidate ultra-compact systems. Although this interpretation is still under debate, we conclude that the detection of these features along with the persistent nature of the source at such low luminosity and the intermediate-long burst that it displayed in the past confirms 1RXS J170854.4-321857as a solid ultra-compact X-ray binary candidate.

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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

Multi-wavelength spectroscopy of the black hole candidate MAXI J1813-095 during its discovery outburst

MAXI J1813-095 is an X-ray transient discovered during an outburst in 2018. We report on X-ray and optical observations obtained during this event, which indicate that the source is a new low-mass X-ray binary. The outburst lasted ~70 d and peaked at Lx(0.5-10keV)~7.6 x 10^36 erg s-1, assuming a distance of 8 kpc. Swift/XRT follow-up covering the whole activity period shows that the X-ray emission was always dominated by a hard power-law component with a photon index in the range of 1.4-1.7. These values are consistent with MAXI J1813-095 being in the hard state, in agreement with the ~30 per cent fractional root-mean-square amplitude of the fast variability (0.1-50 Hz) inferred from the only XMM-Newton observation available. The X-ray spectra are well described by a Comptonization emission component plus a soft, thermal component (kT ~0.2 keV), which barely contributes to the total flux (<8 per cent). The Comptonization y-parameter (~1.5), together with the low temperature and small contribution of the soft component supports a black hole accretor. We also performed optical spectroscopy using the VLT and GTC telescopes during outburst and quiescence, respectively. In both cases the spectrum lack emission lines typical of X-ray binaries in outburst. Instead, we detect the Ca II triplet and H_alpha in absorption. The absence of velocity shifts between the two epochs, as well as the evolution of the H_alpha equivalent width, strongly suggest that the optical emission is dominated by an interloper, likely a G-K star. This favours a distance >3 kpc for the X-ray transient.

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arXiv2018

The complex evolution of the X-ray binary transient MAXI J1807+132 along the decay of its discovery outburst

MAXI J1807+132 is an X-ray transient discovered during the decay of an outburst in 2017. We present optical and X-ray monitoring of the source over more than 125 days, from outburst to quiescence. The outburst decay is characterized by the presence of several re-flares with a quasi-periodic recurrence time of $\sim 6.5$ days. We detect broad H and He emission lines during outburst, characteristic of transient low mass X-ray binaries. These emission lines show strong variability from epoch to epoch and, in particular, during the early stages are found embedded into deep and very broad absorption features. The quiescent spectrum shows H$α$ in emission and no obvious signatures of the donor star. XMM-Newton and Swift spectra can be fitted with standard X-ray models for accreting black-holes and neutron stars, although the obtained spectral parameters favour the latter scenario. Conversely, other observables such as the optical/X-ray flux ratio, the likely systemic velocity ($γ\sim -150$ km s$^{-1}$) and the re-flares recurrence time suggest a black hole nature. We discuss all the above possibilities with emphasis on the strong similarities of MAXI J1807+132 with short orbital period systems.

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arXiv2018

The 1989 and 2015 outbursts of V404 Cygni: a global study of wind-related optical features

The black hole transient V404 Cygni exhibited a bright outburst in June 2015 that was intensively followed over a wide range of wavelengths. Our team obtained high time resolution optical spectroscopy (~90 s), which included a detailed coverage of the most active phase of the event. We present a database consisting of 651 optical spectra obtained during this event, that we combine with 58 spectra gathered during the fainter December 2015 sequel outburst, as well as with 57 spectra from the 1989 event. We previously reported the discovery of wind-related features (P-Cygni and broad-wing line profiles) during both 2015 outbursts. Here, we build diagnostic diagrams that enable us to study the evolution of typical emission line parameters, such as line fluxes and equivalent widths, and develop a technique to systematically detect outflow signatures. We find that these are present throughout the outburst, even at very low optical fluxes, and that both types of outflow features are observed simultaneously in some spectra, confirming the idea of a common origin. We also show that the nebular phases depict loop patterns in many diagnostic diagrams, while P-Cygni profiles are highly variable on time-scales of minutes. The comparison between the three outbursts reveals that the spectra obtained during June and December 2015 share many similarities, while those from 1989 exhibit narrower emission lines and lower wind terminal velocities. The diagnostic diagrams presented in this work have been produced using standard measurement techniques and thus may be applied to other active low-mass X-ray binaries.

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arXiv2017

Bowen emission from Aquila X-1: evidence for multiple components and constraint on the accretion disc vertical structure

We present a detailed spectroscopic study of the optical counterpart of the neutron star X-ray transient Aquila X-1 during its 2011, 2013 and 2016 outbursts. We use 65 intermediate resolution GTC-10.4m spectra with the aim of detecting irradiation-induced Bowen blend emission from the donor star. While Gaussian fitting does not yield conclusive results, our full phase coverage allows us to exploit Doppler mapping techniques to independently constrain the donor star radial velocity. By using the component N III 4640.64/4641.84 A we measure Kem = 102 +-6 km/s. This highly significant detection ( >13 sigma) is fully compatible with the true companion star radial velocity obtained from near-infrared spectroscopy during quiescence. Combining these two velocities we determine, for the first time, the accretion disc opening angle and its associated error from direct spectroscopic measurements and detailed modelling, obtaining alpha = 15.5 +2.5 -5 deg. This value is consistent with theoretical work if significant X-ray irradiation is taken into account and is important in the light of recent observations of GX339-4, where discrepant results were obtained between the donor's intrinsic radial velocity and the Bowen-inferred value. We also discuss the limitations of the Bowen technique when complete phase coverage is not available.

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arXiv2017

The very faint hard state of the persistent neutron star X-ray binary SLX 1737-282 near the Galactic centre

We report on a detailed study of the spectral and temporal properties of the neutron star low mass X-ray binary SLX 1737-282, which is located only ~1degr away from Sgr A. The system is expected to have a short orbital period, even within the ultra-compact regime, given its persistent nature at low X-ray luminosities and the long duration thermonuclear burst that it has displayed. We have analysed a Suzaku (18 ks) observation and an XMM-Newton (39 ks) observation taken 7 years apart. We infer (0.5-10 keV) X-ray luminosities in the range 3-6 x10^35erg s-1, in agreement with previous findings. The spectra are well described by a relatively cool (kTbb = 0.5 keV) black body component plus a Comptonized emission component with Γ ~1.5-1.7. These values are consistent with the source being in a faint hard state, as confirmed by the ~ 20 per cent fractional root mean square amplitude of the fast variability (0.1 - 7 Hz) inferred from the XMM-Newton data. The electron temperature of the corona is >7 keV for the Suzaku observation, but it is measured to be as low as ~2 keV in the XMM-Newton data at higher flux. The latter is significantly lower than expected for systems in the hard state. We searched for X-ray pulsations and imposed an upper limit to their semi-amplitude of 2 per cent (0.001 - 7 Hz). Finally, we investigated the origin of the low frequency variability emission present in the XMM-Newton data and ruled out an absorption dip origin. This constraint the orbital inclination of the system to 65 degr unless the orbital period is longer than 11 hr (i.e. the length of the XMM-Newton observation).

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arXiv2017

Swift and SALT observations of the Multiple Outbursts of MAXI J1957+032

The new recurrent X-ray transient MAXI J1957+032 has had four X-ray outbursts within 16 months, all very briefly detected (lasting <5 days). During the most recent event (Sep/Oct 2016), we obtained with SALT the first optical spectrum of the transient counterpart, showing the classic blue continuum of an X-ray irradiated disc in an LMXB and no other features. At high Galactic latitude below the plane (-13 deg) reddening is low but there is no quiescent counterpart visible on any of the existing sky surveys, nor any other known X-ray source in the region. Swift monitoring of 3 of the 4 events is presented, showing rapidly fading X-ray outbursts together with significant UVOT detections in the UV (W1,M2,W2), U and B bands. The optical properties are most like those of the short-period LMXBs, which, combined with the softening witnessed during the decay to quiescence would place the system at d <13 kpc. The short duration and short recurrence time of the outbursts are reminiscent of the AMXPs, which exhibit peak luminosities of 1 % L_Edd. Assuming this peak luminosity would place MAXI J1957+032 at a distance of d= 5-6 kpc.

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arXiv2017

Suzaku spectroscopy of the neutron star transient 4U 1608-52 during its outburst decay

We test the proposed 3-component spectral model for neutron star low mass X-ray binaries using broad-band X-ray data. We have analysed 4 X-ray spectra (0.8-30 keV) obtained with Suzaku during the 2010 outburst of 4U 1608-52, which have allowed us to perform a comprehensive spectral study covering all the classical spectral states. We use a thermally Comptonized continuum component to account for the hard emission, as well as two thermal components to constrain the accretion disc and neutron star surface contributions. We find that the proposed combination of multicolor disc, single-temperature black body and Comptonization components successfully reproduces the data from soft to hard states. In the soft state, our study supports the neutron star surface (or boundary layer) as the dominant source for the Comptonization seed photons yielding the observed weak hard emission, while in the hard state both solutions, either the disc or the neutron star surface, are equally favoured. The obtained spectral parameters as well as the spectral/timing correlations are comparable to those observed in accreting black holes, which support the idea that black hole and neutron star low mass X-ray binaries undergo a similar state evolution during their accretion episodes.

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arXiv2016

Flares, wind and nebulae: the 2015 December mini-outburst of V404 Cygni

After more than 26 years in quiescence, the black hole transient V404 Cyg went into a luminous outburst in June 2015, and additional activity was detected in late December of the same year. Here, we present an optical spectroscopic follow-up of the December mini-outburst, together with X-ray, optical and radio monitoring that spanned more than a month. Strong flares with gradually increasing intensity are detected in the three spectral ranges during the ~10 days following the Swift trigger. Our optical spectra reveal the presence of a fast outflowing wind, as implied by the detection of a P-Cyg profile (He I - 5876 A) with a terminal velocity of ~2500 km/s. Nebular-like spectra -- with an H_alpha equivalent width of ~500 A -- are also observed. All these features are similar to those seen during the main June 2015 outburst. Thus, the fast optical wind simultaneous with the radio jet is most likely present in every V404 Cyg outburst. Finally, we report on the detection of a strong radio flare in late January 2016, when X-ray and optical monitoring had stopped due to Sun constraints.

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arXiv2016

Regulation of black-hole accretion by a disk wind during a violent outburst of V404 Cygni

Accretion of matter onto black holes is universally associated with strong radiative feedback and powerful outflows. In particular, black hole transients show outflows whose properties are strongly coupled to those of the accretion flow. This includes X-ray winds of ionized material, expelled from the accretion disc encircling the black hole, and collimated radio jets. Very recently, a distinct optical variability pattern has been reported in the transient black hole transient V404 Cyg, and interpreted as disrupted mass flow into the inner regions of its large accretion disc. Here, we report on the discovery of a sustained outer accretion disc wind in V404 Cyg, which is unlike any seen previously. We find that the outflowing wind is neutral, has a large covering factor, expands at 1% of the speed of light and triggers a nebular phase once accretion sharply drops and the ejecta become optically thin. The large expelled mass (> 10^-8 Msun) indicates that the outburst was prematurely ended when a sizeable fraction of the outer disc was depleted by the wind, detaching the inner regions from the rest of the disc. The luminous, but brief, accretion phases shown by transients with large accretion discs imply that this outflow is most likely a new fundamental ingredient regulating mass accretion onto black holes.

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arXiv2016

A "high-hard" outburst of the black hole X-ray binary GS 1354-64

We study in detail the evolution of the 2015 outburst of GS 1354-64 (BW Cir) at optical, UV and X-ray wavelengths using Faulkes Telescope South/LCOGT, SMARTS and Swift. The outburst was found to stay in the hard X-ray state, albeit being anomalously luminous with a peak luminosity of L$_{X} >$ 0.15 L$_{Edd}$, which could be the most luminous hard state observed in a black hole X-ray binary. We found that the optical/UV emission is tightly correlated with the X-ray emission, consistent with accretion disc irradiation and/or a jet producing the optical emission. The X-ray spectra can be fitted well with a Comptonisation model, and show softening towards the end of the outburst. In addition, we detect a QPO in the X-ray lightcurves with increasing centroid frequency during the peak and decay periods of the outburst. The long-term optical lightcurves during quiescence show a statistically significant, slow rise of the source brightness over the 7 years prior to the 2015 outburst. This behaviour as well as the outburst evolution at all wavelengths studied can be explained by the disc instability model with irradiation and disc evaporation/condensation.

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arXiv2015

Low-level accretion in neutron-star X-ray binaries

We search the literature for reports on the spectral properties of neutron-star low-mass X-ray binaries when they have accretion luminosities between 1E34 and 1E36 ergs/s. We found that in this luminosity range the photon index (obtained from fitting a simple absorbed power-law in the 0.5-10 keV range) increases with decreasing 0.5-10 keV X-ray luminosity (i.e., the spectrum softens). Such behaviour has been reported before for individual sources, but here we demonstrate that very likely most (if not all) neutron-star systems behave in a similar manner and possibly even follow a universal relation. When comparing the neutron-star systems with black-hole systems, it is clear that most black-hole binaries have significantly harder spectra at luminosities of 1E34 - 1E35 erg/s. Despite a limited number of data points, there are indications that these spectral differences also extend to the 1E35 - 1E36 erg/s range. This observed difference between the neutron-star binaries and black-hole ones suggests that the spectral properties (between 0.5-10 keV) at 1E34 - 1E35 erg/s can be used to tentatively determine the nature of the accretor in unclassified X-ray binaries. We discuss our results in the context of properties of the accretion flow at low luminosities and we suggest that the observed spectral differences likely arise from the neutron-star surface becoming dominantly visible in the X-ray spectra. We also suggest that both the thermal component and the non-thermal component might be caused by low-level accretion onto the neutron-star surface for luminosities below a few times 1E34 erg/s.

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arXiv2015

Dynamical masses of a nova-like variable on the edge of the period gap

We present the first dynamical determination of the binary parameters of an eclipsing SW Sextantis star in the 3-4 hour orbital period range during a low state. We obtained time-resolved optical spectroscopy and photometry of HS 0220+0603 during its 2004-2005 low brightness state, as revealed in the combined SMARTS, IAC80 and M1 Group long-term optical light curve. The optical spectra taken during primary eclipse reveal a secondary star spectral type of M5.5 $\pm$ 0.5 as derived from molecular band-head indices. The spectra also provide the first detection of a DAB white dwarf in a cataclysmic variable. By modelling its optical spectrum we estimate a white dwarf temperature of 30000 $\pm$ 5000 K. By combining the results of modelling the white dwarf eclipse from ULTRACAM light curves with those obtained by simultaneously fitting the emission- and absorption-line radial velocity curves and I-band ellipsoidal light curves, we measure the stellar masses to be M$_1 = 0.87 \pm 0.09$ M$_\odot$ and M$_2 = 0.47 \pm 0.05$ M$_\odot$ for the white dwarf and the M dwarf, respectively, and an inclination of the orbital plane of $i \approx 79^\mathrm{o}$. A radius of $0.0103 \pm 0.0007$ R$_\odot$ is obtained for the white dwarf. The secondary star in HS 0220+0603 is likely too cool and undersized for its mass.

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arXiv2014

Swift J1357.2-0933: the faintest black hole?

Swift J1357.2-0933 is the first confirmed very faint black hole X-ray transient and has a short estimated orbital period of 2.8 hr. We observed Swift J1357.2-0933 for ~50 ks with XMM-Newton in 2013 July during its quiescent state. The source is clearly detected at a 0.5-10 keV unabsorbed flux of ~3x10^-15 erg cm-2 s-1. If the source is located at a distance of 1.5 kpc (as suggested in the literature), this would imply a luminosity of ~8x10^29 erg s-1, making it the faintest detected quiescent black hole LMXB. This would also imply that there is no indication of a reversal in the quiescence X-ray luminosity versus orbital period diagram down to 2.8 hr, as has been predicted theoretically and recently supported by the detection of the 2.4 hr orbital period black hole MAXI J1659-152 at a 0.5-10 keV X-ray luminosity of ~ 1.2 x 10^31 erg s-1. However, there is considerable uncertainty in the distance of Swift J1357.2-0933 and it may be as distant as 6 kpc. In this case, its quiescent luminosity would be Lx ~ 1.3 x 10^31 erg s-1, i.e., similar to MAXI J1659-152 and hence it would support the existence of such a bifurcation period. We also detected the source in optical at r' ~22.3 mag with the Liverpool telescope, simultaneously to our X-ray observation. The X-ray/optical luminosity ratio of Swift J1357.2-0933 agrees with the expected value for a black hole at this range of quiescent X-ray luminosities.

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