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  • 1
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    A clean sightline to quiescence: multiwavelength observations of the high Galactic latitude black hole X-ray binary Swift J1357.2-0933 (2016)
    Oxford University Press
    Details
    Publication Date: 2016-01-02
    Description: We present coordinated multiwavelength observations of the high Galactic latitude ( b  = +50°) black hole X-ray binary (BHXB) Swift J1357.2–0933 in quiescence. Our broad-band spectrum includes strictly simultaneous radio and X-ray observations, and near-infrared, optical, and ultraviolet data taken 1–2 d later. We detect Swift J1357.2–0933 at all wavebands except for the radio ( f 5 GHz 〈 3.9 μJy beam –1 ; 3 rms ). Given current constraints on the distance (2.3–6.3 kpc), its 0.5–10 keV X-ray flux corresponds to an Eddington ratio L X / L Edd  = 4  x  10 –9 –3  x  10 –8 (assuming a black hole mass of 10 M ). The broad-band spectrum is dominated by synchrotron radiation from a relativistic population of outflowing thermal electrons, which we argue to be a common signature of short-period quiescent BHXBs. Furthermore, we identify the frequency where the synchrotron radiation transitions from optically thick-to-thin ( b 2–5  x  10 14  Hz), which is the most robust determination of a ‘jet break’ for a quiescent BHXB to date. Our interpretation relies on the presence of steep curvature in the ultraviolet spectrum, a frequency window made observable by the low amount of interstellar absorption along the line of sight. High Galactic latitude systems like Swift J1357.2–0933 with clean ultraviolet sightlines are crucial for understanding black hole accretion at low luminosities.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 2
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    A 'high-hard outburst of the black hole X-ray binary GS 1354-64 (2016)
    Oxford University Press
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    Publication Date: 2016-06-01
    Description: 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/Las Cumbres Observatory Global Telescope Network, Small & Moderate Aperture Research Telescope System 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 Comptonization model, and show softening towards the end of the outburst. In addition, we detect a QPO in the X-ray light curves with increasing centroid frequency during the peak and decay periods of the outburst. The long-term optical light curves 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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    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 3
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    Regulation of black-hole accretion by a disk wind during a violent outburst of V404 Cygni (2016)
    Springer Nature
    In: Nature
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    Publication Date: 2016-06-02
    Description: Regulation of black-hole accretion by a disk wind during a violent outburst of V404 Cygni Nature 534, 7605 (2016). doi:10.1038/nature17446 Authors: T. Muñoz-Darias, J. Casares, D. Mata Sánchez, R. P. Fender, M. Armas Padilla, M. Linares, G. Ponti, P. A. Charles, K. P. Mooley & J. Rodriguez Accretion of matter onto black holes is universally associated with strong radiative feedback and powerful outflows. In particular, black-hole transients have outflows whose properties are strongly coupled to those of the accretion flow. This includes X-ray winds of ionized material, expelled from the accretion disk encircling the black hole, and collimated radio jets. Very recently, a distinct optical variability pattern has been reported in the transient stellar-mass black hole V404 Cygni, and interpreted as disrupted mass flow into the inner regions of its large accretion disk. Here we report observations of a sustained outer accretion disk wind in V404 Cyg, which is unlike any seen hitherto. We find that the outflowing wind is neutral, has a large covering factor, expands at one per cent of the speed of light and triggers a nebular phase once accretion drops sharply and the ejecta become optically thin. The large expelled mass (〉10−8 solar masses) indicates that the outburst was prematurely ended when a sizeable fraction of the outer disk was depleted by the wind, detaching the inner regions from the rest of the disk. The luminous, but brief, accretion phases shown by transients with large accretion disks imply that this outflow is probably a fundamental ingredient in regulating mass accretion onto black holes.
    Print ISSN: 0028-0836
    Electronic ISSN: 1476-4687
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Published by Springer Nature
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  • 4
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    Dynamical masses of a nova-like variable on the edge of the period gap (2015)
    Oxford University Press
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    Publication Date: 2015-07-03
    Description: We present the first dynamical determination of the binary parameters of an eclipsing SW Sextantis star in the 3–4 h 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 Small & Moderate Aperture Research Telescope System, 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 ± 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 30 000 ± 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 ± 0.09 M and M 2  = 0.47 ± 0.05 M for the white dwarf and the M dwarf, respectively, and an inclination of the orbital plane of i   79°. A radius of 0.0103 ± 0.0007 R is obtained for the white dwarf. The secondary star in HS 0220+0603 is likely too cool and undersized for its mass.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    Low-level accretion in neutron star X-ray binaries (2015)
    Oxford University Press
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    Publication Date: 2015-10-04
    Description: We search the literature for reports on the spectral properties of neutron star low-mass X-ray binaries when they have accretion luminosities between 10 34 and 10 36  erg s –1 , corresponding to roughly 0.01–1 per cent of the Eddington accretion rate for a neutron star. 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 10 34 –10 35  erg s –1 . Despite a limited number of data points, there are indications that these spectral differences also extend to the 10 35 –10 36  erg s –1 range, but above a luminosity of 10 35  erg s –1 the separation between neutron star and black hole systems is not as clear as below. In addition, the black hole spectra only become softer below luminosities of 10 34  erg s –1 compared to 10 36  erg s –1 for the neutron star systems. This observed difference between the neutron star binaries and black hole ones suggests that the spectral properties (between 0.5 and 10 keV) at 10 34 –10 35  erg s –1 can be used to tentatively determine the nature of the accretor in unclassified X-ray binaries. More observations in this luminosity range are needed to determine how robust this diagnostic tool is and whether or not there are (many) systems that do not follow the general trend. 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 on to the neutron star surface for luminosities below a few times 10 34  erg s –1 .
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    An X-ray view of the very faint black hole X-ray transient Swift J1357.2-0933 during its 2011 outburst (2014)
    Oxford University Press
    Details
    Publication Date: 2014-03-25
    Description: We report on the X-ray spectral (using XMM–Newton data) and timing behaviour [using XMM–Newton and Rossi X-ray Timing Explorer ( RXTE ) data] of the very faint X-ray transient and black hole system Swift J1357.2–0933 during its 2011 outburst. The XMM–Newton X-ray spectrum of this source can be adequately fitted with a soft thermal component with a temperature of ~0.22 keV (using a disc model) and a hard, non-thermal component with a photon index of  ~ 1.6 when using a simple power-law model. In addition, an edge at ~0.73 keV is needed likely due to interstellar absorption. During the first RXTE observation, we find a 6 mHz quasi-periodic oscillation (QPO) which is not present during any of the later RXTE observations or during the XMM–Newton observation which was taken 3 d after the first RXTE observation. The nature of this QPO is not clear, but it could be related to a similar QPO seen in the black hole system H1743–322 and to the so-called 1 Hz QPO seen in the dipping neutron-star X-ray binaries (although this latter identification is quite speculative). The observed QPO has similar frequencies as the optical dips seen previously in this source during its 2011 outburst, but we cannot conclusively determine that they are due to the same underlying physical mechanism. Besides the QPO, we detect strong band-limited noise in the power-density spectra of the source (as calculated from both the RXTE and the XMM–Newton data) with characteristic frequencies and strengths very similar to other black hole X-ray transients when they are at low X-ray luminosities. We discuss the spectral and timing properties of the source in the context of the proposed very high inclination of this source. We conclude that all the phenomena seen from the source cannot, as yet, be straightforwardly explained neither by an edge-on configuration nor by any other inclination configuration of the orbit.
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    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 7
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    Swift J1357.2-0933: the faintest black hole? (2014)
    Oxford University Press
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    Publication Date: 2014-08-25
    Description: Swift J1357.2–0933 is a confirmed very faint black hole X-ray transient and has a short estimated orbital period of 2.8 h. 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 ~3 10 –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 ~8 10 29 erg s –1 , making it the faintest detected quiescent black hole low-mass X-ray binary. 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 h, as has been predicted theoretically and recently supported by the detection of the 2.4 h orbital period black hole MAXI J1659–152 at a 0.5–10 keV X-ray luminosity of ~1.2 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 6.3 kpc. In this case, its quiescent luminosity would be L X  ~ 1.3 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.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 8
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    The optical counterpart of the bright X-ray transient Swift J1745-26 (2013)
    Oxford University Press
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    Publication Date: 2013-05-26
    Description: We present a 30-day monitoring campaign of the optical counterpart of the bright X-ray transient Swift J1745–26, starting only 19 min after the discovery of the source. We observe the system peaking at i '  ~ 17.6 on day six (MJD 561 92) to then decay at a rate of ~0.04 mag d –1 . We show that the optical peak occurs at least 3 d later than the hard X-ray (15–50 keV) flux peak. Our measurements result in an outburst amplitude greater than 4.3 mag, which favours an orbital period 21 h and a companion star with a spectral type later than ~A0. Spectroscopic observations taken with the Gran Telescopio de Canarias 10.4 m telescope reveal a broad (full width at half-maximum ~1100 km s –1 ), double-peaked Hα emission line from which we constrain the radial velocity semi-amplitude of the donor to be K 2  〉 250 km s –1 . The breadth of the line and the observed optical and X-ray fluxes suggest that Swift J1745–26 is a new black hole candidate located closer than ~7 kpc.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 9
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    XMM-Newton and Swift spectroscopy of the newly discovered very faint X-ray transient IGR J17494-3030 (2013)
    Oxford University Press
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    Publication Date: 2013-11-23
    Description: A growing group of low-mass X-ray binaries are found to be accreting at very faint X-ray luminosities of 〈10 36 erg s –1 (2–10 keV). One such system is the new X-ray transient IGR J17494–3030. We present Swift and XMM–Newton observations obtained during its 2012 discovery outburst. The Swift observations trace the peak of the outburst, which reached a luminosity of ~7 10 35 ( D /8 kpc) 2  erg s –1 (2–10 keV). The XMM–Newton data were obtained when the outburst had decayed to an intensity of ~8 10 34 ( D /8 kpc) 2  erg s –1 . The spectrum can be described by a power law with an index of  ~ 1.7 and requires an additional soft component with a blackbody temperature of ~0.37 keV (contributing ~20 per cent to the total unabsorbed flux in the 0.5–10 keV band). Given the similarities with high-quality spectra of very faint neutron-star low-mass X-ray binaries, we suggest that the compact primary in IGR J17494–3030 is a neutron star. Interestingly, the source intensity decreased rapidly during the ~12 h XMM–Newton observation, which was accompanied by a decrease in inferred temperature. We interpret the soft spectral component as arising from the neutron-star surface due to low-level accretion, and propose that the observed decline in intensity was the result of a decrease in the mass-accretion rate on to the neutron star.
    Print ISSN: 1745-3925
    Electronic ISSN: 1745-3933
    Topics: Physics
    Published by Oxford University Press
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  • 10
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    The X-ray spectral properties of very-faint persistent neutron star X-ray binaries (2013)
    Oxford University Press
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    Publication Date: 2013-08-18
    Description: AX J1754.2–2754, 1RXS J171824.2–402934 and 1RXH J173523.7–354013 are three persistent neutron star low-mass X-ray binaries that display a 2–10 keV accretion luminosity L X of only (1–10)  x 10 34 erg s –1 (i.e. only ~=0.005–0.05 per cent of the Eddington limit). The phenomenology of accreting neutron stars which accrete at such low accretion rates is not yet well known and the reason why they have such low accretion rates is also not clear. Therefore, we have obtained XMM–Newton data of these three sources and here we report our analysis of the high-quality X-ray spectra we have obtained for them. We find that AX J1754.2–2754 has L X ~ 10 35 erg s –1 , while the other two have X-ray luminosities about an order of magnitude lower. However, all sources have a similar, relatively soft, spectrum with a photon index of 2.3–2.5, when the spectrum is fitted with an absorbed power-law model. This model fits the data of AX J1754.2–2754 adequately, but it cannot fit the data obtained for 1RXS J171824.2–402934 and 1RXH J173523.7–354013. For those sources, a clear soft thermal component is needed to fit their spectra. This soft component contributes 40–50 per cent to the 0.5–10 keV flux of the sources. The presence of this soft component might be the reason why the spectra of these two sources are soft. When including this additional spectral component, the power-law photon indices are significantly lower. It can be excluded that a similar component with similar contributions to the 2–10 keV X-ray flux is present for AX J1754.2–2754, indicating that the soft spectrum of this source is mostly due to the fact that the power-law component itself is not hard. We note that we cannot exclude that a weaker soft component is present in the spectrum of this source which only contributes up to ~25 per cent to the 0.5–10 keV X-ray flux. We discuss our results in the context of what is known of accreting neutron stars at very low accretion rate.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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