We present optical, X-ray and radio observations of the black hole transient (BHT) XTE J1752−223 towards and in quiescence. Optical photometry shows that the quiescent magnitude of XTE J1752−223 is ...fainter than 24.4 mag in the i′ band. A comparison with measurements of the source during its 2009-2010 outburst shows that the outburst amplitude is more than 8 mag in the i′ band. Known X-ray properties of the source combined with the faintness of the quiescence optical counterpart and the large outburst optical amplitude point towards a short orbital-period system (P
orb≲ 6.8 h) with an M type (or later) mass donor, at a distance of 3.5 ≲d≲ 8 kpc. Simultaneous X-ray and radio data were collected with Chandra and the Expanded Very Large Array (EVLA), allowing constraints to be placed on the quiescent X-ray and radio flux of XTE J1752−223. Furthermore, using data covering the final stage of the outburst decay, we investigated the low-luminosity end of the X-ray-radio correlation for this source and compared it with other BHTs. We found that XTE J1752−223 adds to the number of outliers with respect to the 'standard' X-ray-radio luminosity relation. Furthermore, XTE J1752−223 is the second source, after the BHT H1743−322, that shows a transition from the region of the outliers towards the 'standard' correlation at low luminosity. Finally, we report on a faint, variable X-ray source we discovered with Chandra at an angular distance of ∼2.9 arcsec to XTE J1752−223 and at a position angle consistent with that of the radio jets previously observed from the BHT. We discuss the possibility that we detected X-ray emission associated with a jet from XTE J1752−223.
X-ray disk winds are detected in spectrally soft, disk-dominated phases of stellar-mass black hole outbursts. In contrast, compact, steady, relativistic jets are detected in spectrally hard states ...that are dominated by non-thermal X-ray emission. Although these distinctive outflows appear to be almost mutually exclusive, it is possible that a disk wind persists in hard states but cannot be detected via X-ray absorption lines owing to very high ionization. Here, we present an analysis of a deep, 60 ks Chandra/HETGS observation of the black hole candidate H 1743?322 in the low/hard state. The spectrum shows no evidence of a disk wind, with tight limits, and within the range of ionizing flux levels that were measured in prior Chandra observations wherein a wind was clearly detected. In H 1743?322, at least, disk winds are actually diminished in the low/hard state, and disk winds and jets are likely state dependent and anti-correlated. These results suggest that although the launching radii of winds and jets may differ by orders of magnitude, they may both be tied to a fundamental property of the inner accretion flow, such as the mass accretion rate and/or the magnetic field topology of the disk. We discuss these results in the context of disk winds and jets in other stellar-mass black holes, and possible launching mechanisms for black hole outflows.
The Galactic black hole candidate XTE J1752−223 was observed during the decay of its 2009 outburst with the Suzaku and XMM-Newton observatories. The observed spectra are consistent with the source ...being in the 'intermediate' and 'low-hard' states, respectively. The presence of a strong, relativistic iron emission line is clearly detected in both observations and the line profiles are found to be remarkably consistent and robust to a variety of continuum models. This strongly points to the compact object in XTE J1752−223 being a stellar mass black hole accretor and not a neutron star. Physically motivated and self-consistent reflection models for the Fe Kα emission-line profile and disc reflection spectrum rule out either a non-rotating, Schwarzschild black hole or a maximally rotating, Kerr black hole at greater than 3σ level of confidence. Using a fully relativistic line function in which the black hole spin parameter is a variable, we have formally constrained the spin parameter to be 0.52 ± 0.11(1σ). Furthermore, we show that the source in the low-hard state still requires an optically thick disc component having a luminosity which is consistent with the L∝T
4 relation expected for a thin disc extending down to the innermost stable circular orbit. Our result is in contrast to the prevailing paradigm that the disc is truncated in the low-hard state.
We report the discovery of 42 white dwarfs in the original Kepler mission field, including nine new confirmed pulsating hydrogen-atmosphere white dwarfs (ZZ Ceti stars). Guided by the Kepler-Isaac ...Newton Telescope Survey, we selected white dwarf candidates on the basis of their U − g, g − r, and r − Hα photometric colours. We followed up these candidates with high-signal-to-noise optical spectroscopy from the 4.2-m William Herschel Telescope. Using ground-based, time series photometry, we put our sample of new spectroscopically characterized white dwarfs in the context of the empirical ZZ Ceti instability strip. Prior to our search, only two pulsating white dwarfs had been observed by Kepler. Ultimately, four of our new ZZ Cetis were observed from space. These rich data sets are helping initiate a rapid advancement in the asteroseismic investigation of pulsating white dwarfs, which continues with the extended Kepler mission, K2.
In this paper, we report on radio (Very Large Array and Austrialian Telescope Compact Array) and X-ray (RXTE, Chandra and Swift) observations of the outburst decay of the transient black hole ...candidate H 1743-322 in early 2008. We find that the X-ray light curve followed an exponential decay, levelling off towards its quiescent level. The exponential decay time-scale is ≈4 days and the quiescent flux corresponds to a luminosity of erg s−1. This together with the relation between quiescent X-ray luminosity and orbital period reported in the literature suggests that H 1743-322 has an orbital period longer than ≈10 h. Both the radio and X-ray light curve show evidence for flares. The radio–X-ray correlation can be well described by a power-law with index ≈0.18. This is much lower than the index of ≈0.6–0.7 found for the decay of several black hole transients before. The radio spectral index measured during one of the radio flares while the source is in the low–hard state is −0.5 ± 0.15, which indicates that the radio emission is optically thin. This is unlike what has been found before in black hole sources in the low–hard state. We attribute the radio flares and the low index for the radio–X-ray correlation to the presence of shocks downstream the jet flow, triggered by ejection events earlier in the outburst. We find no evidence for a change in X-ray power-law spectral index during the decay, although the relatively high extinction of NH≈ 2.3 × 1022 cm−2 limits the detected number of soft photons and thus the accuracy of the spectral fits.
We present Gemini spectroscopy for 21 candidate optical counterparts to X-ray sources discovered in the Galactic Bulge Survey (GBS). For the majority of the 21 sources, the optical spectroscopy ...establishes that they are indeed the likely counterparts. One of the criteria we used for the identification was the presence of an Hα emission line. The spectra of several sources revealed an Hα emission line only after careful subtraction of the F or G stellar spectral absorption lines. In a subclass of three of these sources, the residual Hα emission line is broad (≳ 400 km s−1) which suggests that it is formed in an accretion disc, whereas in other cases the line width is such that we currently cannot determine whether the line emission is formed in an active star/binary or in an accretion disc. GBS source CX377 shows this hidden-accretion behaviour most dramatically. The previously identified broad Hα emission of this source is not present in its Gemini spectra taken ∼1 yr later. However, broad emission is revealed after subtracting an F6 template star spectrum. The Gemini spectra of three sources (CX446, CX1004, and CXB2) as well as the presence of possible eclipses in light curves of these sources suggest that these sources are accreting binaries viewed under a high inclination.
We present phase-resolved optical spectroscopy of the counterpart to the neutron star (NS) low-mass X-ray binary EXO 0748−676, almost one year after it turned into quiescence. The spectra display ...prominent Hβ and Hγ and weak Fe ii lines in emission. An average of all the spectra (corrected for the orbital motion) also exhibits a very weak line from Mg i. Tomographic reconstructions show that the accretion disc is not contributing to the optical line emission, which is instead dominated by the irradiated hemisphere of the companion star facing the NS. We could not detect absorption features from the mass donor star in the spectra. The emission lines appear broad, with an intrinsic full width at half-maximum (FWHM) of 255 ± 22 km s−1. Under the assumption that the width of the Fe ii emission lines is dominated by rotational broadening, we obtain a lower limit on the compact object mass which is inconsistent with an NS accretor. We discuss this incongruity and conclude that either the lines are blends of unresolved features (although this requires some fine-tuning) or they are broadened by additional effects such as bulk gas motion in an outflow. The fact that the Fe ii lines slightly lag in phase with respect to the companion star can be understood as outflowing gas consistent with a black-widow-like scenario. Nevertheless, we cannot rule out the possibility that blends of various emission lines cause the apparent phase lag of the Fe ii emission lines as well as their large width.
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