ALBERT

Description: We use mosaics to provide a simple, sound, complete and terminating tableau reasoning procedure for the temporal logic of until and since over general linear time.

Description: We present a spectroscopic analysis of the black hole binary Nova Muscae 1991 in quiescence using data obtained in 2009 with Magellan Echelette on the Magellan Clay telescope and in 2010 with IMACS on the Magellan Baade telescope at the Las Campanas Observatory. Emission from the disc is observed in Hα, Hβ and Ca  ii (8662). A prominent hotspot is observed in the Doppler maps of all three emission lines. The existence of this spot establishes ongoing mass transfer from the donor star in 2009–2010 and, given its absence in the 1993–1995 observations, demonstrates the presence of a variable hotspot in the system. We find the radial distance to the hotspot from the black hole to be consistent with the circularization radius. Our tomograms are suggestive of stream–disc overflow in the system. We also detect possible Ca  ii (8662) absorption from the donor star.

Description: Cultured human cells are invaluable biological models for mechanistic studies of genotoxic chemicals and drugs. Continuing replacement of animals in toxicity testing will further increase the importance of in vitro cell systems, which should accurately reproduce key in vivo characteristics of toxicants such as their profiles of metabolites and DNA lesions. In this work, we examined how a common severe deficiency of cultured cells in ascorbate (Asc) impacts the formation of oxidative DNA damage by hexavalent chromium (chromate). Cr(VI) is reductively activated inside the cells by both Asc and small thiols but with different rates and spectra of intermediates and DNA adducts. We found that Cr(VI) exposure of H460 human lung epithelial cells in standard culture (〈0.01 mM cellular Asc) induced biologically significant amounts of oxidative DNA damage. Inhibition of oxidative damage repair in these cells by stable XRCC1 knockdown strongly enhanced cytotoxic effects of Cr(VI) and led to depletion of cells from G 1 and accumulation in S and G 2 phases. However, restoration of physiological levels of Asc (~1 mM) completely eliminated Cr(VI) hypersensitivity of XRCC1 knockdown. The induction of chromosomal breaks assayed by the micronucleus test in Asc-restored H460, primary human lung fibroblasts, and CHO cells was also unaffected by the XRCC1 status. Centromere-negative (clastogenic) micronuclei accounted for 80–90% of all Cr(VI)-induced micronuclei. Consistent with the micronuclei results, Asc-restored cells also showed no increase in the levels of poly(ADP-ribose), which is a biochemical marker of single-stranded breaks. Asc had no effect on cytotoxicity of O 6 -methylguanine, a lesion produced by direct DNA alkylation. Overall, our results indicate that the presence of physiological levels of Asc strongly suppresses pro-oxidant pathways in Cr(VI) metabolism and that the use of standard cell cultures creates a distorted profile of its genotoxic properties.

Description: We outline the purpose, strategy and first results of a deep, high-cadence, photometric survey of the Kepler field using the Isaac Newton Telescope on La Palma and the MDM 1.3 m Telescope on Kitt Peak. Our goal was to identify sources located in the Kepler field of view which are variable on a time-scale of a few minutes to 1 h. The astrophysically most-interesting sources would then have been candidates for observation using Kepler using 1 min sampling. Our survey covered ~42 per cent of the Kepler field of view, and we have obtained light curves for 7.1 10 5 objects in the range 13 〈 g 〈 20. We have discovered more than 100 variable sources which have passed our two stage identification process. As a service to the wider community, we make our data products and cleaned CCD images available to download. We obtained Kepler data of 18 sources which we found to be variable using our survey, and we give an overview of the currently available data here. These sources include a pulsating DA white dwarf, 11 Sct stars which have dominant pulsation periods in the range 24 min to 2.35 h, three contact binaries, and a cataclysmic variable (V363 Lyr). One of the Sct stars is in a contact binary.

Description: The quiescent state is the dominant accretion mode for black holes on all mass scales. Our knowledge of the X-ray spectrum is limited due to the characteristic low luminosity in this state. Herein, we present an analysis of the sample of dynamically confirmed stellar-mass black holes observed in quiescence in the Chandra/XMM–Newton/Suzaku era resulting in a sample of eight black holes with ~570 ks of observations. In contrast to the majority of active galactic nuclei where observations are limited by contamination from diffuse gas, the stellar-mass systems allow for a clean study of the X-ray spectrum resulting from the accretion flow alone. The data are characterized using simple models. We find a model consisting of a power law or thermal bremsstrahlung to both provide excellent descriptions of the data, where we measure  = 2.06 ± 0.03 and $kT = 5.03^{+0.33}_{-0.31}\,\mathrm{keV}$ , respectively, in the 0.3–10 keV bandpass, at a median luminosity of L x  ~ 5.5 10 –7 L Edd . This result in discussed in the context of our understanding of the accretion flow on to stellar and supermassive black holes at low luminosities.

Description: We report observations using the Swift /XRT, NuSTAR , and Chandra X-ray telescopes of the transient X-ray source CXOGC J174540.0-290005 during its 2013 outburst. Due to its location in the field of multiple observing campaigns targeting Sgr A*, this is one of the best-studied outbursts of a very faint X-ray binary (VFXB; peak L X 〈 10 36 erg s –1 ) yet recorded, with detections in 173 ks of X-ray observations over 50 d. VFXBs are of particular interest, due to their unusually low outburst luminosities and time-averaged mass transfer rates, which are hard to explain within standard accretion physics and binary evolution. The 2013 outburst of CXOGC J174540.0-290005 peaked at L X (2–10 keV) = 5.0 10 35 erg s –1 , and all data above 10 34 erg s –1 were well fitted by an absorbed power law of photon index ~1.7, extending from 2 keV out to 70 keV. We discuss the implications of these observations for the accretion state of CXOGC J174540.0-290005.

Description: Quiescent emission from the neutron star low-mass X-ray binary Cen X-4 is seen to be variable on time-scales from hundreds of seconds to years, suggesting that at least in this object, low-level accretion is important during quiescence. Here, we present results from recent XMM–Newton and Swift observations of Cen X-4, where the X-ray flux (0.5–10 keV) varies by a factor of 6.5 between the brightest and faintest states. We find a positive correlation between the X-ray flux and the simultaneous near-ultraviolet (UV) flux, where as there is no significant correlation between the X-ray and simultaneous optical ( V , B ) fluxes. This suggests that while the X-ray and UV emitting regions are somehow linked, the optical region originates elsewhere. Comparing the luminosities, it is plausible that the UV emission originates due to reprocessing of the X-ray flux by the accretion disc, with the hot inner region of the disc being a possible location for the UV emitting region. The optical emission, however, could be dominated by the donor star. The X-ray/UV correlation does not favour the accretion stream impact point as the source of the UV emission.

Description: GRS 1915+105 has the largest mass function of any Galactic black hole system, although the error is relatively large. Here we present spectroscopic analysis of medium-resolution infrared Very Large Telescope archival data of GRS 1915+105 in the K band. We find an updated ephemeris, and report on attempts to improve the mass function by a refinement of the radial velocity estimate. We show that the spectra are significantly affected by the presence of phase-dependent CO bandhead emission, possibly originating from the accretion disc: we discuss the impact this has on efforts to better constrain the black hole mass. We report on a possible way to measure the radial velocity utilizing apparent H -band atomic absorption features and also discuss the general uncertainty of the system parameters of this well-studied object.

Description: We conducted the first long-term (60 d), multiwavelength (optical, ultraviolet, UV, and X-ray) simultaneous monitoring of Cen X-4 with daily Swift observations from 2012 June to August, with the goal of understanding variability in the low-mass X-ray binary Cen X-4 during quiescence. We found Cen X-4 to be highly variable in all energy bands on time-scales from days to months, with the strongest quiescent variability a factor of 22 drop in the X-ray count rate in only 4 d. The X-ray, UV and optical ( V band) emission are correlated on time-scales down to less than 110 s. The shape of the correlation is a power law with index about 0.2–0.6. The X-ray spectrum is well fitted by a hydrogen neutron star (NS) atmosphere ( kT  = 59–80 eV) and a power law (with spectral index  = 1.4–2.0), with the spectral shape remaining constant as the flux varies. Both components vary in tandem, with each responsible for about 50 per cent of the total X-ray flux, implying that they are physically linked. We conclude that the X-rays are likely generated by matter accreting down to the NS surface. Moreover, based on the short time-scale of the correlation, we also unambiguously demonstrate that the UV emission cannot be due to either thermal emission from the stream impact point, or a standard optically thick, geometrically thin disc. The spectral energy distribution shows a small UV emitting region, too hot to arise from the accretion disc, that we identified as a hotspot on the companion star. Therefore, the UV emission is most likely produced by reprocessing from the companion star, indeed the vertical size of the disc is small and can only reprocess a marginal fraction of the X-ray emission. We also found the accretion disc in quiescence to likely be UV faint, with a minimal contribution to the whole UV flux.

Description: The neutron star low-mass X-ray binary and intermittent millisecond X-ray pulsar HETE J1900.1–2455 returned to quiescence in late 2015, after a prolonged accretion outburst of ~=10 yr. Using a Chandra observation taken ~=180 d into quiescence, we detect the source at a luminosity of ~=4.5 x 10 31 ( D /4.7 kpc) 2 erg s –1 (0.5–10 keV). The X-ray spectrum can be described by a neutron star atmosphere model with a temperature of ~=54 eV for an observer at infinity. We perform thermal evolution calculations based on the 2016 quiescent data and a 98 eV temperature upper limit inferred from a Swift observation taken during an unusually brief (2 weeks) quiescent episode in 2007. We find no evidence in the present data that the thermal properties of the crust, such as the heating rate and thermal conductivity, are different than those of non-pulsating neutron stars. Finding this neutron star so cold after its long outburst imposes interesting constraints on the heat capacity of the stellar core; these become even stronger if further cooling were to occur.