Mark T. Reynolds

Broad relativistic iron emission line observed in sax j1808.4-3658

During the 2008 September-October outburst of the accreting millisecond pulsar SAX J1808.4–3658, the source was observed by both Suzaku and XMM-Newton approximately 1 day apart. Spectral analysis reveals a broad relativistic Fe K? emission line which is present in both data sets, as has recently been reported for other neutron star low-mass X-ray binaries. The properties of the Fe K line observed during each observation are very similar. From modeling the Fe line, we determine the inner accretion disk radius to be 13.2 ± 2.5 GM c–2. The inner disk radius measured from the Fe K line suggests that the accretion disk is not very receded in the hard state. If the inner disk (as measured by the Fe line) is truncated at the magnetospheric radius this implies a magnetic field strength of ~3 × 108 G at the magnetic poles, consistent with other independent estimates.

Flows of X-ray gas reveal the disruption of a star by a massive black hole

Tidal forces close to massive black holes can violently disrupt stars that make a close approach. These extreme events are discovered via bright X-ray and optical/ultraviolet flares in galactic centres. Prior studies based on modelling decaying flux trends have been able to estimate broad properties, such as the mass accretion rate. Here we report the detection of flows of hot, ionized gas in high-resolution X-ray spectra of a nearby tidal disruption event, ASASSN-14li in the galaxy PGC 043234. Variability within the absorption-dominated spectra indicates that the gas is relatively close to the black hole. Narrow linewidths indicate that the gas does not stretch over a large range of radii, giving a low volume filling factor. Modest outflow speeds of a few hundred kilometres per second are observed; these are below the escape speed from the radius set by variability. The gas flow is consistent with a rotating wind from the inner, super-Eddington region of a nascent accretion disk, or with a filament of disrupted stellar gas near to the apocentre of an elliptical orbit. Flows of this sort are predicted by fundamental analytical theory and more recent numerical simulations.

Multimessenger astronomy with pulsar timing and X-ray observations of massive black hole binaries

In the decade of the dawn of gravitational wave astronomy, the concept of multimessenger astronomy, combining gravitational wave signals to conventional electromagnetic observation, has attracted the attention of the astrophysical community. So far, most of the effort has been focused on ground and space based laser interferometer sources, with little attention devoted to the ongoing and upcoming pulsar timing arrays (PTAs). We argue in this paper that PTA sources, being very massive (> 10 8 M⊙), cosmologically nearby (z < 1) black hole binaries (MBHBs), are particularly appealing multimessenger carriers. According to current models for massive black hole formation and evolution, the planned Square Kilometer Array (SKA) will observe thousands of such massive systems, being able to individually resolve and locate in the sky several of them (maybe up to a hundred). MBHBs form in galaxy mergers, which are usually accompanied by strong inflows of gas in the center of the merger remnant. By employing a standard model for the evolution of MBHBs in circumbinary discs, with the aid of dedicated numerical simulations, we characterize the gas-binary interplay, identifying possible electromagnetic signatures of the PTA sources. We concentrate our investigation on two particularly promising scenarios in the high energy domain, )

A 200-second quasi-periodicity after the tidal disruption of a star by a dormant black hole.

Oscillating Black Hole The massive black holes that reside in the centers of galaxies can occasionally capture and tidally disrupt stars that wander too close. One such tidal disruption event was detected last year by the Swift satellite. Follow-up x-ray observations analyzed by Reis et al. (p. 949, published online 2 August; see the Perspective by McKinney) show quasi-periodic oscillations that suggest that an accretion disk formed around the black hole shortly after the tidal disruption event. This type of oscillation is commonly seen in the x-ray light from the much lighter black holes that result from the gravitational collapse of stars, but has been seen only once in a massive black hole residing in the center of a galaxy. Oscillations in x-ray emission from a galaxy’s central black hole imply that a disc formed after the hole captured a star. Supermassive black holes (SMBHs; mass is greater than or approximately 105 times that of the Sun) are known to exist at the center of most galaxies with sufficient stellar mass. In the local universe, it is possible to infer their properties from the surrounding stars or gas. However, at high redshifts we require active, continuous accretion to infer the presence of the SMBHs, which often comes in the form of long-term accretion in active galactic nuclei. SMBHs can also capture and tidally disrupt stars orbiting nearby, resulting in bright flares from otherwise quiescent black holes. Here, we report on a ~200-second x-ray quasi-periodicity around a previously dormant SMBH located in the center of a galaxy at redshift z = 0.3534. This result may open the possibility of probing general relativity beyond our local universe.

The light curve of the companion to PSR B1957+20

We present a new analysis of the light curve for the secondary star in the eclipsing binary millisecond pulsar system PSR B1957+20. Combining previous data and new data points at minimum from the Hubble Space Telescope, we have 100 per cent coverage in the R-band. We also have a number of new K s -band data points, which we use to constrain the infrared magnitude of the system. We model this with the eclipsing light-curve (ELC) code. From the modelling with the ELC code we obtain colour information about the secondary at minimum light in BVRI and K. For our best-fitting model we are able to constrain the system inclination to 65° ± 2° for pulsar masses ranging from 1.3 to 1.9 M⊙. The pulsar mass is unconstrained. We also find that the secondary star is not filling its Roche lobe. The temperature of the unirradiated side of the companion is in agreement with previous estimates and we find that the observed temperature gradient across the secondary star is physically sustainable.

SUZAKU OBSERVATION OF THE BLACK HOLE CANDIDATE MAXI J1836-194 IN A HARD/INTERMEDIATE SPECTRAL STATE

We report on a Suzaku observation of the newly discovered X-ray binary MAXI J1836-194. The source is found to be in the hard/intermediate spectral state and displays a clear and strong relativistically broadened iron emission line. We fit the spectra with a variety of phenomenological, as well as physically motivated disk reflection models, and find that the breadth and strength of the iron line are always characteristic of emission within a few gravitational radii around a black hole. This result is independent of the continuum used and strongly points toward the central object in MAXI J1836-194 being a stellar mass black hole rotating with a spin of a = 0.88 {+-} 0.03 (90% confidence). We discuss this result in the context of spectral state definitions, physical changes (or lack thereof) in the accretion disk, and on the potential importance of the accretion disk corona in state transitions.

Optical spectroscopy and photometry of SAX J1808.4−3658 in outburst

We present phase resolved optical spectroscopy and photometry of V4580 Sagittarii, the optical counterpart to the accretion powered millisecond pulsar SAX J1808.4−3658, obtained during the 2008 September/October outburst. Doppler tomography of the N iii λ4640.64 Bowen blend emission line reveals a focused spot of emission at a location consistent with the secondary star. The velocity of this emission occurs at 324 ± 15 km s 1 ; applying a “K-correction”, we find the velocity of the secondary star projected onto the line of sight to be 370 ± 40 km s 1 . Based on existing pulse timing measurements, this constrains the mass ratio of the system to be 0.044 +0.005 0.004, and the mass function for the pulsar to be 0.44 +0.16 0.13 M⊙. Combining this mass function with various inclination estimates from other authors, we find no evidence to suggest that the neutron star in SAX J1808.4−3658 is more massive than the canonical value of 1.4 M⊙. Our optical light curves exhibit a possible superhump modulation, expected for a system with such a low mass ratio. The equivalent width of the Ca ii H and K interstellar absorption lines suggest that the distance to the source is ∼2.5 kpc. This is consistent with previous distance estimates based on type-I Xray bursts which assume cosmic abundances of hydrogen, but lower than more recent estimates which assume helium-rich bursts.

Daily multiwavelength Swift monitoring of the neutron star low-mass X-ray binary Cen X-4: evidence for accretion and reprocessing during quiescence

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 γ 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.

High Resolution Chandra HETG spectroscopy of V404 Cygni in Outburst

As one of the best-characterized stellar-mass black holes, with good measurements of its mass, distance and inclination, V404 Cyg is the ideal candidate to study Eddington-limited accretion episodes. After a long quiescent period, V404 Cyg underwent a new outburst in June 2015. We obtained two Chandra HETG exposures of 20 ksec and 25 ksec. Many strong emission lines are observed; the ratio of Si He-like triplet lines gives an estimate for the formation region distance of

Reflection from the strong gravity regime in a lensed quasar at redshift z = 0.658

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