The spin of Cygnus X-1 is measured by fitting reflection models to Suzaku data covering the energy band 0.9-400 keV. The inner radius of the accretion disc is found to lie within 2 gravitational ...radii (r
g=GM/c
2), and a value of
is obtained for the dimensionless black hole spin. This agrees with recent measurements using the continuum fitting method by Gou et al. and of the broad iron line by Duro et al. The disc inclination is measured at
, which is consistent with the recent optical measurement of the binary system inclination by Orosz et al. of 27°± 0°.8. We pay special attention to the emissivity profile caused by irradiation of the inner disc by the hard power-law source. The X-ray observations and simulations show that the index q of that profile deviates from the commonly used, Newtonian, value of 3 within 3r
g, steepening considerably within 2r
g, as expected in the strong gravity regime.
When neutron stars reside in transient X-ray binaries, their crustal layers become heated during accretion outbursts and subsequently cool in quiescence. Observing and modelling this thermal response ...has yielded valuable insight into the physics of neutron star crusts. However, one unresolved problem is the evidence in several neutron stars for an extra energy source, located at shallow depth in the crust, that is not accounted for by standard heating models. Its origin remains puzzling, and it is currently unclear whether this additional heating occurs in all neutron stars, and if the magnitude is always the same. Here, we report on Chandra observations that cover two years after the 2012 outburst of the transient neutron star X-ray binary Swift J174805.3–244637 in the globular cluster Terzan 5. The temperature of the neutron star was elevated during the first two months following its ≃8 week accretion episode, but had decayed to the pre-outburst level within ≃100 d. Interpreting this as rapid cooling of the accretion-heated crust, we model the observed temperature curve with a thermal evolution code. We find that there is no need to invoke shallow heating for this neutron star, although an extra energy release up to ≃1.4 MeV nucleon−1 is allowed by the current data (2σ confidence). We also present two new data points on the crust-cooling curve of the 11-Hz X-ray pulsar IGR J17480–2446 in Terzan 5, which was active in 2010. The temperature of this neutron star remains significantly above the pre-outburst level, but we detect no change in the thermal emission since the previous measurements of 2013 February. This is consistent with the slow crust cooling expected several years post-outburst.
Atoll sources are accreting neutron star (NS) low-mass X-ray binaries. We present a spectral analysis of four persistent atoll sources (GX 3+1, 4U 1702−429, 4U 0614+091, and 4U 1746−371) observed for ...∼20 ks each with NuSTAR to determine the extent of the inner accretion disk. These sources range from an apparent luminosity of 0.006-0.11 of the Eddington limit (assuming the empirical limit of 3.8 × 1038 erg s−1). Broad Fe emission features shaped by Doppler and relativistic effects close to the NS were firmly detected in three of these sources. The position of the disk appears to be close to the innermost stable circular orbit (ISCO) in each case. For GX 3+1, we determine (90% confidence level) and an inclination of 27°-31°. For 4U 1702−429, we find a and inclination of 53°-64°. For 4U 0614+091, the disk has a position of and inclination of 50°-62°. If the disk does not extend to the innermost stable circular orbit, we can place conservative limits on the magnetic field strength in these systems in the event that the disk is truncated at the Alfvén radius. This provides the limit at the poles of B ≤ 6.7 × 108 G, 3.3 × 108 G, and 14.5 × 108 G for GX 3+1, 4U 1702−429, and 4U 0614+091, respectively. For 4U 1746−371, we argue that the most plausible explanation for the lack of reflection features is a combination of source geometry and strong Comptonization. We place these sources among the larger sample of NSs that have been observed with NuSTAR.
We conducted the first long-term (75 d) X-ray monitoring of the black hole low-mass X-ray binary V404 Cyg, with the goal of understanding and characterizing its variability during quiescence. The ...X-ray light curve of V404 Cyg shows several flares on time-scales of hours with a count rate change of a factor of about 5-8. The root-mean-square variability is F
var = 57.0 ± 3.2 per cent. The first-order structure function is consistent with both a power spectrum of index −1 (flicker noise), or with a power spectrum of index 0 (white noise), implying that the light curve is variable on time-scales from days to months. The X-ray spectrum is well fitted by a power law with spectral index Γ = 2.10-2.35, and we found that the spectral shape remains roughly constant as the flux changes. A constant spectral shape with respect to a change in the X-ray flux may favour a scenario in which the X-ray emission is dominated by synchrotron radiation produced in a jet.
We present NuSTAR observations of neutron star (NS) low-mass X-ray binaries: 4U 1636-53, GX 17+2, and 4U 1705-44. We observed 4U 1636-53 in the hard state, with an Eddington fraction, , of 0.01; GX ...17+2 and 4U 1705-44 were in the soft state with fractions of 0.57 and 0.10, respectively. Each spectrum shows evidence for a relativistically broadened Fe K line. Through accretion disk reflection modeling, we constrain the radius of the inner disk in 4U 1636-53 to be ISCO (innermost stable circular orbit), assuming a dimensionless spin parameter , and ISCO for (errors quoted at 1 ). This value proves to be model independent. For and , for example, 1.08 0.06 ISCO translates to a physical radius of km, and the NS would have to be smaller than this radius (other outcomes are possible for allowed spin parameters and masses). For GX 17+2, ISCO for and ISCO for . For and , ISCO translates to km. The inner accretion disk in 4U 1705-44 may be truncated just above the stellar surface, perhaps by a boundary layer or magnetosphere; reflection models give a radius of 1.46-1.64 ISCO for and 1.69-1.93 ISCO for . We discuss the implications our results may have on the equation of state of ultradense, cold matter and our understanding of the innermost accretion flow onto NSs with low surface magnetic fields, and systematic errors related to the reflection models and spacetime metric around less idealized NSs.
ABSTRACT Reverberation-mapping-based scaling relations are often used to estimate the masses of black holes from single-epoch spectra of active galactic nuclei (AGNs). While the radius-luminosity ...relation that is the basis of these scaling relations is determined using reverberation mapping of the Hβ line in nearby AGNs, the scaling relations are often extended to use other broad emission lines, such as Mg ii, in order to get black hole masses at higher redshifts when Hβ is redshifted out of the optical waveband. However, there is no radius-luminosity relation determined directly from Mg ii. Here, we present an attempt to perform reverberation mapping using Mg ii in the well-studied nearby Seyfert 1 NGC 5548. We used Swift to obtain UV grism spectra of NGC 5548 once every two days from 2013 April to September. Concurrent photometric UV monitoring with Swift provides a well determined continuum light curve that shows strong variability. The Mg ii emission line, however, is not strongly correlated with the continuum variability, and there is no significant lag between the two. We discuss these results in the context of using Mg ii scaling relations to estimate high-redshift black hole masses.
ABSTRACT
We present the first intensive continuum reverberation mapping study of the high accretion-rate Seyfert galaxy Mrk 110. The source was monitored almost daily for more than 200 d with the ...Swift X-ray and ultraviolet (UV)/optical telescopes, supported by ground-based observations from Las Cumbres Observatory, the Liverpool Telescope, and the Zowada Observatory, thus extending the wavelength coverage to 9100 Å. Mrk 110 was found to be significantly variable at all wavebands. Analysis of the intraband lags reveals two different behaviours, depending on the time-scale. On time-scales shorter than 10 d the lags, relative to the shortest UV waveband (∼1928 Å), increase with increasing wavelength up to a maximum of ∼2 d lag for the longest waveband (∼9100 Å), consistent with the expectation from disc reverberation. On longer time-scales, however, the g-band lags the Swift BAT hard X-rays by ∼10 d, with the z-band lagging the g-band by a similar amount, which cannot be explained in terms of simple reprocessing from the accretion disc. We interpret this result as an interplay between the emission from the accretion disc and diffuse continuum radiation from the broad-line region.
Reverberation lags in active galactic nuclei (AGN) were first discovered in the NLS1 galaxy, 1H0707−495. We present a follow-up analysis using 1.3 Ms of data, which allows for the closest ever look ...at the reverberation signature of this remarkable source. We confirm previous findings of a hard lag of ∼100 s at frequencies ν ∼ 0.5-4 × 10−4 Hz, and a soft lag of ∼30 s at higher frequencies, ν ∼ 0.6-3 × 10−3 Hz. These two frequency domains clearly show different energy dependences in their lag spectra. We also find evidence for a signature from the broad Fe Kα line in the high-frequency lag spectrum. We use Monte Carlo simulations to show how the lag and coherence measurements respond to the addition of Poisson noise and to dilution by other components. With our better understanding of these effects on the lag, we show that the lag-energy spectra can be modelled with a scenario in which low-frequency hard lags are produced by a compact corona responding to accretion rate fluctuations propagating through an optically thick accretion disc, and high-frequency soft lags are produced by short light-travel delay associated with reflection of coronal power-law photons off the disc.
The Galactic black hole X-ray binary MAXI J1820+070 had a bright outburst in 2018 when it became the second brightest X-ray source in the sky. It was too bright for X-ray CCD instruments such as ...XMM–Newton and Chandra, but was well observed by photon counting instruments such as Neutron star Inner Composition Explorer (NICER) and Nuclear Spectroscopic Telescope Array (NuSTAR). We report here on the discovery of an excess emission component during the soft state. It is best modelled with a blackbody spectrum in addition to the regular disc emission, modelled as either diskbb or kerrbb. Its temperature varies from about 0.9 to 1.1 keV, which is about 30–80 per cent higher than the inner disc temperature of diskbb. Its flux varies between 4 and 12 per cent of the disc flux. Simulations of magnetized accretion discs have predicted the possibility of excess emission associated with a non-zero torque at the innermost stable circular orbit (ISCO) about the black hole, which, from other NuSTAR studies, lies at about 5 gravitational radii or about 60 km (for a black hole, mass is 8Msun). In this case, the emitting region at the ISCO has a width varying between 1.3 and 4.6 km and would encompass the start of the plunge region where matter begins to fall freely into the black hole.
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