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Register a new userGRS 1739-278 observed at very low luminosity with XMM-Newton and NuSTAR
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We present a detailed spectral analysis of XMM-Newton and NuSTAR observations of the accreting transient black hole GRS 1739-278 during a very faint low hard state at ~0.02% of the Eddington luminosity (for a distance of 8.5 kpc and a mass of 10 M_sun ). The broad-band X-ray spectrum between 0.5-60 keV can be well-described by a power law continuum with an exponential cutoff. The continuum is unusually hard for such a low luminosity, with a photon index of Gamma = 1.39 +/- 0.04. We find evidence for an additional reflection component from an optically thick accretion disk at the 98% likelihood level. The reflection fraction is low with R_refl = 0.043(+0.033,-0.023). In combination with measurements of the spin and inclination parameters made with NuSTAR during a brighter hard state by Miller and co-workers, we seek to constrain the accretion disk geometry. Depending on the assumed emissivity profile of the accretion disk, we find a truncation radius of 15-35 Rg (5-12 R_ISCO ) at the 90% confidence limit. These values depend strongly on the assumptions and we discuss possible systematic uncertainties.
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We report on the X-ray spectral analysis and time evolution of GRS 1739$-$278 during its 2014 outburst based on MAXI/GSC and Swift/XRT observations. Over the course of the outburst, a transition from the low/hard state to the high/soft state and then back to the low/hard state was seen. During the high/soft state, the innermost disk temperature mildly decreased, while the innermost radius estimated with the multi-color disk model remained constant at $sim18 (frac{D}{8.5 mathrm{kpc}}) {(frac{cos i}{cos 30^{circ}})}^{-1/2}$ km, where $D$ is the source distance and $i$ is the inclination of observation. This small innermost radius of the accretion disk suggests that the central object is more likely to be a Kerr black hole rather than a Schwardzschild black hole. Applying a relativistic disk emission model to the high/soft state spectra, a mass upper limit of $18.3 mathrm{M_{odot}}$ was obtained based on the inclination limit $i<60^{circ}$ for an assumed distance of 8.5 kpc. Using the empirical relation of the transition luminosity to the Eddington limit, the mass is constrained to $4.0-18.3 mathrm{M_{odot}}$ for the same distance. The mass can be further constrained to be no larger than $9.5 mathrm{M_{odot}}$ by adopting the constraints based on the fits to the NuSTAR spectra with relativistically blurred disk reflection models (Miller et al. 2015).
During the scanning observations of the Galactic Center region in August - September 2016 we detected the new outburst of the historical X-ray nova GRS 1739-278, the black hole candidate LMXB system. In this letter we present results of INTEGRAL and Swift-XRT observations taken during the outburst. In hard X-ray band (20-60 keV) the flux from the source raised from $sim$11 to $sim$30 mCrab between 3 and 14 of September. For nearly 8 days the source has been observed at this flux level and then faded to $sim$15 mCrab. The broadband quasi-simultaneous spectrum obtained during the outburst is well described by the absorbed powerlaw with the photon index $Gamma=1.86pm0.07$ in broad energy range 0.5-150 keV, with absorption corresponding to ${N_{H}}=2.3times10^{22}$ cm$^{-2}$ assuming solar abundance. Based on this we can conclude that the source was in the low/hard state. From the lightcurve and spectra we propose that this outburst was `failed, i.e. amount of accreted matter was not sufficient to achieve the high/soft spectral state with dominant soft blackbody component as seen in normal outbursts of black hole candidates.
We present results from NuSTAR and XMM-Newton observations of the new black hole X-ray binary MAXI J1820+070 at low accretion rates (below 1% of the Eddington luminosity). We detect a narrow Fe K$alpha$ emission line, in contrast to the broad and asymmetric Fe K$alpha$ line profiles commonly present in black hole binaries at high accretion rates. The narrow line, with weak relativistic broadening, indicates that the Fe K$alpha$ line is produced at a large disk radius. Fitting with disk reflection models assuming standard disk emissivity finds a large disk truncation radius (a few tens to a few hundreds of gravitational radii, depending on the disk inclination). In addition, we detect a quasi-periodic oscillation (QPO) varying in frequency between $11.6pm0.2$~mHz and $2.8pm0.1$~mHz. The very low QPO frequencies suggest a large size for the optically-thin Comptonization region according to the Lense-Thirring precession model, supporting that the accretion disk recedes from the ISCO and is replaced by advection-dominated accretion flow at low accretion rates. We also discuss the possibility of an alternative accretion geometry that the narrow Fe K$alpha$ line is produced by a lamppost corona with a large height illuminating the disk.
MCG-6-30-15, at a distance of 37 Mpc (z=0.008), is the archetypical Seyfert 1 galaxy showing very broad Fe K$alpha$ emission. We present results from a joint NuSTAR and XMM-Newton observational campaign that, for the first time, allows a sensitive, time-resolved spectral analysis from 0.35 keV up to 80 keV. The strong variability of the source is best explained in terms of intrinsic X-ray flux variations and in the context of the light bending model: the primary, variable emission is reprocessed by the accretion disk, which produces secondary, less variable, reflected emission. The broad Fe K$alpha$ profile is, as usual for this source, well explained by relativistic effects occurring in the innermost regions of the accretion disk around a rapidly rotating black hole. We also discuss the alternative model in which the broadening of the Fe K$alpha$ is due to the complex nature of the circumnuclear absorbing structure. Even if this model cannot be ruled out, it is disfavored on statistical grounds. We also detected an occultation event likely caused by BLR clouds crossing the line of sight.
GRB031203 was observed by XMM-Newton twice, first with an observation beginning 6 hours after the burst, and again after 3 days. The afterglow had average 0.2-10.0keV fluxes for the first and second observations of 4.2+/-0.1x10^-13 and 1.8+/-0.1x10^-13 erg/cm^2/s respectively, decaying very slowly according to a power-law with an index of -0.55+/-0.05. The prompt soft X-ray flux, inferred from a detection of the dust echo of the prompt emission, strongly implies that this burst is very soft and should be classified as an X-ray flash (XRF) and further, implies a steep temporal slope (<~-1.7) between the prompt and afterglow phases or in the early afterglow, very different from the later afterglow decay slope. A power-law (Gamma=1.90+/-0.05) with absorption at a level consistent with the Galactic foreground absorption fits the afterglow spectrum well. A bright, low-redshift (z=0.105) galaxy lies within 0.5 arcsec of the X-ray position and is likely to be the GRB host. At this redshift, GRB031203 is the closest GRB or XRF known after GRB980425. It has a very low equivalent isotropic gamma-ray energy in the burst (~3x10^49 erg) and X-ray luminosity in the afterglow (9x10^42 erg/s at 10 hours), 3-4 orders of magnitude less than typical bursts, though higher than either the faint XRF020903 or GRB980425. The rapid initial decline and subsequent very slow fading of the X-ray afterglow is also similar to that observed in GRB980425, indicating that GRB031203 may be representative of low luminosity bursts.
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