Identification of an Extended Accretion Disk Corona in the Hercules X-1 Low State: Moderate Optical Depth, Precise Density Determination, and Verification of CNO Abundances

We identify an accretion disk atmosphere and corona from the high resolution X-ray spectrum of Hercules X-1, and we determine its detailed physical properties. More than two dozen recombination emission lines (from Fe XXVI at 1.78 A to N VI at 29.08 A) and Fe K-alpha, K-beta fluorescence lines were detected in a 50 ks observation with the Chandra High-Energy Transmission Grating Spectrometer (HETGS). They allow us to measure the density, temperature, spatial distribution, elemental composition, and kinematics of the plasma. We exclude HZ Her as the source of the recombination emission. We compare accretion disk model atmospheres with the observed spectrum in order to constrain the stratification of density and ionization, disk atmosphere area, elemental composition, and energetics. The atmospheric spectrum observed during the low state is photoionized by the main-on X-ray continuum, indicating that the disk is observed edge-on during the low state. We infer the mean number of scatterings N of Ly-alpha and Ly-beta line photons from H-like ions. We derive N < 69 for O VIII Ly_alpha_1, which rules out the presence of a mechanism modeled by Sako (2003) to enhance N VII emission via a line overlap with O VIII. The line optical depth diagnostics are consistent with a flattened atmosphere. Our spectral analysis, the disk atmosphere model, and the presence of intense N VII and N VI lines (plus N V in the UV), confirm the over-abundance of nitrogen relative to other metals, which was shown to be indicative of CNO cycle processing in a massive progenitor.Comment: 38 pages, 14 figures, accepted for publication in Ap

ISM conditions in z~0.2 Lyman-Break Analogs

We present an analysis of far--infrared (FIR) [CII] and [OI] fine structure line and continuum observations obtained with $Herschel$/PACS, and CO(1-0) observations obtained with the IRAM Plateau de Bure Interferometer, of Lyman Break Analogs (LBAs) at $z\sim 0.2$. The principal aim of this work is to determine the typical ISM properties of $z\sim 1-2$ Main Sequence (MS) galaxies, with stellar masses between $10^{9.5}$ and $10^{11}$ $M_{\odot}$, which are currently not easily detectable in all these lines even with ALMA and NOEMA. We perform PDR modeling and apply different IR diagnostics to derive the main physical parameters of the FIR emitting gas and dust and we compare the derived ISM properties to those of galaxies on and above the MS at different redshifts. We find that the ISM properties of LBAs are quite extreme (low gas temperature, high density and thermal pressure) with respect to those found in local normal spirals and more active local galaxies. LBAs have no [CII] deficit despite having the high specific star formation rates (sSFRs) typical of starbursts. Although LBAs lie above the local MS, we show that their ISM properties are more similar to those of high-redshift MS galaxies than of local galaxies above the main sequence. This data set represents an important reference for planning future ALMA [CII] observations of relatively low-mass MS galaxies at the epoch of the peak of the cosmic star formation.Comment: 19 pages, 12 Figures,8 Tables, Accepted for publication in A&

High temperature superconducting infrared imaging satellite

A low earth orbiting platform for an infrared (IR) sensor payload is examined based on the requirements of a Naval Research Laboratory statement of work. The experiment payload is a 1.5-meter square by 0.5-meter high cubic structure equipped with the imaging system, radiators, and spacecraft mounting interface. The orbit is circular at 509 km (275 nmi) altitude and 70 deg. inclination. The spacecraft is three-axis stabilized with pointing accuracy of plus or minus 0.5 deg. in each axis. The experiment payload requires two 15-minute sensing periods over two contiguous orbit periods for 30 minutes of sensing time per day. The spacecraft design is presented for launch via a Delta 2 rocket. Subsystem designs include attitude control, propulsion, electric power, telemetry, tracking and command, thermal design, structure, and cost analysis

New Constraints on the Black Hole Low/Hard State Inner Accretion Flow with NuSTAR

We report on an observation of the Galactic black hole candidate GRS 1739-278 during its 2014 outburst, obtained with NuSTAR. The source was captured at the peak of a rising "low/hard" state, at a flux of ~0.3 Crab. A broad, skewed iron line and disk reflection spectrum are revealed. Fits to the sensitive NuSTAR spectra with a number of relativistically blurred disk reflection models yield strong geometrical constraints on the disk and hard X-ray "corona". Two models that explicitly assume a "lamppost" corona find its base to have a vertical height above the black hole of h = 5 (+7, -2) GM/c^2 and h = 18 +/-4 GM/c^2 (90% confidence errors); models that do not assume a "lamppost" return emissivity profiles that are broadly consistent with coronae of this size. Given that X-ray microlensing studies of quasars and reverberation lags in Seyferts find similarly compact coronae, observations may now signal that compact coronae are fundamental across the black hole mass scale. All of the models fit to GRS 1739-278 find that the accretion disk extends very close to the black hole - the least stringent constraint is r = 5 (+3,-4) GM/c^2. Only two of the models deliver meaningful spin constraints, but a = 0.8 +/-0.2 is consistent with all of the fits. Overall, the data provide especially compelling evidence of an association between compact hard X-ray coronae and the base of relativistic radio jets in black holes.Comment: Accepted for publication in ApJ Letter

EXIST: The Ultimate Spatial/Temporal Hard X-ray Survey

The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed mission to conduct an all-sky imaging hard x-ray (HX) survey (~5–600 keV) with ~0.05mCrab sensitivity (5σ; 6mo.; ~5–100keV) comparable to the ROSAT soft x-ray survey, and to provide the maximum sensitivity and resolution (spatial and temporal) HX imager as the Next Generation GRB mission. Its primary science goals are to i) identify and measure obscured AGN and constrain the accretion luminosity of the universe as well as the cosmic IR background from Blazar spectra coincident with GeV-TeV observations, ii) measure spectra, variability and locations for the faintest GRBs to study the most energetic events in the universe and the earliest epoch of star formation, and iii) study black holes on all scales, from x-ray transients to luminous AGN. EXIST would incorporate a very large area (~8m^2) imaging Cd-Zn-Te detector and coded aperture telescope array with nearly half-sky instantaneous view which images the full sky each orbit. With fixed zenith pointing, it could be mounted on the ISS or a free flyer and would complement both GLAST and Constellation-X science if launched before 2010, as recommended by the Astronomy and Astrophysics Decadal Survey

A Hard X-Ray Study of Ultraluminous X-ray Source NGC 5204 X-1 with NuSTAR and XMM-Newton

We present the results from coordinated X-ray observations of the ultraluminous X-ray source NGC 5204 X-1 performed by NuSTAR and XMM-Newton in early 2013. These observations provide the first detection of NGC 5204 X-1 above 10 keV, extending the broadband coverage to 0.3-20 keV. The observations were carried out in two epochs separated by approximately 10 days, and showed little spectral variation, with an observed luminosity of Lx = (4.95+/-0.11)e39 erg/s. The broadband spectrum confirms the presence of a clear spectral downturn above 10 keV seen in some previous observations. This cutoff is inconsistent with the standard low/hard state seen in Galactic black hole binaries, as would be expected from an intermediate mass black hole accreting at significantly sub-Eddington rates given the observed luminosity. The continuum is apparently dominated by two optically thick thermal-like components, potentially accompanied by a faint high energy tail. The broadband spectrum is likely associated with an accretion disk that differs from a standard Shakura & Sunyaev thin disk.Comment: 7 pages, 5 figures. Accepted for publication in Ap

NuSTAR + XMM-Newton monitoring of the neutron star transient AX J1745.6-2901

AX J1745.6-2901 is a high-inclination (eclipsing) transient neutron star (NS) Low Mass X-ray Binary (LMXB) showcasing intense ionised Fe K absorption. We present here the analysis of 11 XMM-Newton and 15 NuSTAR new data-sets (obtained between 2013-2016), therefore tripling the number of observations of AX J1745.6-2901 in outburst. Thanks to simultaneous XMM-Newton and NuSTAR spectra, we greatly improve on the fitting of the X-ray continuum. During the soft state the emission can be described by a disk black body ($kT\sim1.1-1.2$ keV and inner disc radius $r_{DBB}\sim14$ km), plus hot ($kT\sim2.2-3.0$ keV) black body radiation with a small emitting radius ($r_{BB}\sim0.5-0.8$ km) likely associated with the boundary layer or NS surface, plus a faint Comptonisation component. Imprinted on the spectra are clear absorption features created by both neutral and ionised matter. Additionally, positive residuals suggestive of an emission Fe K$\alpha$ disc line and consistent with relativistic ionised reflection are present during the soft state, while such residuals are not significant during the hard state. The hard state spectra are characterised by a hard ($\Gamma\sim1.9-2.1$) power law, showing no evidence for a high energy cut off ($kT_e>60-140$ keV) and implying a small optical depth ($\tau<1.6$). The new observations confirm the previously witnessed trend of exhibiting strong Fe K absorption in the soft state, that significantly weakens during the hard state. Optical (GROND) and radio (GMRT) observations suggest for AX J1745.6-2901 a standard broad band SED as typically observed in accreting neutron stars.Comment: Accepted for publication in MNRA

NuSTAR Discovery of a Cyclotron Line in the Be/X-ray Binary RX J0520.5-6932 During Outburst

We present spectral and timing analysis of NuSTAR observations of RX J0520.5$-$6932 in the 3-79 keV band collected during its outburst in January 2014. The target was observed on two epochs and we report the detection of a cyclotron resonant scattering feature with central energies of $E_\mathrm{CRSF} = 31.3_{-0.7}^{+0.8}$ keV and $31.5_{-0.6}^{+0.7}$ keV during the two observations, respectively, corresponding to a magnetic field of $B \approx 2 \times10^{12}$ G. The 3-79 keV luminosity of the system during the two epochs assuming a nominal distance of 50 kpc was $3.667\pm0.007\times 10^{38}\,\mathrm{erg\,s^{-1}}$ and $3.983\pm0.007\times10^{38}\,\mathrm{erg\,s^{-1}}$. Both values are much higher than the critical luminosity of $\approx1.5\times10^{37}\,\mathrm{erg\,s^{-1}}$ above which a radiation dominated shock front may be expected. This adds a new object to the sparse set of three systems that have a cyclotron line observed at luminosities in excess of $10^{38}\,\mathrm{erg\,s^{-1}}$. A broad ($\sigma\approx0.45$ keV) Fe emission line is observed in the spectrum at a central energy of $6.58_{-0.05}^{+0.05}$ keV in both epochs. The pulse profile of the pulsar was observed to be highly asymmetric with a sharply rising and slowly falling profile of the primary peak. We also observed minor variations in the cyclotron line energy and width as a function of the rotation phase.% As in observations of other cyclotron absorption line sources, there is a small ($\Delta\phi\lesssim0.1$) phase difference between the peak of the cyclotron energy variation and the peak of the flux variation.Comment: 9 pages, 4 figures, 6 tables. Accepted to Ap