We report the precise optical and X-ray localization of the 3.2 ms accretion-powered X-ray pulsar XTE J1814-338 with data from the Chandra X-Ray Observatory as well as optical observations conducted ...during the 2003 June discovery outburst. Optical imaging of the field during the outburst of this soft X-ray transient reveals an R = 18 star at the X-ray position. This star is absent (R > 20) from an archival 1989 image of the field and brightened during the 2003 outburst, and we therefore identify it as the optical counterpart of XTE J1814-338. The best source position derived from optical astrometry is R.A. = 18h13m39.s04, Dec.= -33d46m22.3s (J2000). The featureless X-ray spectrum of the pulsar in outburst is best fit by an absorbed power-law (with photon index = 1.41 +- 0.06) plus blackbody (with kT = 0.95 +- 0.13 keV) model, where the blackbody component contributes approximately 10% of the source flux. The optical broad-band spectrum shows evidence for an excess of infrared emission with respect to an X-ray heated accretion disk model, suggesting a significant contribution from the secondary or from a synchrotron-emitting region. A follow-up observation performed when XTE J1814-338 was in quiescence reveals no counterpart to a limiting magnitude of R = 23.3. This suggests that the secondary is an M3 V or later-type star, and therefore very unlikely to be responsible for the soft excess, making synchroton emission a more reasonable candidate.
We present the long term X-ray light curves, detailed spectral and timing analyses of XTE J1908+094 using the Rossi X-ray Timing Explorer Proportional Counter Array observations covering two ...outbursts in 2002 and early 2003. At the onset of the first outburst, the source was found in a spectrally low/hard state lasting for ~40 days, followed by a three day long transition to the high/soft state. The source flux (in 2\(-\)10 keV) reached \(\sim\)100 mCrab on 2002 April 6, then decayed rapidly. In power spectra, we detect strong band-limited noise and varying low-frequency quasi periodic oscillations that evolved from ~0.5 Hz to ~5 Hz during the initial low/hard state of the source. We find that the second outburst closely resembled the spectral evolution of the first. The X-ray transient's overall outburst characteristics lead us to classify XTE J1908+094 as a black-hole candidate. Here we also derive precise X-ray position of the source using Chandra observations which were performed during the decay phase of the first outburst and following the second outburst.
We have monitored the pulse frequencies of the two soft gamma repeaters SGR 1806-20 and SGR 1900+14 through the beginning of year 2001 using primarily Rossi X-ray Timing Explorer Proportional Counter ...Array observations. In both sources, we observe large changes in the spin-down torque up to a factor of ~4, which persist for several months. Using long baseline phase-connected timing solutions as well as the overall frequency histories, we construct torque noise power spectra for each SGR. The power spectrum of each source is very red (power-law slope ~-3.5). The torque noise power levels are consistent with some accreting systems on time scales of ~1 year, yet the full power spectrum is much steeper in frequency than any known accreting source. To the best of our knowledge, torque noise power spectra with a comparably steep frequency dependence have only been seen in young, glitching radio pulsars (e.g. Vela). The observed changes in spin-down rate do not correlate with burst activity, therefore, the physical mechanisms behind each phenomenon are also likely unrelated. Within the context of the magnetar model, seismic activity cannot account for both the bursts and the long-term torque changes unless the seismically active regions are decoupled from one another.
We present the results of five NuSTAR observations of the type 2 active galactic nucleus (AGN) in IC 751, three of which were performed simultaneously with XMM-Newton or Swift/XRT. We find that the ...nuclear X-ray source underwent a clear transition from a Compton-thick (\(N_{\rm\,H}\simeq 2\times 10^{24}\rm\,cm^{-2}\)) to a Compton-thin (\(N_{\rm\,H}\simeq 4\times 10^{23}\rm\,cm^{-2}\)) state on timescales of \(\lesssim 3\) months, which makes IC 751 the first changing-look AGN discovered by NuSTAR. Changes of the line-of-sight column density at a \(\sim2\sigma\) level are also found on a time-scale of \(\sim 48\) hours (\(\Delta N_{\rm\,H}\sim 10^{23}\rm\,cm^{-2}\)). From the lack of spectral variability on timescales of \(\sim 100\) ks we infer that the varying absorber is located beyond the emission-weighted average radius of the broad-line region, and could therefore be related either to the external part of the broad-line region or a clumpy molecular torus. By adopting a physical torus X-ray spectral model, we are able to disentangle the column density of the non-varying absorber (\(N_{\rm\,H}\sim 3.8\times 10^{23}\rm\,cm^{-2}\)) from that of the varying clouds \(N_{\rm\,H}\sim(1-150)\times10^{22}\rm\,cm^{-2}\), and to constrain that of the material responsible for the reprocessed X-ray radiation (\(N_{\rm\,H} \sim 6 \times 10^{24}\rm\,cm^{-2}\)). We find evidence of significant intrinsic X-ray variability, with the flux varying by a factor of five on timescales of a few months in the 2-10 and 10-50 keV band.
We present X-ray spectral analyses for three Seyfert 2 active galactic nuclei, NGC 424, NGC 1320, and IC 2560, observed by NuSTAR in the 3-79 keV band. The high quality hard X-ray spectra allow ...detailed modeling of the Compton reflection component for the first time in these sources. Using quasi-simultaneous NuSTAR and Swift/XRT data, as well as archival XMM-Newton data, we find that all three nuclei are obscured by Compton-thick material with column densities in excess of ~5 x \(10^{24}\) cm\(^{-2}\), and that their X-ray spectra above 3 keV are dominated by reflection of the intrinsic continuum on Compton-thick material. Due to the very high obscuration, absorbed intrinsic continuum components are not formally required by the data in any of the sources. We constrain the intrinsic photon indices and the column density of the reflecting medium through the shape of the reflection spectra. Using archival multi-wavelength data we recover the intrinsic X-ray luminosities consistent with the broadband spectral energy distributions. Our results are consistent with the reflecting medium being an edge-on clumpy torus with a relatively large global covering factor and overall reflection efficiency of the order of 1%. Given the unambiguous confirmation of the Compton-thick nature of the sources, we investigate whether similar sources are likely to be missed by commonly used selection criteria for Compton-thick AGN, and explore the possibility of finding their high-redshift counterparts.
We present the X-ray timing and spectral evolution of the Galactic Center magnetar SGR J1745-2900 for the first ~4 months post-discovery using data obtained with the Nuclear Spectroscopic Telescope ...Array (NuSTAR)} and Swift observatories. Our timing analysis reveals a large increase in the magnetar spin-down rate by a factor of 2.60 +/- 0.07 over our data span. We further show that the change in spin evolution was likely coincident with a bright X-ray burst observed in 2013 June by Swift, and if so, there was no accompanying discontinuity in the frequency. We find that the source 3-10 keV flux has declined monotonically by a factor of ~2 over an 80-day period post-outburst accompanied by a ~20% decrease in the source's blackbody temperature, although there is evidence for both flux and kT having levelled off. We argue that the torque variations are likely to be magnetospheric in nature and will dominate over any dynamical signatures of orbital motion around Sgr A*.
Broad-line radio galaxies (BLRGs) are active galactic nuclei that produce powerful, large-scale radio jets, but appear as Seyfert 1 galaxies in their optical spectra. In the X-ray band, BLRGs also ...appear like Seyfert galaxies, but with flatter spectra and weaker reflection features. One explanation for these properties is that the X-ray continuum is diluted by emission from the jet. Here, we present two NuSTAR observations of the BLRG 3C 382 that show clear evidence that the continuum of this source is dominated by thermal Comptonization, as in Seyfert 1 galaxies. The two observations were separated by over a year and found 3C 382 in different states separated by a factor of 1.7 in flux. The lower flux spectrum has a photon-index of \(\Gamma=1.68^{+0.03}_{-0.02}\), while the photon-index of the higher flux spectrum is \(\Gamma=1.78^{+0.02}_{-0.03}\). Thermal and anisotropic Comptonization models provide an excellent fit to both spectra and show that the coronal plasma cooled from \(kT_e=330\pm 30\) keV in the low flux data to \(231^{+50}_{-88}\) keV in the high flux observation. This cooling behavior is typical of Comptonizing corona in Seyfert galaxies and is distinct from the variations observed in jet-dominated sources. In the high flux observation, simultaneous Swift data are leveraged to obtain a broadband spectral energy distribution and indicates that the corona intercepts \(\sim 10\)% of the optical and ultraviolet emitting accretion disk. 3C 382 exhibits very weak reflection features, with no detectable relativistic Fe K\(\alpha\) line, that may be best explained by an outflowing corona combined with an ionized inner accretion disk.
Hot, Dust-Obscured Galaxies (Hot DOGs), selected from the WISE all sky
infrared survey, host some of the most powerful Active Galactic Nuclei (AGN)
known, and might represent an important stage in ...the evolution of galaxies.
Most known Hot DOGs are at $z> 1.5$, due in part to a strong bias against
identifying them at lower redshift related to the selection criteria. We
present a new selection method that identifies 153 Hot DOG candidates at $z\sim
1$, where they are significantly brighter and easier to study. We validate this
approach by measuring a redshift $z=1.009$, and an SED similar to higher
redshift Hot DOGs for one of these objects, WISE J1036+0449
($L_{\rm\,Bol}\simeq 8\times 10^{46}\rm\,erg\,s^{-1}$), using data from
Keck/LRIS and NIRSPEC, SDSS, and CSO. We find evidence of a broadened component
in MgII, which, if due to the gravitational potential of the supermassive black
hole, would imply a black hole mass of $M_{\rm\,BH}\simeq 2 \times 10^8
M_{\odot}$, and an Eddington ratio of $\lambda_{\rm\,Edd}\simeq 2.7$. WISE
J1036+0449 is the first Hot DOG detected by NuSTAR, and the observations show
that the source is heavily obscured, with a column density of
$N_{\rm\,H}\simeq(2-15)\times10^{23}\rm\,cm^{-2}$. The source has an intrinsic
2-10 keV luminosity of $\sim 6\times 10^{44}\rm\,erg\,s^{-1}$, a value
significantly lower than that expected from the mid-infrared/X-ray correlation.
We also find that the other Hot DOGs observed by X-ray facilities show a
similar deficiency of X-ray flux. We discuss the origin of the X-ray weakness
and the absorption properties of Hot DOGs. Hot DOGs at $z\lesssim1$ could be
excellent laboratories to probe the characteristics of the accretion flow and
of the X-ray emitting plasma at extreme values of the Eddington ratio.
Hot, Dust-Obscured Galaxies (Hot DOGs), selected from the WISE all sky infrared survey, host some of the most powerful Active Galactic Nuclei (AGN) known, and might represent an important stage in ...the evolution of galaxies. Most known Hot DOGs are at \(z> 1.5\), due in part to a strong bias against identifying them at lower redshift related to the selection criteria. We present a new selection method that identifies 153 Hot DOG candidates at \(z\sim 1\), where they are significantly brighter and easier to study. We validate this approach by measuring a redshift \(z=1.009\), and an SED similar to higher redshift Hot DOGs for one of these objects, WISE J1036+0449 (\(L_{\rm\,Bol}\simeq 8\times 10^{46}\rm\,erg\,s^{-1}\)), using data from Keck/LRIS and NIRSPEC, SDSS, and CSO. We find evidence of a broadened component in MgII, which, if due to the gravitational potential of the supermassive black hole, would imply a black hole mass of \(M_{\rm\,BH}\simeq 2 \times 10^8 M_{\odot}\), and an Eddington ratio of \(\lambda_{\rm\,Edd}\simeq 2.7\). WISE J1036+0449 is the first Hot DOG detected by NuSTAR, and the observations show that the source is heavily obscured, with a column density of \(N_{\rm\,H}\simeq(2-15)\times10^{23}\rm\,cm^{-2}\). The source has an intrinsic 2-10 keV luminosity of \(\sim 6\times 10^{44}\rm\,erg\,s^{-1}\), a value significantly lower than that expected from the mid-infrared/X-ray correlation. We also find that the other Hot DOGs observed by X-ray facilities show a similar deficiency of X-ray flux. We discuss the origin of the X-ray weakness and the absorption properties of Hot DOGs. Hot DOGs at \(z\lesssim1\) could be excellent laboratories to probe the characteristics of the accretion flow and of the X-ray emitting plasma at extreme values of the Eddington ratio.
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