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Calvera (1RXS J141256.0+792204) is a pulsar of characteristic age 285 kyr at a high Galactic latitude of b=+37{\deg}, detected only in soft thermal X-rays. We measure a new and precise proper motion for Calvera using Chandra HRC-I observations obtained 10 years apart. We also derive a new phase-connected ephemeris using 6 years of NICER data, including the astrometric position and proper motion as fixed parameters in the timing solution. Calvera is located near the center of a faint, circular radio ring that was recently discovered by LOFAR and confirmed as a supernova remnant (SNR) by the detection of gamma-ray emission with Fermi/LAT. The proper motion of $78.5 \pm 2.9$ mas/yr at position angle $241{\deg}.3 \pm 2{\deg}.2$ (in Galactic coordinates) points away from the center of the ring, a result which differs markedly from a previous low-significance measurement, and greatly simplifies the interpretation of the SNR/pulsar association. It argues that the supernova indeed birthed Calvera <10 kyr ago, with an initial spin period close to its present value of 59 ms. The tangential velocity of the pulsar depends on its uncertain distance, $v_t=(372 \pm 14) d_{1 kpc}$ km/s, but is probably dominated by the supernova kick, while its progenitor could have been a runaway O or B star from the Galactic disk., Comment: Accepted for publication in ApJ, 8 pages, 3 figures

We present a high-resolution spectral study of Fe L-shell extinction by the diffuse interstellar medium (ISM) in the direction of the X-ray binaries Cygnus X-1 and GX 339-4, using the XMM-Newton reflection grating spectrometer. The majority of interstellar Fe is suspected to condense into dust grains in the diffuse ISM, but the compounds formed from this process are unknown. Here, we use the laboratory cross sections from Kortright & Kim (2000) and Lee et al. (2009) to model the absorption and scattering profiles of metallic Fe, and the crystalline compounds fayalite (Fe$_2$SiO$_4$), ferrous sulfate (FeSO$_4$), hematite ($\alpha$-Fe$_2$O$_3$), and lepidocrocite ($\gamma$-FeOOH), which have oxidation states ranging from Fe$^{0}$ to Fe$^{3+}$. We find that the observed Fe L-shell features are systematically offset in energy from the laboratory measurements. An examination of over two dozen published measurements of Fe L-shell absorption finds a 1-2 eV scatter in energy positions of the L-shell features. Motivated by this, we fit for the best energy-scale shift simultaneously with the fine structure of the Fe L-shell extinction cross sections. Hematite and lepidocrocite provide the best fits ($\approx +1.1$ eV shift), followed by fayalite ($\approx +1.8$ eV shift). However, fayalite is disfavored, based on the implied abundances and knowledge of ISM silicates gained by infrared astronomical observations and meteoritic studies. We conclude that iron oxides in the Fe$^{3+}$ oxidation state are good candidates for Fe-bearing dust. To verify this, new absolute photoabsorption measurements are needed on an energy scale accurate to better than 0.2 eV., Comment: accepted to AAS Journals

We present updated measurements of the X-ray properties of the pulsar wind nebula associated with the TeV $\gamma$-ray source HESS J1640-465 derived from Chandra & NuSTAR data. We report a high $N_{H}$ value along line of sight, consistent with previous work, which led us to incorporate effects of dust scattering in our spectral analysis. Due to uncertainties in the dust scattering, we report a range of values for the PWN properties (photon index and un-absorbed flux). In addition, we fit the broadband spectrum of this source and found evidence for spectral softening and decreasing unabsorbed flux as we go to higher photon energies. We then used a one zone time dependent evolutionary model to reproduce the dynamical and multi-wavelength spectral properties of our source. Our model suggests a short spin-down time scale, a relatively higher than average magnetized pulsar wind, a strong PWN magnetic field and maximum electron energy up to PeV, suggesting HESS J1640-465 could be a PeVatron candidate., Comment: 14 pages, 7 figures, 10 tables. Accepted for publication in ApJ

We present a broadband X-ray study of W50 (`the Manatee nebula'), the complex region powered by the microquasar SS 433, that provides a test-bed for several important astrophysical processes. The W50 nebula, a Galactic PeVatron candidate, is classified as a supernova remnant but has an unusual double-lobed morphology likely associated with the jets from SS 433. Using NuSTAR, XMM-Newton, and Chandra observations of the inner eastern lobe of W50, we have detected hard non-thermal X-ray emission up to $\sim$30 keV, originating from a few-arcminute size knotty region (`Head') located $\lesssim$ 18$^{\prime}$ (29 pc for a distance of 5.5 kpc) east of SS 433, and constrain its photon index to 1.58$\pm$0.05 (0.5-30 keV band). The index gradually steepens eastward out to the radio `ear' where thermal soft X-ray emission with a temperature $kT$$\sim$0.2 keV dominates. The hard X-ray knots mark the location of acceleration sites within the jet and require an equipartition magnetic field of the order of $\gtrsim$12$\mu$G. The unusually hard spectral index from the `Head' region challenges classical particle acceleration processes and points to particle injection and re-acceleration in the sub-relativistic SS 433 jet, as seen in blazars and pulsar wind nebulae., Comment: 19 pages, 9 figures, 2 tables, accepted for publication in the Astrophysical Journal

The Nuclear Spectroscopic Telescope Array (NuSTAR) mission is the first focusing X-ray telescope in the hard X-ray (3-79 keV) band. Among the phenomena that can be studied in this energy band, some require high time resolution and stability: rotation-powered and accreting millisecond pulsars, fast variability from black holes and neutron stars, X-ray bursts, and more. Moreover, a good alignment of the timestamps of X-ray photons to UTC is key for multi-instrument studies of fast astrophysical processes. In this Paper, we describe the timing calibration of the NuSTAR mission. In particular, we present a method to correct the temperature-dependent frequency response of the on-board temperature-compensated crystal oscillator. Together with measurements of the spacecraft clock offsets obtained during downlinks passes, this allows a precise characterization of the behavior of the oscillator. The calibrated NuSTAR event timestamps for a typical observation are shown to be accurate to a precision of ~65 microsec., Comment: 13 pages, 10 figures. Comments welcome

We perform a sub-threshold follow-up search for continuous nearly-monochromatic gravitational waves from the central compact objects associated with the supernova remnants Vela Jr., Cassiopeia A, and SNR G347.3$-$0.5. Across the three targets, we investigate the most promising ~ 10,000 combinations of gravitational wave frequency and frequency derivative values, based on the results from an Einstein@Home search of the LIGO O1 observing run data, dedicated to these objects. The selection threshold is set so that a signal could be confirmed using the newly released O2 run LIGO data. In order to achieve best sensitivity we perform two separate follow-up searches, on two distinct stretches of the O2 data. Only one candidate survives the first O2 follow-up investigation, associated with the central compact object in SNR G347.3-0.5, but it is not conclusively confirmed. In order to assess a possible astrophysical origin we use archival X-ray observations and search for amplitude modulations of a pulsed signal at the putative rotation frequency of the neutron star and its harmonics. This is the first extensive electromagnetic follow-up of a continuous gravitational wave candidate performed to date. No significant associated signal is identified. New X-ray observations contemporaneous with the LIGO O3 run will enable a more sensitive search for an electromagnetic counterpart. A focused gravitational wave search in O3 data based on the parameters provided here should be easily able to shed light on the nature of this outlier. Noise investigations on the LIGO instruments could also reveal the presence of a coherent contamination., Comment: 13 pages, 4 figures, accepted for publication in The Astrophysical Journal

We present a Chandra and XMM-Newton study of the SNR CTB 37B, along with archival radio observations. In radio wavelengths, CTB 37B is an incomplete shell showing bright emission from the eastern side while the X-ray morphology shows diffuse emission from regions surrounding the magnetar CXOU J171405.7-381031. We used archival HI absorption measurements to constrain the distance to the remnant as 9.8+/-1.5 kpc. The X-ray spectrum of the remnant is described by a thermal model in the 1-5 keV energy range, with a temperature of 1.3+/-0.1 keV. The abundances from the spectral fits are consistent with being solar or sub-solar. A small region of diffuse emission is seen to the southern side of the remnant, best fitted by a nonthermal spectrum with an unusually hard photon index of 1.3+/-0.3. Assuming a distance of 9.8 kpc to the SNR, we infer a shock velocity of 915+/-70 km/s and explosion energy of (1.8+/-0.6)e50 ergs. The overall imaging and spectral properties of CTB 37B favor the interpretation of a young SNR (6200 yr) propagating in a low-density medium, under the assumption of a Sedov evolutionary phase., Comment: Accepted for publication in MNRAS, 10 pages, 9 figures, 4 tables

We present the earliest available soft X-ray observations of XTE J1810-187, the prototypical transient magnetar, obtained 75--84 days after its 2018 outburst with the Neutron Star Interior Composition Explorer (NICER). Using a series of observations covering eight days we find that its decreasing X-ray flux is well-described by either a blackbody plus power-law or a two-blackbody spectral model. The 2-10 keV flux of the source varied from (1.206+/-0.007)x10^{-10} to (1.125+/-0.004)x10^{-10} erg s^{-1} cm^{-2}, a decrease of about 7% within our observations and 44% from that measured 7-14 days after the outburst with NuSTAR. We confirm that the pulsed fraction and spin pulse phase of the neutron star are energy dependent up to at least 8 keV. Phase resolved spectroscopy of the pulsar suggests magnetospheric variations relative to the line of sight., Comment: Accepted for publication in the Astrophysical Journal Letters

Supernova remnants serve as nearby laboratories relevant to many areas in Astrophysics, from stellar and galaxy evolution to extreme astrophysics and the formation of the heavy elements in the Universe. The Chandra X-ray mission has enabled a giant leap forward in studying both SNRs and their compact stellar remnants on sub-arcsecond scale. However, such high-resolution imaging studies have been mostly limited to the nearby and/or relatively bright objects. There is no question that we are missing a large population, especially in external galaxies. Within our own Galaxy, we are presented with new fundamental questions related to neutron stars' diversity, kicks, relativistic winds and the way these objects interact with, and impact, their host environments. In this white paper, we highlight some of the breakthroughs to be achieved with future X-ray missions (such as the proposed AXIS probe) equipped with sub-arcsecond imaging resolution and an order of magnitude improvement in sensitivity., Comment: Astro2020 Science White Paper. 9 pages, 2 figures

We report on a broadband study of a complex X-ray source (1SAX J0618.0+2227) associated with the interaction site of the supernova remnant (SNR) IC 443 and ambient molecular cloud (MC) using NuSTAR, XMM_Newton, and Chandra observations. Its X-ray spectrum is composed of both thermal and non-thermal components. The thermal component can be equally well represented by either a thin plasma model with kT=0.19 keV or a blackbody model with kT=0.11 keV. The non-thermal component can be fit with either a power-law with Gamma~1.7 or a cutoff power-law with Gamma~1.5 and a cutoff energy at E_cut~18 keV. Using the newly obtained NuSTAR dataset, we test three possible scenarios for isolated X-ray sources in the SNR-MC interaction site: 1) pulsar wind nebula (PWN); 2) SNR ejecta fragment; 3) shocked molecular clump. We conclude that this source is most likely composed of a SNR ejecta (or a PWN) and surrounding shocked molecular clumps. The nature of this hard X-ray source in the SNR-MC interaction site of IC 443 may shed light on unidentified X-ray sources with hard X-ray spectra in rich environments for star forming regions, such as the Galactic center., Comment: 10 pages, 6 figures. Accepted by ApJ

We present a catalog of hard X-ray sources in a square-degree region surveyed by NuSTAR in the direction of the Norma spiral arm. This survey has a total exposure time of 1.7 Ms, and typical and maximum exposure depths of 50 ks and 1 Ms, respectively. In the area of deepest coverage, sensitivity limits of $5\times10^{-14}$ and $4\times10^{-14}$ erg s$^{-1}$ cm$^{-2}$ in the 3-10 and 10-20 keV bands, respectively, are reached. Twenty-eight sources are firmly detected and ten are detected with low significance; eight of the 38 sources are expected to be active galactic nuclei. The three brightest sources were previously identified as a low-mass X-ray binary, high-mass X-ray binary, and pulsar wind nebula. Based on their X-ray properties and multi-wavelength counterparts, we identify the likely nature of the other sources as two colliding wind binaries, three pulsar wind nebulae, a black hole binary, and a plurality of cataclysmic variables (CVs). The CV candidates in the Norma region have plasma temperatures of $\approx$10-20 keV, consistent with the Galactic Ridge X-ray emission spectrum but lower than temperatures of CVs near the Galactic Center. This temperature difference may indicate that the Norma region has a lower fraction of intermediate polars relative to other types of CVs compared to the Galactic Center. The NuSTAR log$N$-log$S$ distribution in the 10-20 keV band is consistent with the distribution measured by Chandra at 2-10 keV if the average source spectrum is assumed to be a thermal model with $kT\approx15$~keV, as observed for the CV candidates., Comment: 42 pages, 12 figures, 11 tables

We present the first survey results of hard X-ray point sources in the Galactic Center (GC) region by NuSTAR. We have discovered 70 hard (3-79 keV) X-ray point sources in a 0.6 deg^2 region around Sgr A* with a total exposure of 1.7 Ms, and 7 sources in the Sgr B2 field with 300 ks. We identify clear Chandra counterparts for 58 NuSTAR sources and assign candidate counterparts for the remaining 19. The NuSTAR survey reaches X-ray luminosities of ~4 x and ~8 x 10^32 erg s^-1 at the GC (8 kpc) in the 3-10 and 10-40 keV bands, respectively. The source list includes three persistent luminous X-ray binaries and the likely run-away pulsar called the Cannonball. New source-detection significance maps reveal a cluster of hard (>10 keV) X-ray sources near the Sgr A diffuse complex with no clear soft X-ray counterparts. The severe extinction observed in the Chandra spectra indicates that all the NuSTAR sources are in the central bulge or are of extragalactic origin. Spectral analysis of relatively bright NuSTAR sources suggests that magnetic cataclysmic variables constitute a large fraction (>40-60%). Both spectral analysis and logN-logS distributions of the NuSTAR sources indicate that the X-ray spectra of the NuSTAR sources should have kT > 20 keV on average for a single temperature thermal plasma model or an average photon index of Gamma = 1.5 - 2 for a power-law model. These findings suggest that the GC X-ray source population may contain a larger fraction of X-ray binaries with high plasma temperatures than the field population., Comment: 32 pages, 15 figures, 7 tables, accepted for publication in ApJ

We present a catalog of 1415 X-ray sources identified in the Norma arm region Chandra survey (NARCS), which covers a 2 deg x 0.8 deg region in the direction of the Norma spiral arm to a depth of $\approx$20 ks. Of these sources, 1130 are point-like sources detected with $\geq3\sigma$ confidence in at least one of three energy bands (0.5-10, 0.5-2, and 2-10 keV), five have extended emission, and the remainder are detected at low significance. Since most sources have too few counts to permit individual classification, they are divided into five spectral groups defined by their quantile properties. We analyze stacked spectra of X-ray sources within each group, in conjunction with their fluxes, variability, and infrared counterparts, to identify the dominant populations in our survey. We find that $\sim$50% of our sources are foreground sources located within 1-2 kpc, which is consistent with expectations from previous surveys. Approximately 20% of sources are likely located in the proximity of the Scutum-Crux and near Norma arm, while 30% are more distant, in the proximity of the far Norma arm or beyond. We argue that a mixture of magnetic and nonmagnetic CVs dominates the Scutum-Crux and near Norma arms, while intermediate polars (IPs) and high-mass stars (isolated or in binaries) dominate the far Norma arm. We also present the cumulative number count distribution for sources in our survey that are detected in the hard energy band. A population of very hard sources in the vicinity of the far Norma arm and active galactic nuclei dominate the hard X-ray emission down to $f_X\approx10^{-14}$ erg cm$^{-2}$ s$^{-1}$, but the distribution curve flattens at fainter fluxes. We find good agreement between the observed distribution and predictions based on other surveys., Comment: 27 pages, 19 figures, 12 tables, catalog will be made available in published ApJ version and by personal request

We report on the first hard X-ray detection of the Geminga pulsar above 10 keV using a 150 ks observation with the NuSTAR observatory. The double-peaked pulse profile of non-thermal emission seen in the soft X-ray band persists at higher energies. Broadband phase-integrated spectra over the 0.2-20 keV band with NuSTAR and archival XMM-Newton data do not fit to a conventional two-component model of a blackbody plus power-law, but instead exhibit spectral hardening above ~5 keV. We find two spectral models fit the data well: (1) a blackbody (kT1 ~ 42 eV) with a broken power-law (Gamma1 ~ 2.0, Gamma2 ~ 1.4 and Ebreak ~ 3.4 keV), and (2) two blackbody components (kT1 ~ 44 eV and kT2 ~ 195 eV) with a power-law component (Gamma ~ 1.7). In both cases, the extrapolation of the Rayleigh-Jeans tail of the thermal component is consistent with the UV data, while the non-thermal component overpredicts the near-infrared data, requiring a spectral flattening at E ~ 0.01 - 1 keV. While strong phase variation of the power-law index is present below ~5 keV, our phase-resolved spectroscopy with NuSTAR indicates that another hard non-thermal component with Gamma ~ 1.3 emerges above ~5 keV. The spectral hardening in non-thermal X-ray emission as well as spectral flattening between the optical and X-ray bands argue against the conjecture that a single power-law may account for multi-wavelength non-thermal spectra of middle-aged pulsars., Comment: Accepted to ApJ

We report on new broad band spectral and temporal observations of the magnetar 1E 2259+586, which is located in the supernova remnant CTB 109. Our data were obtained simultaneously with the Nuclear Spectroscopic Telescope Array (NuSTAR) and Swift, and cover the energy range from 0.5-79 keV. We present pulse profiles in various energy bands and compare them to previous RXTE results. The NuSTAR data show pulsations above 20 keV for the first time and we report evidence that one of the pulses in the double-peaked pulse profile shifts position with energy. The pulsed fraction of the magnetar is shown to increase strongly with energy. Our spectral analysis reveals that the soft X-ray spectrum is well characterized by an absorbed double-blackbody or blackbody plus power-law model in agreement with previous reports. Our new hard X-ray data, however, suggests that an additional component, such as a power-law, is needed to describe the NuSTAR and Swift spectrum. We also fit the data with the recently developed coronal outflow model by Beloborodov for hard X-ray emission from magnetars. The outflow from a ring on the magnetar surface is statistically preferred over outflow from a polar cap., Comment: 37 pages, 9 figures, corresponding author, vogel9@llnl.gov

Results are presented for an initial survey of the Norma Arm gathered with the focusing hard X-ray telescope NuSTAR. The survey covers 0.2 deg$^2$ of sky area in the 3-79 keV range with a minimum and maximum raw depth of 15 ks and 135 ks, respectively. Besides a bright black-hole X-ray binary in outburst (4U 1630-47) and a new X-ray transient (NuSTAR J163433-473841), NuSTAR locates three sources from the Chandra survey of this region whose spectra are extended above 10 keV for the first time: CXOU J163329.5-473332, CXOU J163350.9-474638, and CXOU J163355.1-473804. Imaging, timing, and spectral data from a broad X-ray range (0.3-79 keV) are analyzed and interpreted with the aim of classifying these objects. CXOU J163329.5-473332 is either a cataclysmic variable or a faint low-mass X-ray binary. CXOU J163350.9-474638 varies in intensity on year-long timescales, and with no multi-wavelength counterpart, it could be a distant X-ray binary or possibly a magnetar. CXOU J163355.1-473804 features a helium-like iron line at 6.7 keV and is classified as a nearby cataclysmic variable. Additional surveys are planned for the Norma Arm and Galactic Center, and those NuSTAR observations will benefit from the lessons learned during this pilot study., Comment: Accepted for publication in the Astrophysical Journal, 10 pages, 4 figures, 3 tables

We report the detection of eight bright X-ray bursts from the 6.5-s magnetar 1E 1048.1-5937, during a 2013 July observation campaign with the Nuclear Spectroscopic Telescope Array (NuSTAR). We study the morphological and spectral properties of these bursts and their evolution with time. The bursts resulted in count rate increases by orders of magnitude, sometimes limited by the detector dead time, and showed blackbody spectra with kT=6-8 keV in the T90 duration of 1-4 s, similar to earlier bursts detected from the source. We find that the spectra during the tail of the bursts can be modeled with an absorbed blackbody with temperature decreasing with flux. The bursts flux decays followed a power-law of index 0.8-0.9. In the burst tail spectra, we detect a ~13 keV emission feature, similar to those reported in previous bursts from this source as well as from other magnetars observed with the Rossi X-ray Timing Explorer (RXTE). We explore possible origins of the spectral feature such as proton cyclotron emission, which implies a magnetic field strength of B~2X10^15 G in the emission region. However, the consistency of the energy of the feature in different objects requires further explanation., Comment: 10 pages, 6 figures, accepted for publication in ApJ

We present NuSTAR high energy X-ray observations of the pulsar wind nebula (PWN)/supernova remnant G21.5-0.9. We detect integrated emission from the nebula up to ~40 keV, and resolve individual spatial features over a broad X-ray band for the first time. The morphology seen by NuSTAR agrees well with that seen by XMM-Newton and Chandra below 10 keV. At high energies NuSTAR clearly detects non-thermal emission up to ~20 keV that extends along the eastern and northern rim of the supernova shell. The broadband images clearly demonstrate that X-ray emission from the North Spur and Eastern Limb results predominantly from non-thermal processes. We detect a break in the spatially integrated X-ray spectrum at ~9 keV that cannot be reproduced by current SED models, implying either a more complex electron injection spectrum or an additional process such as diffusion compared to what has been considered in previous work. We use spatially resolved maps to derive an energy-dependent cooling length scale, $L(E) \propto E^{m}$ with $m = -0.21 \pm 0.01$. We find this to be inconsistent with the model for the morphological evolution with energy described by Kennel & Coroniti (1984). This value, along with the observed steepening in power-law index between radio and X-ray, can be quantitatively explained as an energy-loss spectral break in the simple scaling model of Reynolds (2009), assuming particle advection dominates over diffusion. This interpretation requires a substantial departure from spherical magnetohydrodynamic (MHD), magnetic-flux-conserving outflow, most plausibly in the form of turbulent magnetic-field amplification., Comment: 13 pages, 8 figures, 1 table, Accepted for publication in the Astrophysical Journal

We present the first analysis of the extended source CXOU J163802.6--471358, which was discovered serendipitously during the {\em Chandra} X-ray survey of the Norma region of the Galactic spiral arms. The X-ray source exhibits a cometary appearance with a point source and an extended tail region. The complete source spectrum is fitted well with an absorbed power law model and jointly fitting the {\em Chandra} spectrum of the full source with one obtained from an archived {\em XMM-Newton} observation results in best fit parameters $N_{\rm H}$ $=1.5^{+0.7}_{-0.5}\times10^{23} \text{cm}{^{-2}}$ and $\Gamma=1.1^{+0.7}_{-0.6}$ (90$%$ confidence uncertainties). The unabsorbed luminosity of the full source is then $L_X\sim 4.8\times10^{33}d_{10}^2$ergs s$^{-1}$ with $d_{10}=d/10$kpc, where a distance of 10 kpc is a lower bound inferred from the large column density. The radio counterpart found for the source using data from the Molonglo Galactic Plane Survey epoch-2 (MGPS-2) shows an elongated tail offset from the X-ray emission. No infrared counterpart was found. The results are consistent with the source being a previously unknown pulsar driving a bow shock through the ambient medium.

During hard X-ray observations of the Norma spiral arm region by the Nuclear Spectroscopic Telescope Array (NuSTAR) in 2013 February, a new transient source, NuSTAR J163433-4738.7, was detected at a significance level of 8-sigma in the 3-10 keV bandpass. The source is consistent with having a constant NuSTAR count rate over a period of 40 ks and is also detected simultaneously by Swift at lower significance. The source is not significantly detected by NuSTAR, Swift, or Chandra in the days before or weeks after the discovery of the transient, indicating that the strong X-ray activity lasted for between ~0.5 and 1.5 days. Near-IR imaging observations were carried out before and after the X-ray activity, but we are not able to identify the counterpart. The combined NuSTAR and Swift energy spectrum is consistent with a power-law with a photon index of Gamma = 4.1(+1.5)(-1.0) (90% confidence errors), a blackbody with kT = 1.2+/-0.3 keV, or a bremsstrahlung model with kT = 3.0(+2.1)(-1.2) keV. The reduced-chi2 values for the three models are not significantly different, ranging from 1.23 to 1.44 for 8 degrees of freedom. The spectrum is strongly absorbed with NH = 2.8(+2.3)(-1.4)e23 cm-2, 9(+15)(-7)e22 cm-2, and 1.7(+1.7)(-0.9)e23 cm-2, for the power-law, blackbody, and bremsstrahlung models, respectively. Although the high column density could be due to material local to the source, it is consistent with absorption from interstellar material along the line of sight at a distance of 11 kpc, which would indicate an X-ray luminosity >1e34 erg/s. Although we do not reach a definitive determination of the nature of NuSTAR J163433-4738.7, we suggest that it may be an unusually bright active binary or a magnetar., Comment: 6 pages, 4 figures, published as ApJ, 785, 4. The name of the source has been revised to NuSTAR J163433-4738.7 in order to comply with the IAU format for NuSTAR sources

The Nuclear Spectroscopic Telescope Array (NuSTAR) is the first focusing hard X-ray mission in orbit and operates in the 3-79 keV range. NuSTAR's sensitivity is roughly two orders of magnitude better than previous missions in this energy band thanks to its superb angular resolution. Since its launch in 2012 June, NuSTAR has performed excellently and observed many interesting sources including four magnetars, two rotation-powered pulsars and the cataclysmic variable AE Aquarii. NuSTAR also discovered 3.76-s pulsations from the transient source SGR J1745-29 recently found by Swift very close to the Galactic Center, clearly identifying the source as a transient magnetar. For magnetar 1E 1841-045, we show that the spectrum is well fit by an absorbed blackbody plus broken power-law model with a hard power-law photon index of ~1.3. This is consistent with previous results by INTEGRAL and RXTE. We also find an interesting double-peaked pulse profile in the 25-35 keV band. For AE Aquarii, we show that the spectrum can be described by a multi-temperature thermal model or a thermal plus non-thermal model; a multi-temperature thermal model without a non-thermal component cannot be ruled out. Furthermore, we do not see a spiky pulse profile in the hard X-ray band, as previously reported based on Suzaku observations. For other magnetars and rotation-powered pulsars observed with NuSTAR, data analysis results will be soon available., Comment: 5 pages, 1 figure. Accepted for publication in Astronomische Nachrichten (AN), Proceedings of the XMM-Newton 2013 Science Workshop "The Fast and the Furious: Energetic Phenomena in Isolated Neutron Stars, Pulsar Wind Nebulae and Supernova Remnants"

We report the first detection of high-energy X-ray (E>10 keV) emission from the Galactic Center non-thermal filament G359.89-0.08 (Sgr A-E) using data acquired with the Nuclear Spectroscopic Telescope Array (NuSTAR). The bright filament was detected up to ~50 keV during a NuSTAR Galactic Center monitoring campaign. The featureless power-law spectrum with a photon index of ~2.3 confirms a non-thermal emission mechanism. The observed flux in the 3-79 keV band is ~ 2.0e-12 erg/cm^2/s, corresponding to an unabsorbed X-ray luminosity of ~2.6e34 erg/s assuming a distance of 8.0 kpc. Based on theoretical predictions and observations, we conclude that Sgr A-E is unlikely to be a pulsar wind nebula (PWN) or supernova remnant-molecular cloud (SNR-MC) interaction, as previously hypothesized. Instead, the emission could be due to a magnetic flux tube which traps TeV electrons. We propose two possible TeV electron sources: old PWNe (up to ~100 kyr) with low surface brightness and radii up to ~30 pc or molecular clouds (MCs) illuminated by cosmic rays (CRs) from CR accelerators such as SNRs or Sgr A*., Comment: 6 pages, 2 figures, accepted for publication in ApJ

AE Aquarii is a cataclysmic variable with the fastest known rotating magnetized white dwarf (P_spin = 33.08 s). Compared to many intermediate polars, AE Aquarii shows a soft X-ray spectrum with a very low luminosity (L_X ~ 10^{31} erg/s). We have analyzed overlapping observations of this system with the NuSTAR and the Swift X-ray observatories in September of 2012. We find the 0.5-30 keV spectra to be well fitted by either an optically thin thermal plasma model with three temperatures of 0.75 +0.18 -0.45, 2.29 +0.96 -0.82, and 9.33 +6.07 -2.18 keV, or an optically thin thermal plasma model with two temperatures of 1.00 +0.34 -0.23 and 4.64 +1.58 -0.84 keV plus a power-law component with photon index of 2.50 +0.17 -0.23. The pulse profile in the 3-20 keV band is broad and approximately sinusoidal, with a pulsed fraction of 16.6 +/- 2.3%. We do not find any evidence for a previously reported sharp feature in the pulse profile., Comment: 7 pages, 5 figures, accepted for publication in ApJ

We report the unambiguous detection of non-thermal X-ray emission up to 30 keV from the Cannonball, a few-arcsecond long diffuse X-ray feature near the Galactic Center, using the NuSTAR X-ray observatory. The Cannonball is a high-velocity (vproj~500 km/s) pulsar candidate with a cometary pulsar wind nebula (PWN) located ~2' north-east from Sgr A*, just outside the radio shell of the supernova remnant Sagittarius A (Sgr A) East. Its non-thermal X-ray spectrum, measured up to 30 keV, is well characterized by a Gamma~1.6 power-law, typical of a PWN, and has an X-ray luminosity of L(3-30 keV)=1.3e34 erg/s. The spectral and spatial results derived from X-ray and radio data strongly suggest a runaway neutron star born in the Sgr A East supernova event. We do not find any pulsed signal from the Cannonball. The NuSTAR observations allow us to deduce the PWN magnetic field and show that it is consistent with the lower limit obtained from radio observations., Comment: 6 pages, 3 figures, 2 tables, Accepted for publication in The Astrophysical Journal Letters

We report new spectral and temporal observations of the magnetar 1E 1841-045 in the Kes 73 supernova remnant obtained with the Nuclear Spectroscopic Telescope Array (NuSTAR). Combined with new Swift and archival XMM-Newton and Chandra observations, the phase-averaged spectrum is well characterized by a blackbody plus double power-law model, in agreement with previous, multi-mission X-ray results. However, we are unable to reproduce the spectral results reported using Suzaku observations. The pulsed fraction of the source is found to increase with photon energy. The measured rms pulsed fraction is ~12% and ~17% at ~20 keV and ~50 keV, respectively. We detect a new feature in the 24--35 keV band pulse profile that is uniquely double-peaked. This feature may be associated with a possible absorption or emission feature in the phase-resolved spectrum. We fit the X-ray data using the recently developed electron-positron outflow model of Beloborodov (2013) for the hard X-ray emission from magnetars. This produces a satisfactory fit allowing a constraint on the angle between the rotation and magnetic axes of the neutron star of ~20 degrees and on the angle between the rotation axis and line-of-sight of ~50 degrees. In this model, the soft X-ray component is inconsistent with a single blackbody; adding a second blackbody or a power-law component fits the data. The two-blackbody interpretation suggests a hot spot of temperature kT~0.9 keV occupying ~1% of the stellar surface., Comment: 9 pages, 4 figures and 4 tables, accepted for publication in ApJ

We report the discovery of 3.76-s pulsations from a new burst source near Sgr A* observed by the NuSTAR Observatory. The strong signal from SGR J1745-29 presents a complex pulse profile modulated with pulsed fraction 27+/-3 % in the 3-10 keV band. Two observations spaced 9 days apart yield a spin-down rate of Pdot = (6.5+/-1.4)x10^{-12}. This implies a magnetic field B = 1.6x10^14 G, spin-down power Edot = 5x10^33 erg/s, and characteristic age P/2Pdot = 9x10^3 yr, for the rotating dipole model. However, the current Pdot may be erratic, especially during outburst. The flux and modulation remained steady during the observations and the 3-79 keV spectrum is well fitted by a combined blackbody plus power-law model with temperature kT_BB = 0.96+/-0.02 keV and photon index = 1.5+/-0.4, respectively. The neutral hydrogen column density (nH ~ 1.4x10^23 cm^{-2}) measured by NuSTAR and Swift suggests that SGR J1745-29 is located at or near the Galactic Center. The lack of an X-ray counterpart in the published Chandra survey catalog sets a quiescent 2-8 keV luminosity limit of Lx ~< 10^32 erg/s. The bursting, timing, and spectral properties indicate a transient magnetar undergoing an outburst with 2-79 keV luminosity up to 3.5x10^35 erg/s for a distance of 8 kpc. SGR J1745-29 joins a growing subclass of transient magnetars, indicating that many magnetars in quiescence remain undetected in the X-ray band or have been detected as high-B radio pulsars. The peculiar location of SGR J1745-29 has important implications for the formation and dynamics of neutron stars in the Galactic Center region., Comment: accepted to ApJ letter

The Nuclear Spectroscopic Telescope Array (NuSTAR) mission, launched on 13 June 2012, is the first focusing high-energy X-ray telescope in orbit. NuSTAR operates in the band from 3 -- 79 keV, extending the sensitivity of focusing far beyond the ~10 keV high-energy cutoff achieved by all previous X-ray satellites. The inherently low-background associated with concentrating the X-ray light enables NuSTAR to probe the hard X-ray sky with a more than one-hundred-fold improvement in sensitivity over the collimated or coded-mask instruments that have operated in this bandpass. Using its unprecedented combination of sensitivity, spatial and spectral resolution, NuSTAR will pursue five primary scientific objectives, and will also undertake a broad program of targeted observations. The observatory consists of two co-aligned grazing-incidence X-ray telescopes pointed at celestial targets by a three-axis stabilized spacecraft. Deployed into a 600 km, near-circular, 6degree inclination orbit, the Observatory has now completed commissioning, and is performing consistent with pre-launch expectations. NuSTAR is now executing its primary science mission, and with an expected orbit lifetime of ten years, we anticipate proposing a guest investigator program, to begin in Fall 2014., Comment: 20 pages, 12 figures, submitted to the Astrophysical Journal

We report on an observation of SGR 1627-41 made with the Chandra X-ray Observatory on 2011 June 16. Approximately three years after its outburst activity in 2008, the source's flux has been declining, as it approaches its quiescent state. For an assumed power-law spectrum, we find that the absorbed 2--10 keV flux for the source is $1.0^{+0.3}_{-0.2} \times 10^{-13} erg cm^{-2} s^{-1}$ with a photon index of $2.9 \pm 0.8$ ($N_H=1.0\times10^{23}$ cm^{-2}). This flux is approximately consistent with that measured at the same time after the source's outburst in 1998. With measurements spanning 3 years after the 2008 outburst, we analyze the long-term flux and spectral evolution of the source. The flux evolution is well described by a double exponential with decay times of 0.5 $\pm$ 0.1 and 59 $\pm$ 6 days, and a thermal cooling model fit suggests that SGR 1627-41 may have a hot core ($T_c ~ 2\times 10^8$ K). We find no clear correlation between flux and spectral hardness as found in other magnetars. We consider the quiescent X-ray luminosities of magnetars and the subset of rotation-powered pulsars with high magnetic fields ($B >~ 10^{13}$ G) in relation to their spin-inferred surface magnetic-field strength, and find a possible trend between the two quantities., Comment: 25 pages, 5 figures, 3 tables. Accepted for publication in ApJ

The return to the quiescent state of the Anomalous X-ray pulsar XTE J1810-197 following its 2003 outburst represents a unique opportunity to probe the surface emission properties of a magnetar. The quiescent emission of XTE J1810-197 is composed of two thermal components, one arising from the whole star surface, and the other from a small warm spot on it. By modeling the magnitude and shape of the pulse profile in narrow spectral bands, we have been able to constrain the physical characteristics and geometrical parameters of the system: the two angles that the line of sight and the spin axis make with respect to the warm spot axis (\psi\ and \xi\ respectively), the angular size of the spot, and the overall surface temperature distribution. Our modeling accounts for the general relativistic effects of gravitational redshift and light bending near the stellar surface, and allows for local anisotropic emission. We found that the surface temperature distribution on the neutron star is consistent with the expectations of a dipole magnetic field configuration; the local radiation requires a pencil-beamed emission pattern, suggesting the presence of a magnetized atmosphere. For a typical value of the radius, R=13 km, the viewing parameters (symmetric for an interchange between \psi\ and \xi), range from \psi=\xi=38 deg to (\psi,\xi)=(52 deg, 29 deg). These angles are consistent with those obtained by modeling the AXP in outburst, with uncertainty contours reduced by a factor of 2.5., Comment: 11 pages, 7 figures, Accepted 2011 July 26 by MNRAS

The temporal decay of the flux components of Transient Anomalous X-ray Pulsar XTE J1810-197 following its 2002 outburst presents a unique opportunity to probe the emission geometry of a magnetar. Toward this goal, we model the magnitude of the pulsar's modulation in narrow spectral bands over time. Following previous work, we assume that the post-outburst flux is produced in two distinct thermal components arising from a hot spot and a warm concentric ring. We include general relativistic effects on the blackbody spectra due to gravitational redshift and light bending near the stellar surface, which strongly depend on radius. This affects the model fits for the temperature and size of the emission regions. For the hot spot, the observed temporal and energy-dependent pulse modulation is found to require an anisotropic, pencil-beamed radiation pattern. We are able to constrain an allowed range for the angles that the line-of-sight (psi) and the hot spot pole (xi) make with respect to the spin-axis. Within errors, this is defined by the locus of points in the xi-psi plane that lie along the line (xi+beta(R))(psi+beta(R)) ~ constant, where beta(R) is a function of the radius R of the star. For a canonical value of R=12 km, the viewing parameters range from psi=xi=37 deg to (psi,xi)=(85 deg,15 deg). We discuss our results in the context of magnetar emission models., Comment: 8 pages, accepted to ApJ

We report on more than 7 years of monitoring of PSR J0537-6910, the 16 ms pulsar in the Large Magellanic Cloud, using data acquired with the RXTE. During this campaign the pulsar experienced 23 sudden increases in frequency (``glitches'') amounting to a total gain of over six ppm of rotation frequency superposed on its gradual spindown of d(nu)/d(t) = -2e-10 Hz/s. The time interval from one glitch to the next obeys a strong linear correlation to the amplitude of the first glitch, with a mean slope of about 400 days ppm (6.5 days per uHz), such that these intervals can be predicted to within a few days, an accuracy which has never before been seen in any other pulsar. There appears to be an upper limit of ~40 uHz for the size of glitches in_all_ pulsars, with the 1999 April glitch of J0537 as the largest so far. The change in the spindown of J0537 across the glitches, Delta(d(nu)/d(t)), appears to have the same hard lower limit of -1.5e-13 Hz/s, as, again, that observed in all other pulsars. The spindown continues to increase in the long term, d(d(nu)/d(t))/d(t) = -1e-21 Hz/s/s, and thus the timing age of J0537 (-0.5 nu d(nu)/d(t)) continues to decrease at a rate of nearly one year every year, consistent with movement of its magnetic moment away from its rotational axis by one radian every 10,000 years, or about one meter per year. J0537 was likely to have been born as a nearly-aligned rotator spinning at 75-80 Hz, with a |d(nu)/d(t)| considerably smaller than its current value of 2e-10 Hz/s. The pulse profile of J0537 consists of a single pulse which is found to be flat at its peak for at least 0.02 cycles., Comment: 54 pages, 12 figures. Accepted for publication in The Astrophysical Journal. Cleaner figure 2. V4 -- in line with version accepted by ApJ

We present Chandra ACIS observations of N157B, a young supernova remnant located in the 30 Doradus star-formation region of the LMC. This remnant contains the most energetic pulsar known (PSR J0537-6910), which is surrounded by a bright nonthermal nebula that likely represents a toroidal pulsar wind terminal shock observed edge-on. We confirm the non-thermal nature of the comet-shaped X-ray emission feature and show that the spectral steepening of this feature away from the pulsar is quantitatively consistent with synchrotron cooling of shocked pulsar wind particles flowing downstream at a bulk velocity close to the speed of light. Around the cometary nebula we unambiguously detect a thermal component, which accounts for about 1/3 of the total 0.5 - 10 keV flux from the remnant. This thermal component is distributed among various clumps of metal-enriched plasma embedded in the low surface brightness X-ray-emitting diffuse gas. The relative metal enrichment pattern suggests that the mass of the supernova progenitor is >~ 20M_sun. A comparison of the X-ray data with HST optical images suggests that the explosion site is close to a dense cloud, against which a reflection shock is launched. The interaction between this reflection shock and the nebula has likely produced both its cometary shape and the surrounding thermal emission enhancement. SNR N157B is apparently expanding into the hot low-density interior of a surrounding superbubble formed by the young OB association LH99, as revealed by Spitzer mid-infrared images. This scenario naturally explains the exceptionally large sizes of both the thermal and nonthermal components as well as the lack of an outer shell of the SNR. These results provide a rare glimpse into the SNR structure and evolution in a region of recent star-formation., Comment: Slightly revised to match the ApJ version. Fig.1 of high resolution available at http://astronomy.nju.edu.cn/~ygchen/paper/N157B/f1.eps.gz

PSR J0205+6449 is a young pulsar in the Crab-like pulsar wind nebula 3C 58 which is thought to be a result of the historical supernova SN1181 CE. The 65.7-ms pulsar is the second most energetic of the known Galactic pulsars and has been shown to be remarkably cool for its age, implying non-standard cooling processes in the neutron star core. We report on RXTE timing observations taken during AO7 and supplemented by monthly radio observations of the pulsar made with the Green Bank Telescope (GBT). The total duration covered with the timing solutions is 450 days. We measure very high levels of timing noise from the source and find evidence for a ``giant'' glitch of magnitude Delta nu/nu ~ 1x10^-6 that occurred in 2002 October. We have also measured the phase-resolved spectra of the pulsations and find them to be surprisingly hard, with photon indices Gamma = 0.84 +0.06/-0.15 for the main pulse and Gamma = 1.0 +0.4/-0.3 for the interpulse assuming an absorbed power-law model., Comment: 4 pages, 6 figures. To appear in the proceedings of X-ray Timing 2003: Rossi and Beyond, ed. P. Kaaret, F.K. Lamb, & J.H. Swank (Melville, NY: American Institute of Physics)

We present an X-ray and radio study of the wind nebula surrounding the central pulsar PSR J1811-1925 in the supernova remnant G11.2-0.3. Using high resolution data obtained with the Chandra X-ray observatory and with the VLA radio telescope we show the X-ray and radio emission is asymmetric around the pulsar, despite the latter's central position in the very circular shell. The new X-ray data allow us to separate the synchrotron emission of the pulsar wind nebula from the surrounding thermal emission and that from the pulsar itself. Based on X-ray data from two epochs, we observe temporal variation of the location of X-ray hot spots near the pulsar, indicating relativistic motion. We compare thermal emission observed within the shell, which may be associated with the forward shock of the pulsar wind nebula, to thermal emission from a nearby portion of the remnant shell, the temperature of which implies an expansion velocity consistent with the identification of the remnant with the historical event of 386 A.D. The measured X-ray and radio spectral indices of the nebula synchrotron emission are found to be consistent with a single synchrotron cooling break. The magnetic field implied by the break frequency is anomalously large given the apparent size and age of the nebula if a spherical morphology is assumed, but is consistent with a bipolar morphology., Comment: 14 pages, 7 figures, submitted to the Astrophysical Journal

We have observed the Galactic supernova remnant G16.7+0.1 for 13 ks using the EPIC cameras aboard the XMM-Newton X-ray Observatory, producing the first detection of the SNR outside of the radio band. G16.7+0.1 is one of the faintest radio synchrotron nebulae yet detected, although the core-to-shell flux ratio at 6 cm is typical of composite SNRs. The distance to the object is unknown. Our image is seriously contaminated by single-reflection arcs from the X-ray binary GX17+2, which lies just outside the field of view. Nonetheless, the remnant's synchrotron core is clearly detected. We report on the spectrum and intensity of the core emission as well as on our search for emission from the thermal shell, and describe the constraints these observations place on the SNR's distance, age, and central pulsar properties., Comment: 6 pages, 3 figures, to appear in "Neutron Stars in Supernova Remnants" (ASP Conference Proceedings), eds P.O. Slane and B.M. Gaensler

We report on X-ray observations of the young, energetic radio pulsar PSR J1105-6107 obtained using the Rossi X-ray Timing Explorer. Our timing analysis of Proportional Counter Array data finds no evidence for X-ray pulsations from this source. We set a new upper limit on the pulsed X-ray flux from the pulsar of 18% in the 2-10 keV band, for a duty cycle of 0.5. Our improved upper limit supports the hypothesis that the X-ray emission seen by the ASCA X-ray satellite in the direction of PSR J1105-6107 is the result of a pulsar-powered synchrotron nebula.

We report the results of ROSAT and ASCA X-ray observations of the supernova remnant N157B (or 30 Dor B, SNR 0539-69.1) in the Large Magellanic Cloud. For comparison, we also briefly describe the results on SNR 0540-69.3, the only confirmed Crab-like remnant in the Cloud. The X-ray emission from N157B can be decomposed into a bright comet-shaped feature, superimposed on a diffuse emission region of a dimension $\sim 20$ pc. The flat and nearly featureless spectrum of the remnant is distinctly different from those of young shell-like remnants, suggesting a predominantly Crab-like nature of N157B. Characterized by a power law with an energy slope $\sim 1.5$, the spectrum of N157B above $\sim 2$ keV is, however, considerably steeper than that of SNR 0540-69.3, which has a slope of $\sim 1.0$. At lower energies, the spectrum of N157B presents marginal evidence for emission lines, which if real most likely arise in hot gas of the diffuse emission region. The hot gas has a characteristic thermal temperature of 0.4-0.7 keV. No significant periodic signal is detected from N157B in the period range of $3 \times 10^{-3}-2000$ s. The pulsed fraction is $\lesssim 9%$ (99% confidence) in the $2-7$ keV range. We discuss the nature of the individual X-ray components. In particular, we suggest that the synchrotron radiation of relativistic particles from a fast-moving ($\sim 10^3 km s^{-1}$) pulsar explains the size, morphology, spectrum, and energetics of the comet-shaped X-ray feature. We infer the age of the remnant as $\sim 5 \times 10^3$ yrs. The lack of radio polarization of the remnant may be due to Faraday dispersion by foreground \ion{H}{2} gas., Comment: To be published in The Astrophysical Journal, 21 pages, plus 11 images in the PS, GIF, or jpeg format. Postscript files of images are available at http://www.astro.nwu.edu/astro/wqd/paper/n157b/

We present X-ray images of Tycho's supernova remnant in emission line features of Mg, Si, S, Ar, Ca, and Fe, plus the continuum, using data obtained by the imaging spectrometers onboard the ASCA X-ray satellite. All the images show the shell-like morphology characteristic of previously obtained broad-band X-ray images, but are clearly distinct from each other. We use image reconstruction techniques to achieve a spatial resolution of ~0.8'. Line intensity ratios are used to make inferences about the remnant's physical state, on average for the entire remnant, and with angular position around the rim. The average temperature (T) of the Si and S ejecta in the remnant is (0.8-1.1) X 10^7 K and the average ionization age (nt) is (0.8-1.3) X 10^11 cm^-3 s. For a constant nt, the observed relative brightness variations of Si and S line image profiles with azimuthalangle imply differences of roughly a factor of 1.3-1.8 in the temperature. We compare the radial brightness profiles of our images to simple geometrical models, and find that a spherical emitting geometry is favored over a torus. A spherical geometry is further supported by the absence of systematic Doppler shifts across the remnant. The radial fit results also suggest that some radial mixing of the ejecta has occurred. However, the azimuthally averaged Fe K image peaks at a markedly lower radius than the other images. The average Fe K / Fe L line intensity ratio and the position of the Fe K energy centroid support a T several times higher and an nt approx. a factor of ten lower than for the other elements, and imply that the Fe ejecta must have retained some of its stratification. Although many of the features in the 4-6 keV X-ray continuum correspond to those in the radio, there is no obvious correlation between the relative brightness in these bands., Comment: 17 pages plus 9 figures, LaTeX type, uses aas2pp4.sty. To appear in the Astrophysical Journal

We present a catalog of hard X-ray sources in a square-degree region surveyed by the Nuclear Spectroscopic Telescope Array (NuSTAR) in the direction of the Norma spiral arm. This survey has a total exposure time of 1.7 Ms, and the typical and maximum exposure depths are 50 ks and 1 Ms, respectively. In the area of deepest coverage, sensitivity limits of 5 x 10(exp -14) and 4 x 10(exp -14) ergs/s/sq cm in the 3-10 and 10-20 keV bands, respectively, are reached. Twenty-eight sources are firmly detected, and 10 are detected with low significance; 8 of the 38 sources are expected to be active galactic nuclei. The three brightest sources were previously identified as a low-mass X-ray binary, high-mass X-ray binary, and pulsar wind nebula. Based on their X-ray properties and multiwavelength counterparts, we identify the likely nature of the other sources as two colliding wind binaries, three pulsar wind nebulae, a black hole binary, and a plurality of cataclysmic variables (CVs). The CV candidates in the Norma region have plasma temperatures of approx. 10-20 keV, consistent with the Galactic ridge X-ray emission spectrum but lower than the temperatures of CVs near the Galactic center. This temperature difference may indicate that the Norma region has a lower fraction of intermediate polars relative to other types of CVs compared to the Galactic center. The NuSTAR logN-logS distribution in the 10-20keV band is consistent with the distribution measured by Chandra at 2-10 keV if the average source spectrum is assumed to be a thermal model with kT approx. =15 keV, as observed for the CV candidates.

We present the X-ray timing and spectral evolution of the Galactic Center magnetar SGR J1745-2900 (SGR*) 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 2.6 plus or minus 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 to 10 kiloelectronvolt flux has declined monotonically by a factor of approximately 2 over an 80-day period post-outburst accompanied by an approximately 20 percent decrease in the source's blackbody temperature, although there is evidence for both flux and kiloteslas having leveled 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*.

AE Aquarii is a cataclysmic variable with the fastest known rotating magnetized white dwarf (P(sub spin) = 33.08 s). Compared to many intermediate polars, AE Aquarii shows a soft X-ray spectrum with a very low luminosity (LX (is) approximately 10(exp 31) erg per second). We have analyzed overlapping observations of this system with the NuSTAR and the Swift X-ray observatories in 2012 September. We find the 0.5-30 keV spectra to be well fitted by either an optically thin thermal plasma model with three temperatures of 0.75(+0.18 / -0.45), 2.29(+0.96 / -0.82), and 9.33 (+6.07 / -2.18) keV, or an optically thin thermal plasma model with two temperatures of 1.00 (+0.34 / -0.23) and 4.64 (+1.58 / -0.84) keV plus a power-law component with photon index of 2.50 (+0.17 / -0.23). The pulse profile in the 3-20 keV band is broad and approximately sinusoidal, with a pulsed fraction of 16.6% +/- 2.3%. We do not find any evidence for a previously reported sharp feature in the pulse profile.

The X-ray observation of the supernova remnant N 157B is described. The Rosat High Resolution Imager (HRI) X-ray emission from the remnant was decomposed into point-like sources. The spectra showed abundance-enhanced neon and magnesium lines, indicating that the remnant originated in a massive progenitor. The flat and featureless data from the Advanced Satellite for Cosmology and Astrophysics (ASCA) confirm the Crab-like nature of the remnant. By interpreting both the thermal spectral component and the shell as representing the remnant's outer shock, the age of the remnant was estimated to be 4 x 10(exp 3) yr and the energy release approximately 2 x 10(exp 50) erg.

We report on a day-long ASCA broadband (1-10 keV) spectro-imaging observation of the X-ray emission from the Castor multibinary system. Significant flares were detected from both the flare star system YY Gem (Castor C) and from Castor AB located 73 sec away. Using an optimal viewing geometry and image restoration techniques, we are able to spatially resolve the emission from the two X-ray components. Broadband flare activity from Castor AB is confirmed, and quiescent flux is detected. The quiescent spectrum of YY Gem is a complex blend of emission lines across the ASCA bandpass which requires multitemperature components or two-temperature variable metal-poor abundances (approximately 5-10 below solar) to obtain a satisfactory fit to both the Mewe-Kaastra and Raymond-Smith models. The flare spectrum is consistent with an increase in the emissivity of the hotter component.

The ASCA Observatory provides the first capability to map a diffuse X-ray source in individually resolved emission lines. We have applied this capability to the Cas A supernova remnant, measuring the mean energies and equivalent widths of 14 line features in three distinct spatial regions. The northwest (NW) and southeast (SE) regions provide the large fraction of the X-ray emission at energies below 4 keV, and define the axis of a Doppler map of the remnant constructed by measuring the peak energy of the prominent Si-K feature. Using restoration techniques to obtain image quality equivalent to 30 sec full width at half maximum (FWHM), we present restored images in the two brightest line features, and in two continuum bands. An emission region in the west is increasingly prominent at higher energies, and its markedly lower metallicity and correlation with radio emission suggest that its X-ray emission has a larger component from fore-shocked material than do the NW and SE regions where the emission is presumed to be dominated by reverse-shocked material.