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The astrophysical journal 819(2), 156 (2016). doi:10.3847/0004-637X/819/2/156, We present coordinated multiwavelength observations of the bright, nearby BL Lacertae object Mrk 421 taken in 2013 January–March, involving GASP-WEBT, Swift, NuSTAR, Fermi-LAT, MAGIC, VERITAS, and other collaborations and instruments, providing data from radio to very high energy (VHE) γ-ray bands. NuSTAR yielded previously unattainable sensitivity in the 3–79 keV range, revealing that the spectrum softens when the source is dimmer until the X-ray spectral shape saturates into a steep ${\rm{\Gamma }}\approx 3$ power law, with no evidence for an exponential cutoff or additional hard components up to ~80 keV. For the first time, we observed both the synchrotron and the inverse-Compton peaks of the spectral energy distribution (SED) simultaneously shifted to frequencies below the typical quiescent state by an order of magnitude. The fractional variability as a function of photon energy shows a double-bump structure that relates to the two bumps of the broadband SED. In each bump, the variability increases with energy, which, in the framework of the synchrotron self-Compton model, implies that the electrons with higher energies are more variable. The measured multi band variability, the significant X-ray-to-VHE correlation down to some of the lowest fluxes ever observed in both bands, the lack of correlation between optical/UV and X-ray flux, the low degree of polarization and its significant (random) variations, the short estimated electron cooling time, and the significantly longer variability timescale observed in the NuSTAR light curves point toward in situ electron acceleration and suggest that there are multiple compact regions contributing to the broadband emission of Mrk 421 during low-activity states., Published by Univ.11032, Chicago, Ill. [u.a.]

We present the 3–8 keV and 8–24 keV number counts of active galactic nuclei (AGNs) identified in the Nuclear Spectroscopic Telescope Array (NuSTAR) extragalactic surveys. NuSTAR has now resolved 33%–39% of the X-ray background in the 8–24 keV band, directly identifying AGNs with obscuring columns up to ~10^(25) cm^(-2). In the softer 3–8 keV band the number counts are in general agreement with those measured by XMM-Newton and Chandra over the flux range 5 x 10^(-15) ≾S(3–8 keV)/erg s^(-1) cm^(-2) ≾10^(-12) probed by NuSTAR. In the hard 8–24 keV band NuSTAR probes fluxes over the range 2 x 10^(-14) ≾ S(8–24 keV)/ erg s^(-1) cm^(-2) ≾ 10^_12), a factor ~100 fainter than previous measurements. The 8–24 keV number counts match predictions from AGN population synthesis models, directly confirming the existence of a population of obscured and/or hard X-ray sources inferred from the shape of the integrated cosmic X-ray background. The measured NuSTAR counts lie significantly above simple extrapolation with a Euclidian slope to low flux of the Swift/BAT 15–55 keV number counts measured at higher fluxes (S(15–55 keV) ≳10^(−11) erg s^(-1) cm^(-2)), reflecting the evolution of the AGN population between the Swift/BAT local (z < 0.1) sample and NuSTAR's z ~ 1 sample. CXB synthesis models, which account for AGN evolution, lie above the Swift/BAT measurements, suggesting that they do not fully capture the evolution of obscured AGNs at low redshifts.

We report the first hard X-ray observations with NuSTAR of the BL Lac-type blazar PKS 2155-304, augmented with soft X-ray data from XMM-Newton and γ-ray data from the Fermi Large Area Telescope, obtained in 2013 April when the source was in a very low flux state. A joint NuSTAR and XMM spectrum, covering the energy range 0.5–60 keV, is best described by a model consisting of a log-parabola component with curvature β = 0.3_(-0.1)^(+0.2) and a (local) photon index 3.04 ± 0.15 at photon energy of 2 keV, and a hard power-law tail with photon index 2.2 ± 0.4. The hard X-ray tail can be smoothly joined to the quasi-simultaneous γ-ray spectrum by a synchrotron self-Compton component produced by an electron distribution with index p = 2.2. Assuming that the power-law electron distribution extends down to γ min = 1 and that there is one proton per electron, an unrealistically high total jet power of L_p ~ 10^(47) erg s^(−1) is inferred. This can be reduced by two orders of magnitude either by considering a significant presence of electron–positron pairs with lepton-to-proton ratio n_(e + e-)/n_p ~ 30, or by introducing an additional, low-energy break in the electron energy distribution at the electron Lorentz factor γ_(br1) ~ 100. In either case, the jet composition is expected to be strongly matter-dominated.

© 2016. The American Astronomical Society. All rights reserved.We present coordinated multiwavelength observations of the bright, nearby BL Lacertae object Mrk 421 taken in 2013 January-March, involving GASP-WEBT, Swift, NuSTAR, Fermi-LAT, MAGIC, VERITAS, and other collaborations and instruments, providing data from radio to very high energy (VHE) γ-ray bands. NuSTAR yielded previously unattainable sensitivity in the 3-79 keV range, revealing that the spectrum softens when the source is dimmer until the X-ray spectral shape saturates into a steep Γ ≈ 3 power law, with no evidence for an exponential cutoff or additional hard components up to ∼80 keV. For the first time, we observed both the synchrotron and the inverse-Compton peaks of the spectral energy distribution (SED) simultaneously shifted to frequencies below the typical quiescent state by an order of magnitude. The fractional variability as a function of photon energy shows a double-bump structure that relates to the two bumps of the broadband SED. In each bump, the variability increases with energy, which, in the framework of the synchrotron self-Compton model, implies that the electrons with higher energies are more variable. The measured multi band variability, the significant X-ray-to-VHE correlation down to some of the lowest fluxes ever observed in both bands, the lack of correlation between optical/UV and X-ray flux, the low degree of polarization and its significant (random) variations, the short estimated electron cooling time, and the significantly longer variability timescale observed in the NuSTAR light curves point toward in situ electron acceleration and suggest that there are multiple compact regions contributing to the broadband emission of Mrk 421 during low-activity states.

In core-collapse supernovae, titanium-44 (^(44)Ti) is produced in the innermost ejecta, in the layer of material directly on top of the newly formed compact object. As such, it provides a direct probe of the supernova engine. Observations of supernova 1987A (SN1987A) have resolved the 67.87- and 78.32–kilo–electron volt emission lines from decay of ^(44)Ti produced in the supernova explosion. These lines are narrow and redshifted with a Doppler velocity of ~700 kilometers per second, direct evidence of large-scale asymmetry in the explosion.

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.

We present broadband (radio, optical, and X-ray) light curves and spectra of the afterglows of four long-duration gamma-ray bursts (GRBs 090323, 090328, 090902B, and 090926A) detected by the Gamma-Ray Burst Monitor (GBM) and Large Area Telescope (LAT) instruments on the Fermi satellite. With its wide spectral bandpass, extending to GeV energies, Fermi is sensitive to GRBs with very large isotropic energy releases (10e54 erg). Although rare, these events are particularly important for testing GRB central-engine models. When combined with spectroscopic redshifts, our afterglow data for these four events are able to constrain jet collimation angles, the density structure of the circumburst medium, and both the true radiated energy release and the kinetic energy of the outflows. In agreement with our earlier work, we find that the relativistic energy budget of at least one of these events (GRB 090926A) exceeds the canonical value of 10e51 erg by an order of magnitude. Such energies pose a severe challenge for models in which the GRB is powered by a magnetar or neutrino-driven collapsar, but remain compatible with theoretical expectations for magneto-hydrodynamical (MHD) collapsar models. Our jet opening angles (theta) are similar to those found for pre-Fermi GRBs, but the large initial Lorentz factors (Gamma_0) inferred from the detection of GeV photons imply theta Gamma_0 ~ 70-90, values which are above those predicted in MHD models of jet acceleration. Finally, we find that these Fermi-LAT events preferentially occur in a low-density circumburst environment, and we speculate that this might result from the lower mass-loss rates of their lower-metallicity progenitor stars. Future studies of Fermi-LAT afterglows in the radio with the order-of-magnitude improvement in sensitivity offered by the EVLA should definitively establish the relativistic energy budgets of these events., Comment: 29 pages, 11 figures, submitted to ApJ. Comments welcome. v2 - Minor typos corrected.

We present the results of high resolution spectroscopy of the prototype polar AM Herculis observed with Chandra High Energy Transmission Grating. The X-ray spectrum contains hydrogen-like and helium-like lines of Fe, S, Si, Mg, Ne and O with several Fe L-shell emission lines. The forbidden lines in the spectrum are generally weak whereas the hydrogen-like lines are stronger suggesting that emission from a multi-temperature, collisionally ionized plasma dominates. The helium-like line flux ratios yield a plasma temperature of 2 MK and a plasma density 1 - 9 x10^12 cm^-3, whereas the line flux ratio of Fe XXVI to Fe XXV gives an ionization temperature of 12.4 +1.1 -1.4 keV. We present the differential emission measure distribution of AM Her whose shape is consistent with the volume emission measure obtained by multi-temperature APEC model. The multi-temperature plasma model fit to the average X-ray spectrum indicates the mass of the white dwarf to be ~1.15 M_sun. From phase resolved spectroscopy, we find the line centers of Mg XII, S XVI, resonance line of Fe XXV, and Fe XXVI emission modulated by a few hundred to 1000 km/s from the theoretically expected values indicating bulk motion of ionized matter in the accretion column of AM Her. The observed velocities of Fe XXVI ions are close to the expected shock velocity for a 0.6 M_sun white dwarf. The observed velocity modulation is consistent with that expected from a single pole accreting binary system., Comment: 6 figures, AASTEX style, accepted for publication in ApJ

We present results from four new broadband X-ray observations of the extreme ultraluminous X-ray source Holmberg IX X-1 (L$_{x}$ > 10$^{40}$ erg s$^{-1}$), performed by $\textit{Suzaku}$ and $\textit{Nu}$STAR in coordination. Combined with the archival data, we now have broadband observations of this remarkable source from six separate epochs. Two of these new observations probe lower fluxes than seen previously, allowing us to extend our knowledge of the broadband spectral variability exhibited. The spectra are well fit by two thermal blackbody components that dominate the emission below 10 keV, as well as a steep ($\Gamma$ $\sim$ 3.5) power-law tail that dominates above ~15 keV. Remarkably, while the 0.3–10.0 keV flux varies by a factor of ~3 between all these epochs, the 15–40 keV flux varies by only ~20%. Although the spectral variability is strongest in the ~1–10 keV band, both of the thermal components are required to vary when all epochs are considered. We also revisit the search for iron absorption features by leveraging the high-energy NuSTAR data to improve our sensitivity to extreme velocity outflows in light of the ultra-fast outflow recently detected in NGC 1313 X-1. Iron absorption from a similar outflow along our line of sight can be ruled out in this case. We discuss these results in the context of super-Eddington accretion models that invoke a funnel-like geometry for the inner flow, and propose a scenario in which we have an almost face-on view of a funnel that expands to larger radii with increasing flux, resulting in an increasing degree of geometrical collimation for the emission from intermediate-temperature regions., D.J.W. and M.J.M. acknowledge support from STFC through Ernest Rutherford fellowships, A.C.F. acknowledges support from ERC Advanced Grant 340442, and D.B. acknowledges financial support from the French Space Agency (CNES). This research has made use of data obtained with NuSTAR, a project led by Caltech, funded by NASA and managed by NASA/JPL, and has utilized the NUSTARDAS software package, jointly developed by the ASDC (Italy) and Caltech (USA). This research has also made use of data obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States, and with Suzaku, a collaborative mission between the space agencies of Japan (JAXA) and the USA (NASA).

We present broadband (radio, optical, and X-ray) light curves and spectra of the afterglows of four long-duration gamma-ray bursts (GRBs; GRBs 090323, 090328, 090902B, and 090926A) detected by the Gamma-Ray Burst Monitor and Large Area Telescope (LAT) instruments on the Fermi satellite. With its wide spectral bandpass, extending to GeV energies, Fermi is sensitive to GRBs with very large isotropic energy releases (1054 erg). Although rare, these events are particularly important for testing GRB central-engine models. When combined with spectroscopic redshifts, our afterglow data for these four events are able to constrain jet collimation angles, the density structure of the circumburst medium, and both the true radiated energy release and the kinetic energy of the outflows. In agreement with our earlier work, we find that the relativistic energy budget of at least one of these events (GRB 090926A) exceeds the canonical value of 1051 erg by an order of magnitude. Such energies pose a severe challenge for models in which the GRB is powered by a magnetar or a neutrino-driven collapsar, but remain compatible with theoretical expectations for magnetohydrodynamical collapsar models (e.g., the Blandford-Znajek mechanism). Our jet opening angles (θ) are similar to those found for pre-Fermi GRBs, but the large initial Lorentz factors (Γ0) inferred from the detection of GeV photons imply θΓ0 ≈ 70-90, values which are above those predicted in magnetohydrodynamic models of jet acceleration. Finally, we find that these Fermi-LAT events preferentially occur in a low-density circumburst environment, and we speculate that this might result from the lower mass-loss rates of their lower-metallicity progenitor stars. Future studies of Fermi-LAT afterglows at radio wavelengths with the order-of-magnitude improvement in sensitivity offered by the Extended Very Large Array should definitively establish the relativistic energy budgets of these events.

We present first results from a series of $\textit{NuSTAR}$ observations of the black hole X-ray binary V404 Cyg obtained during its summer 2015 outburst, primarily focusing on observations during the height of this outburst activity. The $\textit{NuSTAR}$ data show extreme variability in both the flux and spectral properties of the source. This is partly driven by strong and variable line-of-sight absorption, similar to previous outbursts. The latter stages of this observation are dominated by strong flares, reaching luminosities close to Eddington. During these flares, the central source appears to be relatively unobscured and the data show clear evidence for a strong contribution from relativistic reflection, providing a means to probe the geometry of the innermost accretion flow. Based on the flare properties, analogies with other Galactic black hole binaries, and also the simultaneous onset of radio activity, we argue that this intense X-ray flaring is related to transient jet activity during which the ejected plasma is the primary source of illumination for the accretion disk. If this is the case, then our reflection modeling implies that these jets are launched in close proximity to the black hole (as close as a few gravitational radii), consistent with expectations for jet launching models that tap either the spin of the central black hole, or the very innermost accretion disk. Our analysis also allows us to place the first constraints on the black hole spin for this source, which we find to be ${a}^{* }\gt 0.92$ (99% statistical uncertainty, based on an idealized lamp-post geometry)., D.J.W., P.G., and M.J.M. acknowledge support from STFC Ernest Rutherford fellowships (grant ST/J003697/2). K.P.M. acknowledges support from the Hintze Foundation. A.L.K. acknowledges support from NASA through an Einstein Postdoctoral Fellowship (grant number PF4-150125) awarded by the Chandra X-ray Center, operated by the Smithsonian Astrophysical Observatory for NASA under contract NAS8-03060. A.C.F. acknowledges support from ERC Advanced Grant 340442. L.N. wishes to acknowledge the Italian Space Agency (ASI) for Financial support by ASI/INAF grant I/037/12/0-011/13. This research has made use of data obtained with NuSTAR, a project led by Caltech, funded by NASA and managed by NASA/JPL, and has utilized the NUSTARDAS software package, jointly developed by the ASDC (Italy) and Caltech (USA). This research has also made use of data from AMI, which is supported by the ERC, and we thank the AMI staff for scheduling these radio observations.