ABSTRACT 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 . 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 S(3-8 keV)/ probed by NuSTAR. In the hard 8-24 keV band NuSTAR probes fluxes over the range S(8-24 keV)/ , 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 ), reflecting the evolution of the AGN population between the Swift/BAT local ( ) sample and NuSTAR's 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.
ABSTRACT
Temperature measurements of galaxy clusters are used to determine their masses, which in turn are used to determine cosmological parameters. However, systematic differences between the ...temperatures measured by different telescopes imply a significant source of systematic uncertainty on such mass estimates. We perform the first systematic comparison between cluster temperatures measured with Chandra and NuSTAR. This provides a useful contribution to the effort of cross-calibrating cluster temperatures due to the harder response of NuSTAR compared with most other observatories. We measure average temperatures for eight clusters observed with NuSTAR and Chandra. We fit the NuSTAR spectra in a hard (3–10 keV) energy band, and the Chandra spectra in both the hard and a broad (0.6–9 keV) band. We fit a power-law cross-calibration model to the resulting temperatures. At a Chandra temperature of 10 keV, the average NuSTAR temperature was $(10.5\pm 3.7)$ and $(15.7\pm 4.6){{\ \rm per\ cent}}$, lower than Chandra for the broad- and hard-band fits, respectively. We explored the impact of systematics from background modelling and multiphase temperature structure of the clusters, and found that these did not affect our results. Our sample are primarily merging clusters with complex thermal structures so are not ideal calibration targets. However, given the harder response of NuSTAR, it would be expected to measure a higher average temperature than Chandra for a non-isothermal cluster, so we interpret our measurement as a lower limit on the difference in temperatures between NuSTAR and Chandra.
We present 15 high-mass X-ray binary (HMXB) candidates in the disk of M31 for which we are able to infer compact object type, spectral type of the donor star, and age using multiwavelength ...observations from NuSTAR, Chandra, and the Hubble Space Telescope. The hard X-ray colors and luminosities from NuSTAR permit the tentative classification of accreting X-ray binary systems by compact object type, distinguishing black hole from neutron star systems. We find hard-state black holes, pulsars, and non-magnetized neutron stars associated with optical point-source counterparts with similar frequency. We also find nine non-magnetized neutron stars coincident with globular clusters and an equal number of pulsars with and without point-source optical counterparts. We perform spectral energy distribution (SED) fitting for the most likely optical counterparts to the HMXB candidates, finding seven likely high-mass stars and one possible red helium-burning star. The remaining seven HMXB optical counterparts have poor SED fits, so their companion stars remain unclassified. Using published star formation histories, we find that the majority of HMXB candidates-X-ray sources with UV-bright point-source optical counterpart candidates-are found in regions with star formation bursts less than 50 Myr ago, and three are associated with young stellar ages (<10 Myr). This is consistent with similar studies of HMXB populations in the Magellanic Clouds, M33, NGC 300, and NGC 2403.
The binary transition-metal monophosphides CrP, MnP, FeP, and CoP have been studied with X-ray photoelectron spectroscopy. The shifts in phosphorus 2p3/2 core line binding energies relative to that ...of elemental phosphorus indicated that the degree of ionicity of the metal−phosphorus bond decreases on progressing from CrP to CoP. The metal 2p3/2 core line binding energies differ only slightly and show similar line shapes to those of the elemental metals, reaffirming the notion that these transition-metal phosphides have considerable metallic character. The satellite structure observed in the Co 2p3/2 X-ray photoelectron spectra of Co metal and CoP was examined by reflection electron energy loss spectroscopy and has been attributed to plasmon loss, not final state effects as has been previously suggested. Valence-band spectra of the transition-metal phosphides agree well with the density of states profiles determined from band structure calculations. The electron populations of the different electronic states were extracted from the fitted valence-band spectra, and these confirm the presence of strong M−P and weak P−P bonding interactions. Atomic charges determined from the P 2p core line spectra and the fitted valence-band spectra support the approximate formulation M1+P1- for these phosphides.
Abstract
We present an updated catalog of
StrayCats
(a catalog of NuSTAR stray light observations of X-ray sources) that includes nearly 18 additional months of observations.
StrayCats
v2 has an ...added 53 sequence IDs, 106 rows, and three new identified stray light (SL) sources in comparison to the original catalog. The total catalog now has 489 unique sequence IDs, 862 entries, and 83 confirmed
StrayCats
sources. Additionally, we provide new resources for the community to gauge the utility and spectral state of the source in a given observation. We have created long-term light curves for each identified SL source using MAXI and Swift/BAT data when available. Further, source extraction regions for 632 identified SL observations were created and are available to the public. In this paper we present an overview of the updated catalog and new resources for each identified
StrayCats
SL source.
A STRONG MERGER SHOCK IN ABELL 665 Dasadia, S.; Sun, M.; Sarazin, C. ...
Astrophysical journal. Letters,
03/2016, Volume:
820, Issue:
1
Journal Article
Peer reviewed
Open access
ABSTRACT Deep (103 ks) Chandra observations of Abell 665 have revealed rich structures in this merging galaxy cluster, including a strong shock and two cold fronts. The newly discovered shock has a ...Mach number of M = 3.0 0.6, propagating in front of a cold disrupted cloud. This makes Abell 665 the second cluster, after the Bullet cluster, where a strong merger shock of has been detected. The shock velocity from jump conditions is consistent with (2.7 0.7) × 103 km s−1. The new data also reveal a prominent southern cold front with potentially heated gas ahead of it. Abell 665 also hosts a giant radio halo. There is a hint of diffuse radio emission extending to the shock at the north, which needs to be examined with better radio data. This new strong shock provides a great opportunity to study the re-acceleration model with the X-ray and radio data combined.
Nearby galaxy surveys have long classified X-ray binaries (XRBs) by the mass category of their donor stars (high-mass and low-mass). The NuSTAR observatory, which provides imaging data at E > 10 keV, ...has enabled the classification of extragalactic XRBs by their compact object type: neutron star (NS) or black hole (BH). We analyzed NuSTAR/Chandra/XMM-Newton observations from a NuSTAR-selected sample of 12 galaxies within 5 Mpc having stellar masses (M ) 107-11M and star formation rates (SFRs) 0.01-15 M yr−1. We detected 128 NuSTAR sources to a sensitivity of 1038 erg s−1. Using NuSTAR color-intensity and color-color diagrams we classified 43 of these sources as candidate NSs and 47 as candidate BHs. We further subdivide BHs by accretion states (soft, intermediate, and hard) and NSs by weak (Z/Atoll) and strong (accreting pulsar) magnetic field. Using eight normal (Milky Way-type) galaxies in the sample, we confirm the relation between the SFR and galaxy X-ray point source luminosity in the 4-25 and 12-25 keV energy bands. We also constrained galaxy X-ray point source luminosity using the relation LX = M + βSFR, finding agreement with previous work. The X-ray luminosity function (XLF) of all sources in the 4-25 and 12-25 keV energy bands matches the = 1.6 slope for high-mass XRBs. We find that NS XLFs suggest a decline beginning at the Eddington limit for a 1.4 M NS, whereas the BH fraction shows an approximate monotonic increase in the 4-25 and 12-25 keV energy bands. We calculate the overall ratio of BH to NS to be 1 for 4-25 keV and 2 for 12-25 keV.
Prior to the launch of NuSTAR, it was not feasible to spatially resolve the hard (E > 10 keV) emission from galaxies beyond the Local Group. The combined NuSTAR data set, comprised of three ~165 ks ...observations, allows spatial characterization of the hard X-ray emission in the galaxy NGC 253 for the first time. As a follow up to our initial study of its nuclear region, we present the first results concerning the full galaxy from simultaneous NuSTAR, Chandra, and Very Long Baseline Array monitoring of the local starburst galaxy NGC 253. Above ~10 keV, nearly all the emission is concentrated within 100" of the galactic center, produced almost exclusively by three nuclear sources, an off-nuclear ultraluminous X-ray source (ULX), and a pulsar candidate that we identify for the first time in these observations. We detect 21 distinct sources in energy bands up to 25 keV, mostly consisting of intermediate state black hole X-ray binaries. The global X-ray emission of the galaxy-dominated by the off-nuclear ULX and nuclear sources, which are also likely ULXs-falls steeply (photon index > ~3) above lOkeV, consistent with other NuSTAR-observed ULXs, and no significant excess above the background is detected at E > 40 keV. We report upper limits on diffuse inverse Compton emission for a range of spatial models. For the most extended morphologies considered, these hard X-ray constraints disfavor a dominant inverse Compton component to explain the gamma -ray emission detected with Fermi and H.E.S.S. If NGC 253 is typical of starburst galaxies at higher redshift, their contribution to the E > 10 keV cosmic X-ray background is < 1%.
ABSTRACT We present the results from sensitive, multi-epoch NuSTAR observations of the late-type star-forming galaxy M83 (d = 4.6 Mpc). This is the first investigation to spatially resolve the hard ( ...E > 10 keV) X-ray emission of this galaxy. The nuclear region and ∼20 off-nuclear point sources, including a previously discovered ultraluminous X-ray source, are detected in our NuSTAR observations. The X-ray hardnesses and luminosities of the majority of the point sources are consistent with hard X-ray sources resolved in the starburst galaxy NGC 253. We infer that the hard X-ray emission is most likely dominated by intermediate accretion state black hole binaries and neutron star low-mass X-ray binaries (Z-sources). We construct the X-ray binary luminosity function (XLF) in the NuSTAR band for an extragalactic environment for the first time. The M83 XLF has a steeper XLF than the X-ray binary XLF in NGC 253, which is consistent with previous measurements by Chandra at softer X-ray energies. The NuSTAR integrated galaxy spectrum of M83 drops quickly above 10 keV, which is also seen in the starburst galaxies NGC 253, NGC 3310, and NGC 3256. The NuSTAR observations constrain any active galactic nucleus (AGN) to be either highly obscured or to have an extremely low luminosity of 1038 erg s−1 (10-30 keV), implying that it is emitting at a very low Eddington ratio. An X-ray point source that is consistent with the location of the nuclear star cluster with an X-ray luminosity of a few times 1038 erg s−1 may be a low-luminosity AGN but is more consistent with being an X-ray binary.
We report the identification of a bright hard X-ray source dominating the M31 bulge above 25 keV from a simultaneous NuSTAR-Swift observation. We find that this source is the counterpart to Swift ...J0042.6+4112, which was previously detected in the Swift BAT All-sky Hard X-ray Survey. This Swift BAT source had been suggested to be the combined emission from a number of point sources; our new observations have identified a single X-ray source from 0.5 to 50 keV as the counterpart for the first time. In the 0.5-10 keV band, the source had been classified as an X-ray Binary candidate in various Chandra and XMM-Newton studies; however, since it was not clearly associated with Swift J0042.6+4112, the previous E < 10 keV observations did not generate much attention. This source has a spectrum with a soft X-ray excess (kT ∼ 0.2 keV) plus a hard spectrum with a power law of and a cutoff around 15-20 keV, typical of the spectral characteristics of accreting pulsars. Unfortunately, any potential pulsation was undetected in the NuSTAR data, possibly due to insufficient photon statistics. The existing deep HST images exclude high-mass (>3 ) donors at the location of this source. The best interpretation for the nature of this source is an X-ray pulsar with an intermediate-mass (<3 ) companion or a symbiotic X-ray binary. We discuss other possibilities in more detail.
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