The Nuclear Spectroscopic Telescope Array (NuSTAR) mission, launched on 2012 June 13, is the first focusing high-energy X-ray telescope in orbit. NuSTAR operates in the band from 3 to 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 100-fold improvement in sensitivity over the collimated or coded mask instruments that have operated in this bandpass. Using its unprecedented combination of sensitivity and spatial and spectral resolution, NuSTAR will pursue five primary scientific objectives: (1) probe obscured active galactic nucleus (AGN) activity out to the

Asymmetry is required by most numerical simulations of stellar core-collapse explosions, but the form it takes differs significantly among models. The spatial distribution of radioactive (44)Ti, synthesized in an exploding star near the boundary between material falling back onto the collapsing core and that ejected into the surrounding medium, directly probes the explosion asymmetries. Cassiopeia A is a young, nearby, core-collapse remnant from which (44)Ti emission has previously been detected but not imaged. Asymmetries in the explosion have been indirectly inferred from a high ratio of observed (44)Ti emission to estimated (56)Ni emission, from optical light echoes, and from jet-like features seen in the X-ray and optical ejecta. Here we report spatial maps and spectral properties of the (44)Ti in Cassiopeia A. This may explain the unexpected lack of correlation between the (44)Ti and iron X-ray emission, the latter being visible only in shock-heated material. The observed spatial distribution rules out symmetric explosions even with a high level of convective mixing, as well as highly asymmetric bipolar explosions resulting from a fast-rotating progenitor. Instead, these observations provide strong evidence for the development of low-mode convective instabilities in core-collapse supernovae.

We use a combined 1.2 Ms of NuSTAR observations of M31 to search for X-ray lines from sterile neutrino dark matter decay. For the first time in a NuSTAR analysis, we consistently take into account the signal contribution from both the focused and unfocused fields of view. We also reduce the modeling systematic uncertainty by performing spectral fits to each observation individually and statistically combining the results, instead of stacking the spectra. We find no evidence of unknown lines, and thus derive limits on the sterile neutrino parameters. Our results place stringent constraints for dark matter masses 12 keV, which reduces the available parameter space for sterile neutrino dark matter produced via neutrino mixing (e.g., in the νMSM) by approximately one-third. Additional NuSTAR observations, together with improved low-energy background modeling, could probe the remaining parameter space in the future. Lastly, we also report model-independent limits on generic dark matter decay rates and annihilation cross sections. I.via a small mixing with active neutrinos [12], which may be enhanced by the presence of primordial lepton asymmetry [13]. As the mixing angle determines both the abundance and decay rate, there is a finite window in the mass-mixing angle parameter plane in which sterile neutrinos could constitute the full DM abundance, thus allowing this scenario to be fully testable. Closing this window would imply additional physics and production mechanisms are needed to make sterile neutrinos a viable DM candidate [14][15][16][17][18][19][20][21]. The existence of sterile neutrino DM could provide strong clues for explaining neutrino mass and baryogenesis [22], such as the scenario advocated in the νMSM model [23][24][25][26].Due to several sensitive X-ray instruments, such as Chandra, Suzaku, XMM-Newton, and INTEGRAL, stringent constraints on X-ray line emission have been obtained using many different observations (e.g., Refs. [27][28][29][30][31][32][33]). Interest in these topics was heightened with the tentative detection of a 3.5-keV line from cluster observations [34], which was followed up by many observational studies . The nature of this line is still inconclusive. The line could be a signature of sterile neutrino DM [57] or other candidates [58][59][60][61][62]. However, as the line flux is weak, astrophysical modeling systematics [37,41] or new astrophysical processes [63,64] could also be the explanation. New detectors [44,56,65,66] or techniques, such as velocity spectroscopy [67,68], are likely required to fully determine its nature. (Recently, Ref. [69] claims that blank-sky observations with XMM-Newton disfavor the DM interpretation of the 3.5-keV line. On the other hand, Ref. [70] claims detection of the 3.5 keV line in the Milky Way halo up to 35 • with XMM-Newton, and arXiv:1901.01262v2 [astro-ph.HE]

The distribution of elements produced in the innermost layers of a supernova explosion is a key diagnostic for studying the collapse of massive stars. Here we present the results of a 2.4 Ms NuSTAR observing campaign aimed at studying the supernova remnant Cassiopeia A (Cas A). We perform spatially resolved spectroscopic analyses of the 44 Ti ejecta, which we use to determine the Doppler shift and thus the three-dimensional (3D) velocities of the 44 Ti ejecta. We find an initial 44 Ti mass of (1.54±0.21) ×10 −4 M e , which has a present-day average momentum direction of 340°±15°projected onto the plane of the sky (measured clockwise from celestial north) and is tilted by 58°±20°into the plane of the sky away from the observer, roughly opposite to the inferred direction of motion of the central compact object. We find some 44 Ti ejecta that are clearly interior to the reverse shock and some that are clearly exterior to it. Where we observe 44 Ti ejecta exterior to the reverse shock we also see shock-heated iron; however, there are regions where we see iron but do not observe 44 Ti. This suggests that the local conditions of the supernova shock during explosive nucleosynthesis varied enough to suppress the production of 44 Ti by at least a factor of two in some regions, even in regions that are assumed to be the result of processes like α-rich freezeout that should produce both iron and titanium.

To provide the census of the sources contributing to the X-ray background peak above 10 keV, NuSTAR is performing extragalactic surveys using a three-tier "wedding cake" approach. We present the NuSTAR survey of the COSMOS field, the medium sensitivity and medium area tier, covering 1.7 deg 2 and overlapping with both Chandra and XMM-Newton data. This survey consists of 121 observations for a total exposure of ∼3 Ms. To fully exploit these data, we developed a new detection strategy, carefully tested through extensive simulations. The survey sensitivity at 20% completeness is 5.9, 2.9 and 6.4 × 10 −14 erg cm −2 s −1 in the 3-24 keV, 3-8 keV and 8-24 keV bands, respectively. By combining detections in 3 bands, we have a sample of 91 NuSTAR sources with 10 42 -10 45.5 erg s −1 luminosities and redshift z=0.04-2.5. Thirty two sources are detected in the 8-24 keV band with fluxes ∼100 times fainter than sources detected by Swift-BAT. Of the 91 detections, all but four are associated with a Chandra and/or XMM-Newton point-like counterpart. One source is associated with an extended lower energy X-ray source. We present the X-ray (hardness ratio and luminosity) and optical-to-X-ray properties. The observed fraction of candidate Compton-thick AGN measured from the hardness ratio is between 13%-20%. We discuss the spectral properties of NuSTAR J100259+0220.6 (ID 330) at z=0.044, with the highest hardness ratio in the entire sample. The measured column density exceeds 10 24 cm −2 , implying the source is Compton-thick. This source was not previously recognized as such without the >10 keV data.

High-resolution X-ray spectroscopy with Hitomi was expected to resolve the origin of the faint unidentified » E 3.5 keV emission line reported in several low-resolution studies of various massive systems, such as galaxies and clusters, including the Perseus cluster. We have analyzed the Hitomi first-light observation of the Perseus cluster. The emission line expected for Perseus based on the XMM-Newton signal from the large cluster sample under the dark matter decay scenario is too faint to be detectable in the Hitomi data. However, the previously reported 3.5 keV flux from Perseus was anomalously high compared to the sample-based prediction. We find no unidentified line at the reported high flux level. Taking into account the XMM measurement uncertainties for this region, the inconsistency with Hitomi is at a 99% significance for a broad dark matter line and at 99.7% for a narrow line from the gas. We do not find anomalously high fluxes of the nearby faint K line or the Ar satellite line that were proposed as explanations for the earlier 3.5 keV detections. We do find a hint of a broad excess near the energies of high-n transitions of S XVI (  E 3.44 keV rest-frame)-a possible signature of charge exchange in the molecular nebula and another proposed explanation for the unidentified line. While its energy is consistent with XMM pn detections, it is unlikely to explain the MOS signal. A confirmation of this interesting feature has to wait for a more sensitive observation with a future calorimeter experiment.

We present high-energy (3-30 keV) NuSTAR observations of the nearest quasar, the ultraluminous infrared galaxy (ULIRG) Markarian 231 (Mrk 231), supplemented with new and simultaneous low-energy (0.5-8 keV) data from Chandra. The source was detected, though at much fainter levels than previously reported, likely due to contamination in the large apertures of previous non-focusing hard X-ray telescopes. The full band (0.5-30 keV) X-ray spectrum suggests the active galactic nucleus (AGN) in Mrk 231 is absorbed by a patchy and Compton-thin (N H ∼ 1.2 +0.3 −0.3 × 10 23 cm −2 ) column. The intrinsic X-ray luminosity (L 0.5−30 keV ∼ 1.0 × 10 43 erg s −1 ) is extremely weak relative to the bolometric luminosity where the 2-10 keV to bolometric luminosity ratio is ∼0.03% compared to the typical values of 2%-15%. Additionally, Mrk 231 has a low X-ray-to-optical power law slope (α OX ∼ −1.7). It is a local example of a low-ionization broad absorption line quasar that is intrinsically X-ray weak. The weak ionizing continuum may explain the lack of mid-infrared [O iv], [Ne v], and [Ne vi] fine-structure emission lines which are present in sources with otherwise similar AGN properties. We argue that the intrinsic X-ray weakness may be a result of the super-Eddington accretion occurring in the nucleus of this ULIRG, and may also be naturally related to the powerful wind event seen in Mrk 231, a merger remnant escaping from its dusty cocoon.

The low-mass X-ray binary Cen X-4 is the brightest and closest (<1.2 kpc) quiescent neutron star transient. Previous 0.5-10 keV X-ray observations of Cen X-4 in quiescence identified two spectral components: soft thermal emission from the neutron star atmosphere and a hard power-law tail of unknown origin. We report here on a simultaneous observation of Cen X-4 with NuSTAR (3-79 keV) and XMM-Newton (0.3-10 keV) in 2013 January, providing the first sensitive hard X-ray spectrum of a quiescent neutron star transient. The 0.3-79 keV luminosity was 1.1 × 10 33 D 2 kpc erg s −1 , with ≃60% in the thermal component. We clearly detect a cutoff of the hard spectral tail above 10 keV, the first time such a feature has been detected in this source class. We show that thermal Comptonization and synchrotron shock origins for the hard X-ray emission are ruled out on physical grounds. However, the hard X-ray spectrum is well fit by a thermal bremsstrahlung model with kT e =18 keV, which can be understood as arising either in a hot layer above the neutron star atmosphere or in a radiativelyinefficient accretion flow (RIAF). The power-law cutoff energy may be set by the degree of Compton cooling of the bremsstrahlung electrons by thermal seed photons from the neutron star surface. Lower thermal luminosities should lead to higher (possibly undetectable) cutoff energies. We compare Cen X-4's behavior with the PSR J1023+0038, IGR J18245−2452, and XSS J12270−4859, which have shown transitions between LMXB and radio pulsar modes at a similar X-ray luminosity.