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.