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In core-collapse supernovae, titanium-44 (44Ti) 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 44Ti 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 Soft X-ray focusing Telescope (SXT), India’s first X-ray telescope based on the principle of grazing incidence, was launched aboard the AstroSat and made operational on October 26, 2015. X-rays in the energy band of 0.3–8.0 keV are focussed on to a cooled charge coupled device thus providing medium resolution X-ray spectroscopy of cosmic X-ray sources of various types. It is the most sensitive X-ray instrument aboard the AstroSat. In its first year of operation, SXT has been used to observe objects ranging from active stars, compact binaries, supernova remnants, active galactic nuclei and clusters of galaxies in order to study its performance and quantify its characteriztics. Here, we present an overview of its design, mechanical hardware, electronics, data modes, observational constraints, pipeline processing and its in-orbit performance based on preliminary results from its characterization during the performance verification phase.

The observation of non-uniformly distributed titanium emission in the interior of Cassiopeia A, a core-collapse supernova, is an indicator of asymmetries in the stellar explosion and provides strong evidence for the development of low-mode convective instabilities in such supernovae. Cassiopeia A — remnant of an asymmetric explosion Most simulations of stellar core collapse events indicate that the explosions are asymmetric, but the resulting shapes differ in the various models. Brian Grefenstette et al . analysed the distribution of radioactive titanium-44 in Cassiopeia A, a young core-collapse remnant, as a proxy for explosion asymmetry. They report a degree of non-uniform distribution in the unshocked interior of Cas A greater than that expected from a spherical explosion, yet not as pronounced as would follow a highly bipolar explosion. On the basis of these findings, the authors conclude that the type of explosion for the Cas A core-collapse explosion was part-way between the two extremes of asymmetry. 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 1 , directly probes the explosion asymmetries. Cassiopeia A is a young 2 , nearby 3 , core-collapse 4 remnant from which 44 Ti emission has previously been detected 5 , 6 , 7 , 8 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 9 , from optical light echoes 10 , and from jet-like features seen in the X-ray 11 and optical 12 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.

The Soft X-ray focusing Telescope (SXT) is a moderate-resolution X-ray imaging spectrometer supplementing the ultraviolet and hard X-ray payloads for broadband studies of cosmic sources with AstroSat. Well suited for observing bright X-ray sources, SXT observations of nearby active galactic nuclei (AGN), binary star systems with compact companions, active stars, etc. are producing long soft X-ray light curves and high-quality spectra. The strong X-ray variability and multiple spectral components exhibited by SXT observations of nearby Seyfert 1 galaxies show excellent promise to probe accretion disks and central engines in AGN through multi-band variability and spectroscopy.

DSRI has initiated a development program of CZT X-ray and gamma raydetectors employing strip readout techniques. A dramatic improvement ofthe energy response was found operating the detectors as so-called driftdetectors. For the electronic readout, modern ASIC chips wereinvestigated. Modular design and the low power electronics will make largearea detectors using the drift strip method feasible. The performance of aprototype CZT system will be presented and discussed. One such detector system has been proposed for future space missions: TheX-Ray Imager (XRI) on the Atmospheric X-ray Observatory (AXO), whichis a mission proposed to the Danish Small Satellite Program and is dedicatedto observations of X-ray generating processes in the Earth's atmosphere. Ofspecial interest will be simultaneous optical and X-ray observations of spritesthat are flashes appearing directly above an active thunderstorm system.Additional objective is a detailed mapping of the auroral X-ray and opticalemission. XRI comprises a coded mask and a 20 cm × 40 cm CZTdetector array covering an energy range from 5 to 200 keV.[PUBLICATION ABSTRACT]

Taking a different look at a familiar star may still yield surprises. Boggs et al. trained the x-ray vision of the NuSTAR observatory on the well-studied supernova 1987A. Core-collapse explosions such as SN 1987A produce a titanium isotope, 44 Ti, whose radioactive decay yields hard x-ray emission lines. All the emission associated with 44Ti appears to be from material moving toward us, with none moving away. This implies that the explosion was not symmetric. These findings help to explain the mechanics of SN 1987A and of core-collapse supernovae in general. Science , this issue p. 670 Asymmetric signatures of radioactive decay are seen from a metal deep within a supernova. 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.

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 ^sup 44^Ti, synthesized in an exploding star near the boundary between material falling back on to 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 ^sup 44^Ti emission has previously been detected but not imaged. Asymmetries in the explosion have been indirectly inferred from a high ratio of observed ^sup 44^Ti emission to estimated ^sup 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 ^sup 44^Ti in Cassiopeia A. This may explain the unexpected lack of correlation between the ^sup 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 in stabilities in core-collapse supernovae. [PUBLICATION ABSTRACT]

We present the 3-8 keV and 8-24 keV number counts of active galactic nuclei (AGN) identified in the NuSTAR extragalactic surveys. NuSTAR has now resolved 33-39% of the X-ray background in the 8-24 keV band, directly identifying AGN with obscuring columns up to approximately 1e25 /cm2. 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 5e-15 < S(3 - 8 keV)/(erg/cm2/s) < 1e-12 probed by NuSTAR. In the hard 8-24 keV band NuSTAR probes fluxes over the range 2e-14 < S(8-24 keV)/(erg/cm2/s) < 1e-12, a factor of approximately 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) > 1e-11 erg/cm2/s, 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 AGN at low redshifts.