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We present X-ray observations of novae V2491 Cyg and KT Eri about 9 years post-outburst, of the dwarf nova and post-nova candidate EY Cyg, and of a VY Scl variable. The first three objects were observed with XMM-Newton, KT Eri also with the Chandra ACIS-S camera, V794 Aql with the Chandra ACIS-S camera and High Energy Transmission Gratings. The two recent novae, similar in outburst amplitude and light curve, appear very different at quiescence. Assuming half of the gravitational energy is irradiated in X-rays, V2491 Cyg is accreting at $dot{m}=1.4times10^{-9}-10^{-8}M_odot/yr$, while for KT Eri, $dot{m}<2times10^{-10}M_odot/yr$. V2491 Cyg shows signatures of a magnetized WD, specifically of an intermediate polar. A periodicity of ~39 minutes, detected in outburst, was still measured and is likely due to WD rotation. EY Cyg is accreting at $dot{m}sim1.8times10^{-11}M_odot/yr$, one magnitude lower than KT Eri, consistently with its U Gem outburst behavior and its quiescent UV flux. The X-rays are modulated with the orbital period, despite the systems low inclination, probably due to the X-ray flux of the secondary. A period of ~81 minutes is also detected, suggesting that it may also be an intermediate polar. V794 Aql had low X-ray luminosity during an optically high state, about the same level as in a recent optically low state. Thus, we find no clear correlation between optical and X-ray luminosity: the accretion rate seems unstable and variable. The very hard X-ray spectrum indicates a massive WD.
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We present a new series of supernova neutrino light curves and spectra calculated by numerical simulations for a variety of progenitor stellar masses (13-50Msolar) and metallicities (Z = 0.02 and 0.004), which would be useful for a broad range of supernova neutrino studies, e.g., simulations of future neutrino burst detection by underground detectors, or theoretical predictions for the relic supernova neutrino background. To follow the evolution from the onset of collapse to 20 s after the core bounce, we combine the results of neutrino-radiation hydrodynamic simulations for the early phase and quasi-static evolutionary calculations of neutrino diffusion for the late phase, with different values of shock revival time as a parameter that should depend on the still unknown explosion mechanism. We here describe the calculation methods and basic results including the dependence on progenitor models and the shock revival time. The neutrino data are publicly available electronically.
Blazars research is one of the hot topics of contemporary extra-galactic astrophysics. That is because these sources are the most abundant type of extra-galactic gamma-ray sources and are suspected to play a central role in multi-messenger astrophysics. We have used swift_xrtproc, a tool to carry out an accurate spectral and photometric analysis of the Swift-XRT data of all blazars observed by Swift at least 50 times between December 2004 and the end of 2020. We present a database of X-ray spectra, best-fit parameter values, count-rates and flux estimations in several energy bands of over 31,000 X-ray observations and single snapshots of 65 blazars. The results of the X-ray analysis have been combined with other multi-frequency archival data to assemble the broad-band Spectral Energy Distributions (SEDs) and the long-term light-curves of all sources in the sample. Our study shows that large X-ray luminosity variability on different timescales is present in all objects. Spectral changes are also frequently observed with a harder-when-brighter or softer-when-brighter behavior depending on the SED type of the blazars. The peak energy of the synchrotron component nu_peak in the SED of HBL blazars, estimated from the log-parabolic shape of their X-ray spectra, also exhibits very large changes in the same source, spanning a range of over two orders of magnitude in Mrk421 and Mrk501, the objects with the best data sets in our sample.
Two observations of V959 Mon, done using the Chandra X-ray gratings during the late outburst phases (2012 September and December), offer extraordinary insight into the physics and chemistry of this Galactic ONe nova. the X-ray flux was 1.7 x 10(-11) erg/cm(2)/s and 8.6 x 10(-12) erg/cm(2)/s, respectively at the two epochs. The first result, coupled with electron density diagnostics and compared with published optical and ultraviolet observations, indicates that most likely in 2012 September the X-rays originate from a very small fraction of the ejecta, concentrated in very dense clumps. We obtained a fairly good fit to the September spectrum with a model of plasma in collisional ionization equilibrium (CIE) with two components; one at a temperature of 0.78 keV, blueshifted by 710-930 km/s, the other at a temperature of 4.5 keV, mostly contributing to the high-energy continuum. However, we cannot rule out a range of plasma temperatures between these two extremes. In December, the central white dwarf (WD) became visible in X-rays. We estimate an effective temperature of about 680,000 K, consistent with a WD mass ~1.1 M(sol). The WD flux is modulated with the orbital period, indicating high inclination, and two quasi-periodic modulations with hour timescales were also observed. No hot plasma component with temperature above 0.5 keV was observed in December, and the blue-shifted component cooled to kT~0.45 keV. Additionally, new emission lines due to a much cooler plasma appeared, which were not observed two months earlier. We estimate abundances and yields of elements in the nova wind that cannot be measured in the optical spectra and confirm the high Ne abundance previously derived for this nova. We also find high abundance of Al, 230 times the solar value, consistently with the prediction that ONe novae contribute to at least 1/3rd of the Galactic yield of Al(26).
X-ray grating spectra have opened a new window on the nova physics. High signal-to-noise spectra have been obtained for 12 novae after the outburst in the last 13 years with the Chandra and XMM-Newton gratings. They offer the only way to probe the temperature, effective gravity and chemical composition of the hydrogen burning white dwarf before it turns off. These spectra also allow an analysis of the ejecta, which can be photoionized by the hot white dwarf, but more often seem to undergo collisional ionization. The long observations required for the gratings have revealed semi-regular and irregular variability in X-ray flux and spectra. Large short term variability is especially evident in the first weeks after the ejecta have become transparent to the central supersoft X-ray source. Thanks to Chandra and XMM-Newton, we have discovered violent phenomena in the ejecta, discrete shell ejection, and clumpy emission regions. As expected, we have also unveiled the white dwarf characteristics. The peak white dwarf effective temperature in the targets of our samples varies between ~400,000 K and over a million K, with most cases closer to the upper end, although for two novae only upper limits around 200,000 K were obtained. A combination of results from different X-ray satellites and instruments, including Swift and ROSAT, shows that the shorter is the supersoft X-ray phase, the lower is the white dwarf peak effective temperature, consistently with theoretical predictions. The peak temperature is also inversely correlated with t(2) the time for a decay by 2 mag in optical. I strongly advocate the use of white dwarf atmospheric models to obtain a coherent physical picture of the hydrogen burning process and of the surrounding ejecta.
We analyzed the 100-yr light curves of Galactic high-mass X-ray binaries using the Harvard photographic plate collection, made accessible through the DASCH project (Digital Access to a Sky Century at Harvard). As scanning is still in progress, we focus on the four objects that are currently well covered: the supergiant X-ray binary Cyg X-1 (V1357 Cyg), and the Be X-ray binaries 1H 1936+541 (BD+53 2262), RX J1744.7-2713 (HD 161103), and RX J2030.5+4751 (SAO 49725), the latter two objects being similar to gamma Cas. The star associated with Cyg X-1 does not show evidence for variability with an amplitude higher than 0.3 magnitude over a hundred years. We found significant variability of one magnitude with timescales of more than 10 years for SAO 49725, as well as a possible period of 500-600 days and an amplitude of 0.05 magnitude that might be the orbital, or super-orbital period of the system. The data is insufficient to conclude for HD 161103 but suggests a similar long-term variability. We thus observe an additional characteristic of gamma Cas-like objects: their long-term variability. This variability seems to be due to the slow evolution of a decretion disk around the Be star, but may be triggered by the presence of a compact object in the system, possibly a white dwarf. This characteristic could be used to identify further similar objects otherwise difficult to detect.
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