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Multimessenger observations of GW170817 have not conclusively established whether the merger remnant is a black hole (BH) or a neutron star (NS). We show that a long-lived magnetized NS with a poloidal field B ≈ 1012 G is fully consistent with the electromagnetic dataset, when spin-down losses are dominated by gravitational wave (GW) emission. The required ellipticity ε ≳ 10−5 can result from a toroidal magnetic field component much stronger than the poloidal component, a configuration expected from an NS newly formed from a merger. Abrupt magnetic dissipation of the toroidal component can lead to the appearance of X-ray flares, analogous to the one observed in gamma-ray burst (GRB) afterglows. In the X-ray afterglow of GW170817, we identify a low-significance (≳3σ) temporal feature at 155 d, consistent with a sudden reactivation of the central NS. Energy injection from the NS spin-down into the relativistic shock is negligible, and the underlying continuum is fully accounted for by a structured jet seen off-axis. Whereas radio and optical observations probe the interaction of this jet with the surrounding medium, observations at X-ray wavelengths, performed with adequate sampling, open a privileged window on to the merger remnant. [ABSTRACT FROM AUTHOR]

We present our broad-band study of GW170817 from radio to hard X-rays, including NuSTAR and Chandra observations up to 165  d after the merger, and a multimessenger analysis including LIGO constraints. The data are compared with predictions from a wide range of models, providing the first detailed comparison between non-trivial cocoon and jet models. Homogeneous and power-law shaped jets, as well as simple cocoon models are ruled out by the data, while both a Gaussian shaped jet and a cocoon with energy injection can describe the current data set for a reasonable range of physical parameters, consistent with the typical values derived from short GRB afterglows. We propose that these models can be unambiguously discriminated by future observations measuring the post-peak behaviour, with Fν ∝ t∼−1.0 for the cocoon and Fν∝ t∼−2.5 for the jet model. [ABSTRACT FROM AUTHOR]

ATHENA is a large X-ray observatory, planned to be launched by ESA in 2028 towards an L2 orbit. One of the two instruments of the payload is the X-IFU: a cryogenic spectrometer based on a large array of TES microcalorimeters, able to perform integral field spectrography in the 0.2-12 keV band (2.5 eV FWHM at 6 keV). The X-IFU sensitivity is highly degraded by the particle background expected in the L2 orbit, which is induced by primary protons of both galactic and solar origin, and mostly by secondary electrons. To reduce the particle background level and enable the mission science goals, the instrument incorporates a Cryogenic AntiCoincidence detector (CryoAC). It is a 4 pixel TES based detector, placed < 1 mm below the main array. In this paper we report a scientific assessment of the CryoAC observational capabilities in the hard X-ray band (E > 10 keV). The aim of the study has been to understand if the present detector design can be improved in order to enlarge the X-IFU scientific capability on an energy band wider than the TES array. This is beyond the CryoAC baseline, being this instrument aimed to operate as anticoincidence particle detector and not conceived to perform X-ray observations. [ABSTRACT FROM AUTHOR]

We report on the discovery of two emission features observed in the x-ray spectrum of the afterglow of the gamma-ray burst (GRB) of 16 December 1999 by the Chandra X-ray Observatory. These features are identified with the Ly[sub alpha] tine and the narrow recombination continuum by hydrogenic ions of iron at a redshift z = 1.00 +/- 0.02, providing an unambiguous measurement of the distance of a GRB. Line width and intensity imply that the progenitor of the GRB was a massive star system that ejected, before the GRB event, a quantity of iron Is similar to 0.01 of the mass of the sun at a velocity Is similar to 0.1 of the speed of light, probably by a supernova explosion. [ABSTRACT FROM AUTHOR]

The results of a BeppoSAX target of opportunity (TOO) observation of the BL Lac object Mrk 421 during a high-intensity state are reported and compared with monitoring X-ray data collected with the BeppoSAX Wide Field Cameras (WFC) and the RXTE All Sky Monitor (ASM). The 0.1-100 keV spectmm of Mrk 421 shows continuous convex curvature that can be interpreted as the high-energy end of the synchrotron emission. The source shows significant short-term temporal and spectral variability, which can be interpreted in terms of synchrotron cooling. The comparison of our results with those of previous observations when the source was a factor 3-5 fainter shows evidence for strong spectral variability, with the maximum of the synchrotron power shifting to higher energy during high states. This behaviour suggests an increase in the number of energetic electrons during high states. [ABSTRACT FROM AUTHOR]

ABSTRACT We preset first results of the long term monitoring of Seyfert Galaxies with BeppoSAX Wide Field Cameras (WFCs). We have firstly examined the data of the Seyfert 1.5 NGC 4151, This object is known to be one of the brightest Seyferts ranging from few to about 15 mCrab in the 2 - 10 keV energy range. As of September 13, 1998, the sky region containing NGC 4151 has been observed by WFCs in secondary mode (i.e., when the narrow field instruments are performing prime observations) about 80 times. We report here the results of the analysis of 21 observations. The minimum flux at which the source was detected is ∼ 4 mCrab, In June of this year we found the source at ∼ 20 mCrab, the highest state ever recorded. The study of the light curve of the very faint and stable source Cas A gives a preliminary assessment of systematic errors of ∼20%. The preliminary results obtained, confirm that the WFCs are powerful tools for the study of bright AGN. [ABSTRACT FROM AUTHOR]

ABSTRACT Preliminary results from a BeppoSAX NFI observation of Mkn421 in a very bright state are reported together with monitoring data collected with the BeppoSAX-WFC and the RXTE-ASM. The BeppoSAX observation was carried out as part of a Target Of Opportunity program aimed at the study of AGN near record level intensity states. The comparison of our results with those of previous observations of Mkn421 shows clear evidence for strong spectral evolution. [ABSTRACT FROM AUTHOR]

Presents research which detected bright optical emission from the gamma-ray burst GRB990123 from January 23, 1999. Afterglow; Brightness variances; Redshift; Production of gamma rays not at the shock front, but near the site of the original explosion.

Simultaneous Swift and Fermi observations of gamma-ray bursts (GRBs) offer a unique broadband view of their afterglow emission, spanning more than 10 decades in energy. We present the sample of X-ray flares observed by both Swift and Fermi during the first three years of Fermi operations. While bright in the X-ray band, X-ray flares are often undetected at lower (optical), and higher (MeV to GeV) energies. We show that this disfavors synchrotron self-Compton processes as the origin of the observed X-ray emission. We compare the broadband properties of X-ray flares with the standard late internal shock model, and find that in this scenario, X-ray flares can be produced by a late-time relativistic (Γ > 50) outflow at radii R ∼ 1013-1014 cm. This conclusion holds only if the variability timescale is significantly shorter than the observed flare duration, and implies that X-ray flares can directly probe the activity of the GRB central engine. [ABSTRACT FROM AUTHOR]