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Article: A Swift view on the HMXB IGR J16283-4838 unveils a 288 days orbital period
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ABSTRACT: The Swift-BAT survey provides an extensive and continuous monitoring of hard X-ray sources and thus it is particularly suited for the study of their long term temporal properties. We have been exploiting these data focusing on the Galactic compact high mass binaries sources, that, in this energy band, can be observed with good sensitivity even in the presence of high local soft X-ray extinction. We aim at giving new results on the temporal and spectral properties of the HMXB IGR J16283-4838 in the hard X-ray band. We performed timing analysis of the first 88-month of the BAT survey data searching for long term modulations. We also investigated the broad band (0.2-150 keV) spectral properties of IGR J16283-4838 complementing the BAT dataset with the soft X-ray data from the available Swift-XRT pointed observations. The BAT light curve of IGR J16283-4838 revealed a periodic modulation at Po=287.6\pm1.9 days (with a significance higher than 5 standard deviations). The profile of the light curve folded at Po shows a sharp peak lasting ~18$ d, over a flat plateau characterized by an intensity level ~10 times lower than the peak level. The broad band 0.2-150 keV spectrum is modeled with an absorbed power-law with a photon index Gamma 1.1 and a steepening in the BAT energy range modeled with a cut-off at an energy of ~25 keV.05/2013; -
Article: Finding a 60.9-day orbital period for the HMXB 4U 1036-56 with the Swift-BAT monitoring
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ABSTRACT: Since November 2004, the Burst Alert Telescope on board Swift is producing a monitoring of the entire sky in the 15-150 keV band, recording the timing and spectral behavior of the detected sources. In this letter we study the properties of the HMXB 4U 1036-56 using both the BAT survey data and those from a Swift-XRT pointed observation. The timing analysis of the BAT light curve unveils a periodic modulation with a period of ~60.9 days, that we explain as the orbital period of the binary system. The position of 4U 1036-56 on the Corbet diagram and the derived semi-major orbit axis (~180 R_dot) are consistent with the Be nature of its companion star. The intensity orbital profile averaged over 88 months of observations shows a large asymmetric shape with a minimum consistent with zero intensity, that could be related to the occultation of the neutron star by the supergiant companion. The source shows also a strong long term variability, going from high intensity states to quiescent states over a time scale of ~2 years. The broad band 0.2-150 keV spectrum is well modeled with a flat absorbed power law with a cutoff at ~14 keV. Finally, we explore the possibility that 4U~1036--56 is associated to the unidentified source AGL J1037-5708, finding that the BAT light curve does not show any intensity enhancement correlated to the outburst of the Gamma-ray transient.05/2013; -
Article: Molecular Hydrogen in the Damped Lyman-alpha System towards GRB 120815A at z=2.36
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ABSTRACT: [Abridged] Molecular hydrogen H_2 is a key requirement for star-formation, frequently observed along sightlines in the Galaxy and to the Magellanic Clouds, but notoriously hard to detect directly beyond z ~ 0. In the DLAs associated with long gamma-ray bursts (GRBs), which are tightly linked to vigorous star-formation, H_2 has remained largely elusive, and has been unequivocally detected only towards GRB 080607 and possibly towards GRB 060206. Here, we present the discovery of H_2-rich gas, including the presence of vibrationally-excited H_2^* in the optical spectrum of the afterglow of GRB 120815A at z=2.36 obtained with X-shooter at the VLT. The galactic environment of GRB 120815A is characterized by a strong DLA with log(N(H)/cm^-2) = 21.95 +/- 0.10, prominent H_2 absorption in the Lyman-Werner bands (log(N(H_2)/cm^-2) = 20.53 +/- 0.04) and thus a molecular gas fraction log f(H_2)=-1.14 +/- 0.10. The distance d between the absorbing neutral gas and GRB 120815A is constrained via photo-excitation modeling of fine-structure and meta-stable transitions of FeII and NiII to d = 0.5 +/- 0.1 kpc. The DLA metallicity ([Zn/H] = -1.15 +/- 0.11), visual extinction (A_V < 0.15 mag) and dust depletion ([Zn/Fe] = 1.01 +/- 0.09) are intermediate between the values of well-studied, H_2-deficient GRB-DLAs observed at high spectral resolution, and the approximately solar metallicity, highly-obscured and H_2-rich GRB 080607 sightline. With respect to N(H), metallicity, as well as dust-extinction and depletion, GRB 120815A is fairly representative of the average properties of GRB-DLAs. This demonstrates that molecular hydrogen is present in at least a fraction of the more typical GRB-DLAs, and H_2 and H_2^* are probably more wide-spread among GRB-selected systems than the few examples of previous detections would suggest.04/2013; -
Article: The ASTRI Project: a mini-array of dual-mirror small Cherenkov telescopes for CTA
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ABSTRACT: ASTRI is a flagship project of the Italian Ministry of Education, University and Research, which aims to develop an end-to-end prototype of the CTA small-size telescope. The proposed design is characterized by a dual-mirror Schwarzschild-Couder configuration and a camera based on Silicon photo-multipliers, two challenging but innovative technological solutions which will be adopted for the first time on a Cherenkov telescope. Here we describe the current status of the project, the expected performance and the possibility to realize a mini-array composed by a few small-size telescopes, which shall be placed at the final CTA Southern Site.04/2013; -
Article: Blazar candidates beyond redshift 4 observed with GROND
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ABSTRACT: The search for extremely massive high redshift blazars is essential to put strong constraints on the supermassive black holes formation theories. Up to now, the few blazars known to have a redshift larger than 4 have been discovered serendipitously. We try a more systematic approach. Assuming radio-loudness as a proxy for the jet orientation, we select a sample of extremely radio-loud quasars. We measure their black hole masses with a method based on fitting the thermal emission from the accretion disc. We achieve a precision of a factor of two for our measures, thanks to the observations performed with the Gamma-Ray Burst Optical Near-Infrared Detector (GROND). The infrared to optical GROND data allow us to observe directly the peak of the disc emission, thus constraining the overall disc luminosity. We obtain a small range of masses, that peaks at 10^{9.3}Msun. If some of our candidates will be confirmed as blazars, these results would introduce interesting constraints on the mass function of extremely massive black holes at very high redshift. Moreover, all our blazar candidates have high accretion rates. This result, along with the high masses, opens an interesting view on the need of a fast growth of the heaviest black holes at very high redshift.03/2013;
