The Near Infrared and Multiwavelength Afterglow of GRB 000301c

We present near-infrared observations of the counterpart of GRB 000301c. The K' filter (2.1 micron) light curve shows a well-sampled break in the decay slope at t=3.5 days post-burst. The early time slope is very shallow (~ -0.1), while the late time slope is steep (-2.2). Comparison with the optical (R band) light curve shows marginally significant differences, especially in the early time decay slope (which is steeper in the optical) and the break time (which occurs later in the optical). This is contrary to the general expectation that light curve breaks should either be achromatic (e.g., for breaks due to collimation effects) or should occur later at longer wavelengths (for most other breaks). The observed color variations might be intrinsic to the afterglow, or might indicate systematic errors of > 0.08 magnitude in all fluxes. Even if the break is achromatic, we argue that its sharpness poses difficulties for explanations that depend on collimated ejecta. The R light curve shows further signs of fairly rapid variability (a bump, steep drop, and plateau) that are not apparent in the K' light curve. In addition, by combining the IR-optical-UV data with millimeter and radio fluxes, we are able to constrain the locations of the self-absorption break and cooling break and to infer the location of the spectral peak at t=3 days: f_nu = 3.4 mJy at nu=1e12 Hz. Using the multiwavelength spectral energy distribution, we are able to constrain the blast wave energy, which was E > 3e53 erg if the explosion was isotropic. This implies a maximum gamma ray production efficiency of ~ 0.15 for GRB 000301C.Comment: Accepted to The Astrophysical Journal. 24 pages, 4 figures, 3 tables; uses AASTeX 5 macros. This version includes a new figure (R-K' color vs. time), a better sampled R band light curve, and more extensive discussion of the optical data and error analysi

Unveiling the nature of INTEGRAL objects through optical spectroscopy. VIII. Identification of 44 newly detected hard X-ray sources

(abridged) Hard X-ray surveys performed by the INTEGRAL satellite have discovered a conspicuous fraction (up to 30%) of unidentified objects among the detected sources. Here we continue our identification program by selecting probable optical candidates using positional cross-correlation with soft X-ray, radio, and/or optical archives, and performing optical spectroscopy on them. As a result, we identified or more accurately characterized 44 counterparts of INTEGRAL sources: 32 active galactic nuclei, with redshift 0.019 < z < 0.6058, 6 cataclysmic variables (CVs), 5 high-mass X-ray binaries (2 of which in the Small Magellanic Cloud), and 1 low-mass X-ray binary. This was achieved by using 7 telescopes of various sizes and archival data from two online spectroscopic surveys. The main physical parameters of these hard X-ray sources were also determined using the available multiwavelength information. AGNs are the most abundant population among hard X-ray objects, and our results confirm this tendency when optical spectroscopy is used as an identification tool. The deeper sensitivity of recent INTEGRAL surveys enables one to begin detecting hard X-ray emission above 20 keV from sources such as LINER-type AGNs and non-magnetic CVs.Comment: 22 pages, 14 figures, 6 tables, accepted for publication on A&A, main journa

Unveiling the nature of INTEGRAL objects through optical spectroscopy. IX. 22 more identifications, and a glance into the far hard X-ray Universe

(Abridged) Since its launch in October 2002, the INTEGRAL satellite has revolutionized our knowledge of the hard X-ray sky thanks to its unprecedented imaging capabilities and source detection positional accuracy above 20 keV. Nevertheless, many of the newly-detected sources in the INTEGRAL sky surveys are of unknown nature. The combined use of available information at longer wavelengths (mainly soft X-rays and radio) and of optical spectroscopy on the putative counterparts of these new hard X-ray objects allows us to pinpoint their exact nature. Continuing our long-standing program that has been running since 2004, and using 6 different telescopes of various sizes, we report the classification through optical spectroscopy of 22 more unidentified or poorly studied high-energy sources detected with the IBIS instrument onboard INTEGRAL. We found that 16 of them are active galactic nuclei (AGNs), while the remaining 6 objects are within our Galaxy. Among the identified extragalactic sources, 14 are Type 1 AGNs; of these, 6 lie at redshift larger than 0.5 and one has z = 3.12, which makes it the second farthest object detected in the INTEGRAL surveys up to now. The remaining AGNs are of type 2, and one of them is a pair of interacting Seyfert 2 galaxies. The Galactic objects are identified as two cataclysmic variables, one high-mass X-ray binary, one symbiotic binary and two chromospherically active stars. We thus still find that AGNs are the most abundant population among hard X-ray objects identified through optical spectroscopy. Moreover, we note that the higher sensitivity of the more recent INTEGRAL surveys is now enabling the detection of high-redshift AGNs, thus allowing the exploration of the most distant hard X-ray emitting sources and possibly of the most extreme blazars.Comment: 18 pages, 9 figures, 8 tables, accepted for publication on Astronomy & Astrophysics, main journa

Unveiling GRB hard X-ray afterglow emission with Simbol-X

Despite the enormous progress occurred in the last 10 years, the Gamma-Ray Bursts (GRB) phenomenon is still far to be fully understood. One of the most important open issues that have still to be settled is the afterglow emission above 10 keV, which is almost completely unexplored. This is due to the lack of sensitive enough detectors operating in this energy band. The only detection, by the BeppoSAX/PDS instrument (15-200 keV), of hard X-ray emission from a GRB (the very bright GRB 990123), combined with optical and radio observations, seriously challenged the standard scenario in which the dominant mechanism is synchrotron radiation produced in the shock of a ultra-relativistic fireball with the ISM, showing the need of a substantial revision of present models. In this respect, thanks to its unprecedented sensitivity in the 10-80 keV energy band, Simbol-X, through follow-up observations of bright GRBs detected and localized by GRB dedicated experiments that will fly in the >2010 time frame, will provide an important breakthrough in the GRB field.Comment: 4 pages, 2 figures. Paper presented at "Simbol-X: the hard X-ray universe in focus", held in Bologna, Italy, on 14-16 May 2007. To be published in Memorie della Societa' Astronomica Italian

Constraining ΩM\Omega_M and Dark Energy with Gamma-Ray Bursts

An $E_{\gamma,{\rm jet}}\propto {E'_p}^{1.5}$ relationship with a small scatter for current $\gamma$-ray burst (GRB) data was recently reported, where $E_{\gamma,{\rm jet}}$ is the beaming-corrected $\gamma$-ray energy and $E'_p$ is the $\nu F_\nu$ peak energy in the local observer frame. By considering this relationship for a sample of 12 GRBs with known redshift, peak energy, and break time of afterglow light curves, we constrain the mass density of the universe and the nature of dark energy. We find that the mass density $\Omega_M=0.35\pm^{0.15}_{0.15}$ (at the $1\sigma$ confident level) for a flat universe with a cosmological constant, and the $w$ parameter of an assumed static dark-energy equation of state $w=-0.84\pm^{0.57}_{0.83}$ ($1\sigma$). Our results are consistent with those from type Ia supernovae. A larger sample established by the upcoming {\em Swift} satellite is expected to provide further constraints.Comment: 8 pages including 4 figures, to appear in ApJ Letters, typos correcte

K-band spectroscopy of IGR J16358-4726 and IGR J16393-4643: two new symbiotic X-ray binaries

Symbiotic X-ray Binaries (SyXBs) are a recently discovered subclass of Low Mass X-ray Binaries. Their growing number makes them an important evolutionary channel of X-ray Binaries. Our goal is to perform spectral analysis and classification of the proposed counterparts to IGR J16358-4726 and IGR J16393-4643 and to establish their nature as X-ray systems. We used the ESO/UT1 ISAAC spectrograph to observe the proposed counterparts to the two sources, obtaining K-band medium resolution spectra (R = 500) with a S/N > 140. Data reduction was performed with the standard procedure. We classified them by means of comparison with published atlases. We performed SED fitting in order to refine the spectral classification. The two counterparts clearly exhibit the typical features of late-type stars, notably strong CO absorption bands in the red part of the spectrum. With information from previous X-ray studies, we classify the two systems as two new members of the SyXB class. For IGR J16393-4643, we considered the most probable counterpart to the system, although three other objects cannot be completely discarded. For this system, we compared our findings with available orbital solutions, constraining the orbital parameters and the mass of the companion star. By including two more systems, we increased to eight the number of known SyXBs, which emerges as a non-negligible category of galactic X-ray binaries.Comment: 7 pages, 3 figures, 4 tables; this version includes referee's comments and language editing. Accepted for publication in Astronomy and Astrophysics

Unveiling the nature of three INTEGRAL sources through optical spectroscopy

The results of an optical spectroscopy campaign performed at the Astronomical Observatory of Bologna in Loiano (Italy) on three hard X-ray sources detected by INTEGRAL (IGR J17303-0601, IGR J18027-1455 and IGR J21247+5058) are presented. These data have allowed a determination of the nature for two of them, with IGR J17303-0601 being a low mass X-ray binary in the Galaxy and IGR J18027-1455 a background Type 1 Seyfert galaxy at redshift z = 0.035. IGR J21247+5058, instead, has a quite puzzling spectroscopic appearance, with a broad, redshifted H_alpha complex superimposed onto a `normal' F/G-type Galactic star continuum: these features, together with the spatially coincident extended radio emission, might suggest a chance alignment between a relatively nearby star and a background radio galaxy. These results underline the still non-negligible importance of smaller telescopes in modern astrophysics.Comment: 4 pages, 2 figures. Accepted for publication in A&A Letter

Accurate classification of 29 objects detected in the 39 months Palermo Swift/BAT hard X-ray catalogue

Through an optical campaign performed at 4 telescopes located in the northern and the southern hemispheres, plus archival data from two on-line sky surveys, we have obtained optical spectroscopy for 29 counterparts of unclassified or poorly studied hard X-ray emitting objects detected with Swift/BAT and listed in the 39 months Palermo catalogue. All these objects have also observations taken with Swift/XRT or XMM-EPIC which not only allow us to pinpoint their optical counterpart, but also to study their X-ray spectral properties (column density, power law photon index and F2-10 keV flux). We find that 28 sources in our sample are AGN; 7 are classified as type 1 while 21 are of type 2; the remaining object is a galactic cataclysmic variable. Among our type 1 AGN, we find 5 objects of intermediate Seyfert type (1.2-1.9) and one Narrow Line Seyfert 1 galaxy; for 4 out of 7 sources, we have been able to estimate the central black hole mass. Three of the type 2 AGN of our sample display optical features typical of the LINER class and one is a likely Compton thick AGN. All galaxies classified in this work are relatively nearby objects since their redshifts lie in the range 0.008-0.075; the only galactic object found lies at an estimated distance of 90 pc. We have also investigated the optical versus X-ray emission ratio of the galaxies of our sample to test the AGN unified model. For them, we have also compared the X-ray absorption (due to gas) with the optical reddening (due to dust): we find that for most of our sources, specifically those of type 1.9-2.0 the former is higher than the latter confirming early results by Maiolino et al. (2001); this is possibly due to the properties of dust in the circumnuclear obscuring torus of the AGN.Comment: 15 pages, 4 figures, 8 tables, accepted for publication on Astronomy and Astrophysic