We have observed with the BeppoSAX satellite the quiescent counterpart of the Soft Gamma-ray Repeater SGR 1806-20. Observations performed in October 1998 and in March 1999 showed that this pulsar ...continued its long term spin-down trend at an average rate of ~8 10e-11 s/s while its flux and spectrum remained remarkably constant between the two observations, despite the soft gamma-ray bursting activity that occurred in this period. We also reanalyzed archival ASCA data, that when compared with the new BeppoSAX observations, show evidence for a long term variation in luminosity.
GRB 080514B is the first gamma ray burst (GRB), since the time of EGRET, for which individual photons of energy above several tens of MeV have been detected with a pair-conversion tracker telescope. ...This burst was discovered with the Italian AGILE gamma-ray satellite. The GRB was localized with a cooperation by AGILE and the interplanetary network (IPN). The gamma-ray imager (GRID) estimate of the position, obtained before the SuperAGILE-IPN localization, is found to be consistent with the burst position. The hard X-ray emission observed by SuperAGILE lasted about 7 s, while there is evidence that the emission above 30 MeV extends for a longer duration (at least ~13 s). Similar behavior was seen in the past from a few other GRBs observed with EGRET. However, the latter measurements were affected, during the brightest phases, by instrumental dead time effects, resulting in only lower limits to the burst intensity. Thanks to the small dead time of the AGILE/GRID we could assess that in the case of GRB 080514B the gamma-ray to X-ray flux ratio changes significantly between the prompt and extended emission phase.
We present the AGILE gamma-ray observations of the field containing the puzzling gamma-ray source 3EG J1835+5918. This source is one of the most remarkable unidentified EGRET sources. An ...unprecedentedly long AGILE monitoring of this source yields important information on the positional error box, flux evolution, and spectrum. 3EG J1835+5918 has been in the AGILE field of view several times in 2007 and 2008 for a total observing time of 138 days from 2007 Sept 04 to 2008 June 30 encompassing several weeks of continuous coverage. With an exposure time approximately twice that of EGRET, AGILE confirms the existence of a prominent gamma-ray source (AGL J1836+5926) at a position consistent with that of EGRET, although with a remarkably lower average flux value for photon energies greater than 100 MeV. A 5-day bin temporal analysis of the whole data set of AGL J1836+5926 shows some evidence for variability of the gamma-ray flux. The source spectrum between 100 MeV and 1 GeV can be fitted with a power law with photon index in the range 1.6-1.7, fully consistent with the EGRET value. The faint X-ray source RX J1836.2+5925 that has been proposed as a possible counterpart of 3EG J1835+5918 is well within the AGILE error box. Future continuous monitoring (both by AGILE and GLAST) is needed to confirm the gamma-ray flux variability and to unveil the source origin, a subject that is currently being pursued through a multiwavelength search for counterparts.
Astrophys.J. 596 (2003) 470-476 After nearly two years of quiescence, the soft gamma-ray repeater SGR 1900+14
again became burst-active on April 18 2001, when it emitted a large flare,
preceded by ...few weak and soft short bursts. After having detected the X and
gamma prompt emission of the flare, BeppoSAX pointed its narrow field X-ray
telescopes to the source in less than 8 hours. In this paper we present an
analysis of the data from this and from a subsequent BeppoSAX observation, as
well as from a set of RossiXTE observations. Our data show the detection of an
X-ray afterglow from the source, most likely related to the large hard X-ray
flare. In fact, the persistent flux from the source, in 2-10 keV, was initially
found at a level $\sim$5 times higher than the usual value. Assuming an
underlying persistent (constant) emission, the decay of the excess flux can be
reasonably well described by a t$^{-0.9}$ law. A temporal feature - a
$\sim$half a day long bump - is observed in the decay light curve approximately
one day after the burst onset. This feature is unprecedented in SGR afterglows.
We discuss our results in the context of previous observations of this source
and derive implications for the physics of these objects.
We report on a 20 ks BeppoSAX observation of the transient 2.5 ms X-ray
pulsar SAX J1808.4-3658 in its quiescent state. A source at a level of ~ 3 x
10^-3 c/s was detected with the BeppoSAX/MECS at a ...position consistent with
that of SAX J1808.4-3658. The inferred 0.5-10 keV luminosity was to that
measured from other quiescent low mass X-ray transients hosting an old neutron
star. This result is discussed in the light of the propeller and radio pulsar
shock emission models for the quiescent emission of these systems.
AIPConf.Proc.836:467-470,2006 Understanding the reasons for the faintness of the optical/near-infrared
afterglows of the so-called dark bursts is essential to assess whether they
form a subclass of ...GRBs, and hence for the use of GRBs in cosmology. With VLT
and other ground-based telescopes, we searched for the afterglows of the
INTEGRAL bursts GRB 040223, GRB 040422 and GRB 040624 in the first hours after
the triggers. A detection of a faint afterglow and of the host galaxy in the K
band was achieved for GRB 040422, while only upper limits were obtained for GRB
040223 and GRB 040624, although in the former case the X-ray afterglow was
observed. A comparison with the magnitudes of a sample of afterglows clearly
shows the faintness of these bursts, which are good examples of a population
that an increasing usage of large diameter telescopes is beginning to unveil.
Astron.Astrophys. 433 (2005) 113-116 GRB 040403 is one of the faintest gamma-ray bursts for which a rapid and
accurate localization has been obtained. Here we report on the gamma-ray
properties of ...this burst, based on observations with the IBIS instrument aboard
INTEGRAL, and the results of searches for its optical afterglow. The steep
spectrum (power law photon index = 1.9 in the 20-200 keV range) implies that
GRB 040403 is most likely an X-ray rich burst. Our optical limit of R > 24.2 at
16.5 hours after the burst, indicates a rather faint afterglow, similar to
those seen in other relatively soft and faint bursts.
The INTEGRAL Burst Alert System (IBAS) is the software for real time detection of Gamma Ray Bursts (GRBs) and the rapid distribution of their coordinates. IBAS has been running almost continuously at ...the INTEGRAL Science Data Center since the beginning of the INTEGRAL mission, yielding up to now accurate localizations for 12 GRBs detected in the IBIS field of view. IBAS is able to provide error regions with radius as small as 3 arcminutes (90% c.l.) within a few tens of seconds of the GRB start. We present the current status of IBAS, review the results obtained for the GRBs localized so far, and briefly discuss future prospects for using the IBAS real time information on other classes of variable sources.
Astron.Astrophys.467:629-639,2007 We analyze high resolution spectroscopic observations of the optical
afterglow of GRB050730, obtained with UVES@VLT about hours after the GRB
trigger. The spectrum ...shows that the ISM of the GRB host galaxy at z = 3.967 is
complex, with at least five components contributing to the main absorption
system. We detect strong CII*, SiII*, OI* and FeII* fine structure absorption
lines associated to the second and third component. For the first three
components we derive information on the relative distance from the site of the
GRB explosion. Component 1, which has the highest redshift, does not present
any fine structure nor low ionization lines; it only shows very high ionization
features, such as CIV and OVI, suggesting that this component is very close to
the GRB site. From the analysis of low and high ionization lines and fine
structure lines, we find evidences that the distance of component 2 from the
site of the GRB explosion is 10-100 times smaller than that of component 3. We
evaluated the mean metallicity of the z=3.967 system obtaining values about
0.01 of the solar metallicity or less. However, this should not be taken as
representative of the circumburst medium, since the main contribution to the
hydrogen column density comes from the outer regions of the galaxy while that
of the other elements presumably comes from the ISM closer to the GRB site.
Furthermore, difficulties in evaluating dust depletion correction can modify
significantly these values. The mean C/Fe ratio agrees well with that
expected by single star-formation event models. Interestingly the C/Fe of
component 2 is smaller than that of component 3, in agreement with GRB dust
destruction scenarios, if component 2 is closer than component 3 to the GRB
site.
Abridged We report on a multiwavelength observation of the blazar 3C 454.3 (which we dubbed "crazy diamond") carried out on November 2007 by means of the astrophysical satellites AGILE, INTEGRAL, ...Swift, the WEBT Consortium, and the optical-NIR telescope REM. 3C 454.3 is detected at a \(\sim 19-\sigma\) level during the 3-week observing period, with an average flux above 100 MeV of \(F_{\rm E>100MeV} = (170 \pm 13) \times 10^{-8}\) \phcmsec. The gamma-ray spectrum can be fit with a single power-law with photon index \(\Gamma_{\rm GRID} = 1.73 \pm 0.16\) between 100 MeV and 1 GeV. We detect significant day-by-day variability of the gamma-ray emission during our observations, and we can exclude that the fluxes are constant at the 99.6% (\(\sim 2.9 \sigma\)) level. The source was detected typically around 40 degrees off-axis, and it was substantially off--axis in the field of view of the AGILE hard X-ray imager. However, a 5-day long ToO observation by INTEGRAL detected 3C 454.3 at an average flux of about \(F_{\rm 20-200 keV} = 1.49 \times 10^{-3}\) \phcmsec with an average photon index of \(\Gamma_{\rm IBIS} = 1.75 \pm 0.24\) between 20--200 keV. Swift also detected 3C 454.3 with a flux in the 0.3--10 keV energy band in the range \((1.23-1.40) \times 10^{-2}\) \phcmsec{} and a photon index in the range \(\Gamma_{\rm XRT} = 1.56-1.73\). In the optical band, both WEBT and REM show an extremely variable behavior in the \(R\) band. A correlation analysis based on the entire data set is consistent with no time-lags between the gamma-ray and the optical flux variations. Our simultaneous multifrequency observations strongly indicate that the dominant emission mechanism between 30 MeV and 30 GeV is dominated by inverse Compton scattering of relativistic electrons in the jet on the external photons from the broad line region.
