ABSTRACT We report the results of an extensive search through the AGILE data for a gamma-ray counterpart to the LIGO gravitational-wave (GW) event GW150914. Currently in spinning mode, AGILE has the ...potential of cover 80% of the sky with its gamma-ray instrument, more than 100 times a day. It turns out that AGILE came within a minute of the event time of observing the accessible GW150914 localization region. Interestingly, the gamma-ray detector exposed ∼65% of this region during the 100 s time intervals centered at −100 and +300 s from the event time. We determine a 2 flux upper limit in the band 50 MeV-10 GeV, UL = 1.9 × 10−8 erg cm−2 s−1, obtained ∼300 s after the event. The timing of this measurement is the fastest ever obtained for GW150914, and significantly constrains the electromagnetic emission of a possible high-energy counterpart. We also carried out a search for a gamma-ray precursor and delayed emission over five timescales ranging from minutes to days: in particular, we obtained an optimal exposure during the interval −150/−30 s. In all these observations, we do not detect a significant signal associated with GW150914. We do not reveal the weak transient source reported by Fermi-GBM 0.4 s after the event time. However, even though a gamma-ray counterpart of the GW150914 event was not detected, the prospects for future AGILE observations of GW sources are decidedly promising.
GRB080319B reached fifth optical magnitude during the burst prompt emission. Thanks to the Very Large Telescope (VLT)/Ultraviolet and Visual Echelle Spectrograph (UVES) rapid response mode, we ...observed its afterglow just 8m:30s after the gamma-ray burst (GRB) onset when the magnitude was R ~ 12. This allowed us to obtain the best signal-to-noise (S/N), high-resolution spectrum of a GRB afterglow ever (S/N per resolution element ~50). The spectrum is rich of absorption features belonging to the main system at z = 0.937, divided in at least six components spanning a total velocity range of 100 km s-1. The VLT/UVES observations caught the absorbing gas in a highly excited state, producing the strongest Fe II fine structure lines ever observed in a GRB. A few hours later, the optical depth of these lines was reduced by a factor of 4-20, and the optical/UV flux by a factor of ~60. This proves that the excitation of the observed fine structure lines is due to 'pumping' by the GRB UV photons. A comparison of the observed ratio between the number of photons absorbed by the excited state and those in the Fe II ground state suggests that the six absorbers are ~2-6 kpc from the GRB site, with component I ~ 3 times closer to the GRB site than components III-VI. Component I is characterized also by the lack of Mg I absorption, unlike all other components. This may be both due to a closer distance and a lower density, suggesting a structured interstellar matter in this galaxy complex.
Establishing the nature of γ-ray bursts is one of the greatest challenges in high-energy astrophysics. The distribution of these bursts is isotropic across the sky, but inhomogeneous in space, with a ...deficit of faint bursts. It is currently unknown whether γ-ray bursts are produced in our Galaxy or at cosmological distances. The detection and identification of counterparts at other wavelengths are seen as crucial for resolving the origin of the events. Here we report the detection by the Beppo-SAX satellite of an X-ray 'afterglow', associated with the γ-ray burst of 28 February 1997 (GRB970228; ref. 3)-the first such detection for any γ-ray burst. The X-ray transient was found to contain a significant fraction of the total energy of the γ-ray burst and, following the initial detection eight hours after the main burst, faded within a few days with a power-law decay function. The rapid locating of this γ-ray burst instigated a multi-wavelength observational campaign that culminated in the identification of a fading optical transient in a position consistent with the X-ray transient reported here.
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Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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•A new PTCR material based on a mixed phase of sodium titanate is report.•A model based on the structural composition of the system is proposed.•The resistivity at room temperature ...can be tune by slightly modifying the structure and microstructure of the system.•Unprecedented physical properties and Novel functionalities for sodium-titanate based ceramics system.
Mixed phases of sodium titanate ceramics are obtained using both conventional and laser sintering methods. It was observed a clear dependence on the structural, microstructural and electrical resistivity with the employed sintering method. The temperature dependence of the resistivity in the ceramics shows an increase of three orders of magnitude in a narrow range of temperatures, thereby indicating a noticeable positive temperature coefficient of resistivity (PTCR) effect. To explain this observed abnormal behavior, a model based on the structure of the Na2Ti6O13/Na2Ti3O7 composite is proposed. A new class of PTCR materials is reported.
We present the results from an ESO/VLT campaign aimed at studying the afterglow properties of the short/hard gamma ray burst GRB 070707. Observations were carried out at ten different epochs from ...~0.5 to ~80 days after the event. The optical flux decayed steeply with a power-law decay index greater than 3, later levelling off at $R\sim 27.3$ mag; this is likely the emission level of the host galaxy, the faintest yet detected for a short GRB. Spectroscopic observations did not reveal any line features/edges that could unambiguously pinpoint the GRB redshift, but set a limit $z < 3.6$. In the range of allowed redshifts, the host has a low luminosity, comparable to that of long-duration GRBs. The existence of such faint host galaxies suggests caution when associating short GRBs with bright, offset galaxies, where the true host might just be too dim for detection. The steepness of the decay of the optical afterglow of GRB 070707 challenges external shock models for the optical afterglow of short/hard GRBs. We argue that this behaviour might result from prolonged activity of the central engine or require alternative scenarios.
We present optical observations of the short/hard gamma-ray burst GRB 050709, the first such event with an identified optical counterpart. The object is coincident with a weak X-ray source and is ...located inside a galaxy at redshift $z = 0.1606 \pm 0.0002$. Multiband photometry allowed us to study the broad-band spectral energy distribution. Late-time monitoring places strong limits on any supernova simultaneous with the GRB. The host galaxy is not of early type. Spectra show that the dominant stellar population is relatively young (∼1 Gyr), and that ongoing star formation is present at a level of $2\mbox{--}3\,L/L_*~M_\odot$ yr-1. This is at least 2 orders of magnitude larger than that observed in the elliptical hosts of the short GRB 050509B and GRB 050724. This shows that at least some short GRBs originate in a young population. Short/hard GRB models based on the merger of a binary degenerate system are compatible with the host galaxy characteristics, although there is still the possibility of a connection between young stars and at least a fraction of such events.
We report the AGILE detection and the results of the multifrequency follow-up observations of a bright γ-ray flare of the blazar 3C 279 in 2015 June. We use AGILE and Fermi gamma-ray data, together ...with Swift X-ray andoptical-ultraviolet data, and ground-based GASP-WEBT optical observations, including polarization information, to study the source variability and the overall spectral energy distribution during the γ-ray flare. The γ-ray flaring data, compared with as yet unpublished simultaneous optical data that will allow constraints on the big blue bump disk luminosity, show very high Compton dominance values of ∼100, with the ratio of γ-ray to optical emission rising by a factor of three in a few hours. The multiwavelength behavior of the source during the flare challenges one-zone leptonic theoretical models. The new observations during the 2015 June flare are also compared with already published data and nonsimultaneous historical 3C 279 archival data.
We present photometric and spectroscopic observations of the gamma-ray burst GRB 021211 obtained during the late stages of its afterglow. The light curve shows a rebrightening occurring ~25 days ...after the GRB. The analysis of a VLT spectrum obtained during the bump (27 days after the GRB) reveals a suggestive resemblance with the spectrum of the prototypical type-Ic SN 1994I, obtained ~10 days past maximum light. Particularly we have measured a strong, broad absorption feature at 3770 Å, which we have identified with Ca II blueshifted by ~$14\,400$ km s-1, thus indicating that a supernova (SN) component is indeed powering the “bump” in the afterglow decay. Assuming SN 1994I as a template, the spectroscopic and photometric data together indicate that the SN and GRB explosions were at most separated by a few days. Our results suggest that GRBs might be associated also to standard type-Ic supernovae.
We report observations of GRB 050525A, for which a Gemini North spectrum shows its redshift to be z = 0.606. This is the third closest long GRB discovered by Swift. We observed its afterglow using ...the VLT, Gemini, and TNG telescopes to search for an associated supernova. We find that the early-time light curve is described by a broken power law with a break at t 6 0.3 days after the burst. About 5 days after the burst, a flattening is apparent, followed by a further dimming. Both the magnitude and the shape of the light curve suggest that a supernova was emerging during the late decay of the afterglow. This supernova, named SN 2005nc, had a rise time faster than SN 1998bw and a long-lasting maximum. A spectrum obtained about 20 days (rest frame) after the GRB resembles the spectrum of SN 1998bw obtained close to maximum light.
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