140 research outputs found
Multiwavelength observations of blazars
The INTEGRAL mission has played a major role in blazar science, thanks to its
sensitive coverage of a spectral region (3-100 keV) that is critical for this
type of sources, to its flexibility of scheduling and to the large field of
view of its cameras. A number of flat-spectrum radio quasars (up to z ~ 3) and
BL Lac objects were observed by INTEGRAL together with facilities at all
wavelengths. These results have advanced our knowledge of blazars from a
physical and cosmological point of view. This paper reviews some of these
outcomes, with particular reference to the INTEGRAL program for blazars in
outburst as targets of opportunity, with a perspective into a future of
multi-messenger astronomyComment: 7 pages, 1 figure, invited talk at the 11th INTEGRAL Conference
"Gamma-Ray Astrophysics in Multi-Wavelength Perspective", Amsterdam, 10-14
October 2016. To be published in the Conf. Proceeding
FERMI constraints on the high energy, ~1 GeV, emission of long GRBs
We investigate the constraints imposed on the luminosity function (LF) of
long duration Gamma Ray Bursts (LGRBs) by the flux distribution of bursts
detected by the GBM at ~1 MeV, and the implications of the non detection of the
vast majority, ~95%, of the LGRBs at higher energy, ~1 GeV, by the LAT
detector. We find a LF that is consistent with those determined by BATSE and
Swift. The non detections by LAT set upper limits on the ratio R of the prompt
fluence at ~1 GeV to that at ~1 MeV. The upper limits are more stringent for
brighter bursts, with R<{0.1,0.3,1} for {5,30,60}% of the bursts. This implies
that for most bursts the prompt ~1 GeV emission may be comparable to the ~1 MeV
emission, but can not dominate it. The value of R is not universal, with a
spread of (at least) an order of magnitude around R~10^(-1). For several bright
bursts with reliable determination of the photon spectral index at ~1 MeV, the
LAT non detection implies an upper limit to the ~100 MeV flux which is <0.1 of
the flux obtained by extrapolating the ~1 MeV flux to high energy. For the
widely accepted models, in which the ~1 MeV power-law photon spectrum reflects
the power-law energy distribution of fast cooling electrons, this suggests that
either the electron energy distribution does not follow a power-law over a wide
energy range, or that the high energy photons are absorbed. Requiring an order
unity pair production optical depth at ~100 MeV sets an upper limit for the
Lorentz factor, Gamma<=10^(2.5).Comment: 12 pages, 6 figures. Submitted to A&
Mergers of binary neutron star systems: a multi-messenger revolution
On 17 August 2017, less than two years after the direct detection of
gravitational radiation from the merger of two ~30 Msun black holes, a binary
neutron star merger was identified as the source of a gravitational wave signal
of ~100 s duration that occurred at less than 50 Mpc from Earth. A short GRB
was independently identified in the same sky area by the Fermi and INTEGRAL
satellites for high energy astrophysics, which turned out to be associated with
the gravitational event. Prompt follow-up observations at all wavelengths led
first to the detection of an optical and infrared source located in the
spheroidal galaxy NGC4993 and, with a delay of ~10 days, to the detection of
radio and X-ray signals. This paper revisits these observations and focusses on
the early optical/infrared source, which was thermal in nature and powered by
the radioactive decay of the unstable isotopes of elements synthesized via
rapid neutron capture during the merger and in the phases immediately following
it. The far-reaching consequences of this event for cosmic nucleosynthesis and
for the history of heavy elements formation in the Universe are also
illustrated.Comment: 24 pages, 1 figure, author's version of paper accepted for
publication in Frontiers in Physics, Nuclear Physic
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