625 research outputs found
A new method to unveil blazars among multi-wavelength counterparts of Unassociated Fermi gamma-ray Sources
We discuss a new method for unveiling the possible blazar AGN nature among
the numerous population of Unassociated Gamma-ray sources (UGS) in the Fermi
catalogues. Our tool relies on positional correspondence of the Fermi object
with X-ray sources (mostly from Swift-XRT), correlated with other radio, IR and
optical data in the field. We built a set of Spectral Energy Distributions
(SED) templates representative of the various blazar classes, and we
quantitatively compared them to the observed multi-wavelength flux density data
for all Swift-XRT sources found within the Fermi error-box, by taking advantage
of some well-recognised regularities in the broad-band spectral properties of
the objects. We tested the procedure by comparison with a few well-known
blazars, and tested the chance for false positive recognition of UGS sources
against known pulsars and other Galactic and extragalactic sources. Based on
our spectral recognition tool, we find the blazar candidate counterparts for 14
2FGL UGSs among 183 selected at high galactic latitudes. Further our tool also
allows us rough estimates of the redshift for the candidate blazar. In a few
cases in which this has been possible (i.e. when the counterpart was a SDSS
object), we verified that our estimate is consistent with the measured
redshift. The estimated redshifts of the proposed UGS counterparts are larger,
on average, than those of known Fermi blazars, a fact that might explain the
lack of previous association or identification in published catalogues.Comment: 41 pages, 40 figures, 3 tables, accepted for publication in MNRA
Searching for Gamma-Ray counterparts to Gravitational Waves from merging binary neutron stars with the Cherenkov Telescope Array
The merger of binary neutron star (BNS) systems are predicted to be
progenitors of short gamma-ray bursts (GRBs); the definitive probe of this
association came with the recent detection of gravitational waves (GWs) from a
BNS merger by Advanced LIGO and Advanced Virgo (GW170817), in coincidence with
the short GRB 170817A observed by Fermi-GBM and INTEGRAL. Short GRBs are also
expected to emit very-high energy (VHE, > 100 GeV) photons and VHE
electromagnetic (EM) upper limits have been set with observations performed by
ground-based gamma-ray detectors and during the intense EM follow-up campaign
associated with GW170817/GRB 170817A. In the next years, the searches for VHE
EM counterparts will become more effective thanks to the Cherenkov Telescope
Array (CTA): this instrument will be fundamental for the EM follow-up of
transient GW events at VHE, owing to its unprecedented sensitivity, rapid
response (few tens of seconds) and capability to monitor large sky areas via
survey-mode operation. We present a comprehensive study on the prospects for
joint GW and VHE EM observations of merging BNSs with Advanced LIGO, Advanced
Virgo and CTA, based on detailed simulations of the multi-messenger emission
and detection. We propose a new observational strategy optimized on the prior
assumptions about the EM emission. The method can be further generalized to
include other electromagnetic emission models. According to this study CTA will
cover most of the region of the GW skymap for the intermediate and most
energetic on-axis GRBs associated to the GW event. We estimate the expected
joint GW and VHE EM detection rates and we found this rate goes from 0.08 up to
0.5 events per year for the most energetic EM sources.Comment: 26 pages, 8 figures. Submitted to JCA
Prospects for joint observations of gravitational waves and gamma rays from merging neutron star binaries
The detection of the events GW150914 and GW151226, both consistent with the
merger of a binary black hole system (BBH), opened the era of gravitational
wave (GW) astronomy. Besides BBHs, the most promising GW sources are the
coalescences of binary systems formed by two neutron stars or a neutron star
and a black hole. These mergers are thought to be connected with short Gamma
Ray Bursts (GRBs), therefore combined observations of GW and electromagnetic
(EM) signals could definitively probe this association. We present a detailed
study on the expectations for joint GW and high-energy EM observations of
coalescences of binary systems of neutron stars with Advanced Virgo and LIGO
and with the \emph{Fermi} gamma-ray telescope. To this scope, we designed a
dedicated Montecarlo simulation pipeline for the multimessenger emission and
detection by GW and gamma-ray instruments, considering the evolution of the GW
detector sensitivities. We show that the expected rate of joint detection is
low during the Advanced Virgo and Advanced LIGO 2016-2017 run; however, as the
interferometers approach their final design sensitivities, the rate will
increase by a factor of ten. Future joint observations will help to
constrain the association between short GRBs and binary systems and to solve
the puzzle of the progenitors of GWs. Comparison of the joint detection rate
with the ones predicted in this paper will help to constrain the geometry of
the GRB jet.Comment: 24 pages, 4 figure
ATel 7267: The TeV HBL blazar 1ES 1011+496 in Gamma-, X-, Optical flaring activity
We detected a gamma-ray flare from the TeV HBL blazar 1ES 1011+496 (z=0.212), triggering on FERMI-LAT data at E > 10 GeV with TS ~50, from 2015-03-12 to 2015-03-16, following the prescription of Pacciani et al
MAGIC observations of Mkn 421 in 2008, and related optical/X-ray/TeV MWL study
The HBL-type blazar Markarian 421 is one of the brightest TeV gamma-ray
sources of the Northern sky. From December 2007 until June 2008 it was
intensively observed in the VHE (E>100 GeV) band by the MAGIC gamma-ray
telescope. The source showed intense and prolonged activity during the whole
period. In some nights the integral flux rose up to 3.6 Crab units (E>200 GeV).
Intra-night rapid flux variations were observed. We compared the optical (KVA)
and X-ray (RXTE-ASM, Swift-XRT) data with the MAGIC VHE data, investigating the
correlations between different energy bands.Comment: 4 pages,4figures, Contribution to the 31st ICRC, Lodz, Poland, July
200
Fermi/LAT detection of extraordinary variability in the gamma-ray emission of the blazar PKS 1510-089
We have reanalyzed the giant outburst of the blazar PKS 1510-089 (z = 0.36) that occurred on 2011 October-November. The γ-ray flux in the 0.1-100 GeV energy range exceeded the value of 10-5 ph cm-2 s -1 for several days. The peak flux was reached on 2011 October 19, with a value of ∼4.4 × 10-5 ph cm-2 s -1, which in turn corresponds to a luminosity of ∼2 × 1049 erg s-1. A very short timescale variability was measured. Particularly on 2011 October 18, the flux-doubling time was as short as ∼20 min. This is the shortest variability ever detected in the MeV-GeV energy band. We compared our analysis with two other outbursts observed in 2009 March and 2012 February-March, when the blazar was also detected by H.E.S.S. and MAGIC to infer information about the emission at hundreds of GeV. © 2013 ESO
Gamma rays from microquasars Cygnus X-1 and Cygnus X-3
Gamma-ray observations of microquasars at high and very-high energies can
provide valuable information of the acceleration processes inside the jets, the
jet-environment interaction and the disk-jet coupling. Two high-mass
microquasars have been deeply studied to shed light on these aspects: Cygnus
X-1 and Cygnus X-3. Both systems display the canonical hard and soft X-ray
spectral states of black hole transients, where the radiation is dominated by
non-thermal emission from the corona and jets and by thermal emission from the
disk, respectively. Here, we report on the detection of Cygnus X-1 above 60 MeV
using 7.5 yr of Pass8 Fermi-LAT data, correlated with the hard X-ray state. A
hint of orbital flux modulation was also found, as the source is only detected
in phases around the compact object superior conjunction. We conclude that the
high-energy gamma-ray emission from Cygnus X-1 is most likely associated with
jets and its detection allow us to constrain the production site. Moreover, we
include in the discussion the final results of a MAGIC long-term campaign on
Cygnus X-1 that reaches almost 100 hr of observations at different X-ray
states. On the other hand, during summer 2016, Cygnus X-3 underwent a flaring
activity period in radio and high-energy gamma rays, similar to the one that
led to its detection in the high-energy regime in 2009. MAGIC performed
comprehensive follow-up observations for a total of about 70 hr. We discuss our
results in a multi-wavelength context.Comment: Proceedings of the 35th International Cosmic Ray Conference (ICRC
2017), Bexco, Busan, Korea (arXiv:1708.05153
Multi-Wavelength Observations of the HBL Object 1ES 1011+496 in Spring 2008
In the spring of 2008 MAGIC organised multi-wavelength (MWL) observations of
the blazar 1ES 1011+496. 1ES 1011+496 is a high-frequency peaked BL Lac object
discovered at VHE gamma-rays by MAGIC in spring 2007 during an optical outburst
reported by the Tuorla Blazar Monitoring Programme. MAGIC re-observed the
source during the 2008 MWL campaign which also included the Mets\"ahovi, KVA,
Swift and AGILE telescopes. This was the first MWL campaign on this source that
also included VHE coverage. MAGIC observed 1ES 1011+496 from March 4th to May
24th 2008 for a total of 27.9 hours, of which 20 h remained after quality cuts.
The observations resulted in a detection of the source a ~7 sigma significance
level with a mean flux and spectral index similar to those during the
discovery.
Here we will present the results of the MAGIC observations of the source in
combination with contemporaneous observations at other wavelengths (radio,
optical, X-rays, high energy gamma-rays) and discuss their implications on the
modelling of the spectral energy distribution.Comment: 4 pages, 5 figures, contribution to the 32nd ICRC, Beijing 201
Evidence Of Quasi Periodic Modulation In The Gamma-Ray Blazar PG1553+113
For the first time a gamma-ray and multi-wavelength nearly-periodic oscillation in an active galactic nucleus is reported by the Fermi Large Area Telescope (LAT). A quasi-periodicity in the gamma-ray flux (E>100 MeV and E>1 GeV) is observed from the well-known GeV/TeV BL Lac object PG 1553+113. The significance of the 2.18 +/- 0.08 year-period gamma-ray modulation, seen in 3.5 oscillation maxima observed, is supported by significant cross-correlated variations observed in radio and optical flux light curves, through data collected in the OVRO, Tuorla, KAIT, and CSS monitoring programs and Swift UVOT and XRT. As a BL Lac object, the mechanism driving the observed modulation could arise from the jet itself or from the process feeding the jet. It might point to interesting physical phenomena such as pulsational accretion flow instabilities, jet precession, or the tantalizing possibility of a milli-parsec scale binary super massive black hole system An intense multi-wavelength campaign aimed at unbiased monitoring of the source activity, from radio to VHE (E>100 GeV) gamma rays, started in 2015. It aims at revealing the physical scenarios that can account for such a variability pattern and at covering the next maximum, expected between the end of 2016 and beginning of 2017
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