2,255 research outputs found
Does the gamma-ray flux of the blazar 3C 454.3 vary on sub-hour timescales?
In the early days of April 2010, the blazar 3C 454.3 (z=0.859) underwent a
strong gamma-ray outburst, reaching fluxes (E > 100 MeV) in excess of 10^-5 ph
cm^-2 s^-1. The Fermi Gamma ray Space Telescope performed a 200 ks long pointed
observation starting from 5 April 2010 19:38 UTC. This allowed us to try
probing the variability of the gamma-ray emission on timescales of hours or
less. We found the variability on a few hours timescale. On sub-hour timescale
we found no evidence of significant variability, although the present
statistics is not yet conclusive and further observations are needed.Comment: 4 pages, 2 figures. Accepted for publication on MNRAS. Section 1
completely rewritten and enlarge
The bright unidentified gamma-ray source 1FGL J1227.9-4852: Can it be associated with an LMXB?
We present an analysis of high energy (HE; 0.1-300 GeV) gamma-ray
observations of 1FGL J1227.9-4852 with the Fermi Gamma-ray Space Telescope,
follow-up radio observations with the Australia Telescope Compact Array, Giant
Metrewave Radio Telescope and Parkes radio telescopes of the same field and
follow-up optical observations with the ESO VLT. We also examine archival
XMM-Newton and INTEGRAL X-ray observations of the region around this source.
The gamma-ray spectrum of 1FGL J1227.9-4852 is best fit with an exponentially
cutoff power-law, reminiscent of the population of pulsars observed by Fermi. A
previously unknown, compact radio source within the 99.7% error circle of 1FGL
J1227.9-4852 is discovered and has a morphology consistent either with an AGN
core/jet structure or with two roughly symmetric lobes of a distant radio
galaxy. A single bright X-ray source XSS J12270-4859, a low-mass X-ray binary,
also lies within the 1FGL J1227.9-4852 error circle and we report the first
detection of radio emission from this source. The potential association of 1FGL
J1227.9-4852 with each of these counterparts is discussed. Based upon the
available data we find the association of the gamma-ray source to the compact
double radio source unlikely and suggest that XSS J12270-4859 is a more likely
counterpart to the new HE source. We propose that XSS J12270-4859 may be a
millisecond binary pulsar and draw comparisons with PSR J1023+0038.Comment: Accepted for publication in MNRAS; 9 pages, 8 figures, 2 table
Cosmic rays in the surroundings of SNR G35.6-0.4
HESS J1858+020 is a TeV gamma-ray source that was reported not to have any
clear cataloged counterpart at any wavelength. However, it has been recently
proposed that this source is indirectly associated with the radio source,
re-identified as a supernova remnant (SNR), G35.6-0.4. The latter is found to
be middle-aged ( kyr) and to have nearby molecular clouds (MCs). HESS
J1858+020 was proposed to be the result of the interaction of protons
accelerated in the SNR shell with target ions residing in the clouds. The Fermi
Large Area Telescope (LAT) First Source Catalog does not list any source
coincident with the position of HESS J1858+020, but some lie close. Here, we
analyse more than 2 years of data obtained with the Fermi-LAT for the region of
interest, and consider whether it is indeed possible that the closest LAT
source, 1FGL J1857.1+0212c, is related to HESS J1858+020. We conclude it is
not, and we impose upper limits on the GeV emission originating from HESS
J1858+020. Using a simplified 3D model for the cosmic-ray propagation out from
the shell of the SNR, we consider whether the interaction between SNR G35.6-0.4
and the MCs nearby could give rise to the TeV emission of HESS J1858+020
without producing a GeV counterpart. If so, the pair of SNR/TeV source with no
GeV detection would be reminiscent of other similarly-aged SNRs, such as some
of the TeV hotspots near W28, for which cosmic-ray diffusion may be used to
explain their multi-frequency phenomenology. However, for HESS J1858+020, we
found that although the phase space in principle allows for such GeV--TeV
non-correlation to appear, usual and/or observationally constrained values of
the parameters (e.g., diffusion coefficients and cloud-SNR likely distances)
would disfavor it.Comment: In press in MNRA
Correlation of Fermi-LAT sources with the AT20GHz radio survey
We cross correlate the Fermi 11 months survey catalogue (1FGL) with the 20
GHz Australia Telescope Compact Array radio survey catalogue (AT20G) composed
by 5890 sources at declination <0 deg. Among the 738 Fermi sources distributed
in the southern sky we find 230 highly probable candidate counterparts in the
AT20G survey. Of these, 222 are already classified as blazars (176 of known
type and 46 of unknown optical class) in the Fermi 1-year LAT AGN Catalogue
(1LAC) and 8 are new associations. By studying the gamma-ray and radio
properties of these associations we find a strong correlation between the
gamma-ray flux (above 100 MeV) and the 20 GHz flux density. This correlation is
more than 3 sigma statistically significant both for the population of BL Lacs
and of FSRQ considered separately. We also find that the radio counterparts
associated to the Fermi sources have on average flat radio spectra between 5
and 20 GHz and that Fermi gamma-ray sources are not preferentially associated
with "ultra inverted spectrum" radio sources. For 2 of the 8 new associations
we build the broad band spectral energy distribution combining Fermi, Swift and
radio observations. One of these two sources is identified with the high
redshift FSRQ Swift J1656.3-3302 (z=2.4) and we classify the other source as a
candidate new FSRQ. We also study the brightest radio source of the 46
associations without an optical classification and classify it as a new BL Lac
candidate "twin" of the prototypical BL Lac OJ 287 if its redshift is somewhat
larger, z~0.4.Comment: 12 pages, 10 figures, 3 tables. Submitted to MNRAS on the 22nd March
2010
Impact of the orbital uncertainties on the timing of pulsars in binary systems
The detection of pulsations from an X-ray binary is an unambiguous signature
of the presence of a neutron star in the system. When the pulsations are missed
in the radio band, their detection at other wavelengths, like X-ray or
gamma-rays, requires orbital demodulation, since the length of the observations
are often comparable to, or longer than the system orbital period. The detailed
knowledge of the orbital parameters of binary systems plays a crucial role in
the detection of the spin period of pulsars, since any uncertainty in their
determination translates into a loss in the coherence of the signal during the
demodulation process. In this paper, we present an analytical study aimed at
unveiling how the uncertainties in the orbital parameters might impact on
periodicity searches. We find a correlation between the power of the signal in
the demodulated arrival time series and the uncertainty in each of the orbital
parameters. This correlation is also a function of the pulsar frequency. We
test our analytical results with numerical simulations, finding good agreement
between them. Finally, we apply our study to the cases of LS 5039 and LS I +61
303 and consider the current level of uncertainties in the orbital parameters
of these systems and their impact on a possible detection of a hosted pulsar.
We also discuss the possible appearance of a sideband ambiguity in real data.
The latter can occur when, due to the use of uncertain orbital parameters, the
power of a putative pulsar is distributed in frequencies lying nearby the
pulsar period. Even if the appearance of a sideband is already a signature of a
pulsar component, it may introduce an ambiguity in the determination of its
period. We present here a method to solve the sideband issue.Comment: Accepted 2012 September 08 by MNRAS. The paper contains 18 figures
and 5 table
On the use of bianisotropic huygens' metasurfaces to build leaky-wave antennas
The Electromagnetics AcademyHuygens' metasurfaces are considered a powerful tool to achieve anomalous electromagnetic field transformations. They consist of an artifcial surface built of pairs of collocated electric and magetic dipoles that force the boundary conditions for the desired transformation to be ful lled [1]. Despite their possibilities, the achievable transformations must ful l some conditions. In [2] it was
shown that Huygens' metasurfaces with passive and lossless particles can achieve an arbitrary field transformation provided that the power is conserved at each point of the metasurface and there is wave impedance matching. However, it was shown in [3], that by introducing bianisotropy of the omega-type, the matching condition can be suppressed, which allows the control of both the transmission and rejection coe cients on the metasurface.Universidad de MĂĄlaga. Campus de Excelencia Internacional AndalucĂa Tech
The radio/gamma-ray connection in Fermi-blazars
We study the correlation between the gamma-ray flux F_g, averaged over the
first 11 months of Fermi survey and integrated above 100 MeV, and the radio
flux density (F_r at 20 GHz) of Fermi sources associated with a radio
counterpart in the AT20G survey. Considering the blazars detected in both
bands, the correlation is highly significant and it is F_g~F_r^0.85+-0.04,
similar for BL Lac and FSRQ sources. However, only a small fraction (~1/15) of
the AT20G radio sources with flat radio spectrum, are detected by Fermi. To
understand if this correlation is real, we examine the selection effects
introduced by the flux limits of the radio and gamma-ray surveys, and the
importance of variability of the gamma-ray flux. We find that the
radio/gamma-ray flux correlation is real, but its slope is steeper than the
observed one, i.e. F_g~F_r^delta with delta in the range 1.25-1.5. The observed
F_g-F_r correlation and the fraction of radio sources detected by Fermi is
reproduced assuming a long term gamma-ray flux variability following a
log-normal probability distribution with standard deviation sigma>0.5
(corresponding to F_g varying by at least a factor 3). Such a variability is
compatible with what observed when comparing, for the sources in common, the
EGRET and the Fermi gamma-ray fluxes (even if the Fermi fluxes are averaged
over ~1 year). Another indication of variability is the non detection of 12 out
of 66 EGRET blazars by Fermi, despite its higher sensitivity.We also study the
strong linear correlation between the gamma-ray and the radio luminosity of the
144 AT20G-Fermi associations with known redshift and show that it is
statistically robust. Two possible implications of these correlations are
discussed: the contribution of blazars to the extragalactic gamma-ray
background and the prediction of blazars that might undergo extremely high
states of gamma-ray emission in the next years.Comment: 12 pages, 8 figures, 2 tables. Accepted for publication in MNRA
Polarization and photometric observations of the gamma-ray blazar PG 1553+113
We present the results of an observational photo-polarimetry campaign of the
blazar PG 1553+113 at optical wavelengths. The blazar was recently detected at
very high energies (> 100 GeV) by the H.E.S.S and MAGIC gamma-ray Cherenkov
telescopes.
Our high-temporal resolution data show significant variations in the linear
polarization percentage and position angle at inter-night time-scales, while at
shorter (intra-night) time-scales both parameters varied less significantly, if
at all. Changes in the polarization angle seem to be common in gamma-ray
emitting blazars. Simultaneous differential photometry (through the B and R
bands) shows no significant variability in the total optical flux. We provide B
and R magnitudes, along with a finding chart, for a set of field stars suitable
for differential photometry.Comment: 4 pages, 3 figures. To be published by Astronomy and Astrophysic
The intergalactic magnetic field constrained by Fermi/LAT observations of the TeV blazar 1ES 0229+200
TeV photons from blazars at relatively large distances, interacting with the
optical-IR cosmic background, are efficiently converted into electron-positron
pairs. The produced pairs are extremely relativistic (Lorentz factors of the
order of 1e6 1e7 and promptly loose their energy through inverse Compton
scatterings with the photons of the microwave cosmic background, producing
emission in the GeV band. The spectrum and the flux level of this reprocessed
emission is critically dependent on the intensity of the intergalactic magnetic
field, B, that can deflect the pairs diluting the intrinsic emission over a
large solid angle. We derive a simple relation for the reprocessed spectrum
expected from a steady source. We apply this treatment to the blazar 1ES
0229+200, whose intrinsic very hard TeV spectrum is expected to be
approximately steady. Comparing the predicted reprocessed emission with the
upper limits measured by the Fermi/Large Area Telescope, we constrain the value
of the intergalactic magnetic field to be larger than Gauss, depending on the model of extragalactic background light.Comment: 5 pages 2 figures, revised version accepted for publication in MNRAS
(Letters
On leptonic models for blazars in the Fermi era
Some questions raised by Fermi-LAT data about blazars are summarized, along
with attempts at solutions within the context of leptonic models. These include
both spectral and statistical questions, including the origin of the GeV breaks
in low-synchrotron peaked blazars, the location of the gamma-ray emission
sites, the correlations in the spectral energy distributions with luminosity,
and the difficulty of synchrotron/SSC models to fit the spectra of some TeV
blazars.Comment: 9 pages, 1 figure, in "Beamed and Unbeamed Gamma Rays from Galaxies,"
Muonio, Finland, 11-15 April, 2011, ed. R. Wagner, L. Maraschi, A. Sillanpaa,
to appear in Journal of Physics: Conference Serie
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