750 research outputs found
Extreme BL Lacs: probes for cosmology and UHECR candidates
High-energy observations of extreme BL Lac objects, such as 1ES0229+200 or
1ES 0347-121, recently focused interest both for blazar and jet physics and for
the implication on the extragalactic background light and intergalactic
magnetic field estimate. Moreover, their enigmatic properties have been
interpreted in a scenario in which their primary high- energy output is through
a beam of high-energy hadrons. However, despite their possible important role
in all these topics, the number of these extreme highly peaked BL Lac objects
(EHBL) is still rather small. Aiming at increase their number, we selected a
group of EHBL candidates considering those undetected (or only barely detected)
by the LAT onboard Fermi and characterized by a high X-ray versus radio flux
ratio. We assembled the multi-wavelength spectral energy distribution of the
resulting 9 sources, using available archival data of Swift, GALEX, and Fermi
satellites, confirming their nature. Through a simple one-zone synchrotron
self-Compton model we estimate the expected very high energy flux, finding that
in the majority of cases it is within the reach of present generation of
Cherenkov arrays or of the forthcoming CTA.Comment: 6 pages, to appear in the Proceedings of the Conference "High-Energy
Phenomena and Relativistic Outflows V", held in La Plata, 5-8 October 201
On the detectability of Lorentz invariance violation through anomalous multi-TeV -ray spectra of blazars
Cosmic opacity for very high-energy gamma rays ( TeV) due to the
interaction with the extragalactic background light can be strongly reduced
because of possible Lorentz-violating terms in the particle dispersion
relations expected, e.g., in several versions of quantum gravity theories. We
discuss the possibility to use very high energy observations of blazars to
detect anomalies of the cosmic opacity induced by LIV, considering in
particular the possibility to use -- besides the bright and close-by BL Lac Mkn
501 -- extreme BL Lac objects. We derive the modified expression for the
optical depth of rays considering also the redshift dependence and we
apply it to derive the expected high-energy spectrum above 10 TeV of Mkn 501 in
high and low state and the extreme BL Lac 1ES 0229+200. We find that, besides
the nearby and well studied BL Lac Mkn 501 -- especially in high state --,
suitable targets are extreme BL Lac objects, characterized by quite hard TeV
intrinsic spectra likely extending at the energies relevant to detect LIV
features.Comment: 9 pages, 9 figures, accepted by A&
Constraining the location of the emitting region in Fermi blazars through rapid gamma-ray variability
We consider the 1.5 years Fermi Large Area Telescope light curves (E > 100
MeV) of the flat spectrum radio quasars 3C 454.3 and PKS 1510-089, which show
high activity in this period of time. We characterise the duty cycle of the
source by comparing the time spent by the sources at different flux levels. We
consider in detail the light curves covering periods of extreme flux. The large
number of high-energy photons collected by LAT in these events allows us to
find evidence of variability on timescales of few hours. We discuss the
implications of significant variability on such short timescales, that
challenge the scenario recently advanced in which the bulk of the gamma-ray
luminosity is produced in regions of the jet at large distances (tens of
parsec) from the black hole.Comment: 5 pages, 5 figures, accpted for publication in MNRAS Letters
The red blazar PMN J2345-1555 becomes blue
The Flat Spectrum Radio Quasar PMN J2345-1555 is a bright gamma-ray source,
that recently underwent a flaring episode in the IR, UV and gamma-ray bands.
The flux changed quasi simultaneously at different frequencies, suggesting that
it was produced by a single population of emitting particles, hence by a single
and well localized region of the jet. While the overall Spectral Energy
Distribution (SED) before the flare was typical of powerful blazars (namely two
broad humps peaking in the far IR and below 100 MeV bands, respectively),
during the flare the peaks moved to the optical-UV and to energies larger than
1 GeV, to resemble low power BL Lac objects, even if the observed bolometric
luminosity increased by more than one order of magnitude. We interpret this
behavior as due to a change of the location of the emission region in the jet,
from within the broad line region, to just outside. The corresponding decrease
of the radiation energy density as seen in the comoving frame of the jet
allowed the relativistic electrons to be accelerated to higher energies, and
thus produce a "bluer" SED.Comment: 5 pages, 4 figures, MNRAS Letters, in pres
The NuSTAR view on Hard-TeV BL Lacs
Hard-TeV BL Lacs are a new type of blazars characterized by a hard intrinsic
TeV spectrum, locating the peak of their gamma-ray emission in the spectral
energy distribution (SED) above 2-10 TeV. Such high energies are problematic
for the Compton emission, using a standard one-zone leptonic model. We study
six examples of this new type of BL Lacs in the hard X-ray band with the NuSTAR
satellite. Together with simultaneous observations with the SWIFT satellite, we
fully constrain the peak of the synchrotron emission in their SED, and test the
leptonic synchrotron self-Compton (SSC) model. We confirm the extreme nature of
5 objects also in the synchrotron emission. We do not find evidence of
additional emission components in the hard X-ray band. We find that a one-zone
SSC model can in principle reproduce the extreme properties of both peaks in
the SED, from X-ray up to TeV energies, but at the cost of i) extreme electron
energies with very low radiative efficiency, ii) conditions heavily out of
equipartition (by 3 to 5 orders of magnitude), and iii) not accounting for the
simultaneous UV data, which then should belong to a different emission
component, possibly the same as the far-IR (WISE) data. We find evidence of
this separation of the UV and X-ray emission in at least two objects. In any
case, the TeV electrons must not "see" the UV or lower-energy photons, even if
coming from different zones/populations, or the increased radiative cooling
would steepen the VHE spectrum.Comment: 13 pages, 2 figures. Version accepted for publication in MNRAS. Fig.
2 corrected for a small plotting erro
On the distribution of fluxes of gamma-ray blazars: hints for a stochastic process?
We examine a model for the observed temporal variability of powerful blazars
in the -ray band in which the dynamics is described in terms of a
stochastic differential equation, including the contribution of a deterministic
drift and a stochastic term. The form of the equation is motivated by the
current astrophysical framework, accepting that jets are powered through the
extraction of the rotational energy of the central supermassive black hole
mediated by magnetic fields supported by a so-called \emph{magnetically
arrested} accretion disk. We apply the model to the -ray light curves
of several bright blazars and we infer the parameters suitable to describe
them. In particular, we examine the differential distribution of fluxes
() and we show that the predicted probability density function
for the assumed stochastic equation naturally reproduces the observed power law
shape at large fluxes with
.Comment: 7 pages, 4 figures, accepted for publication in MNRA
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
Astrophysical hints of axion-like particles
After reviewing three astrophysical hints of the existence of axionlike particles (ALPs), we describe in more detail a new similar hint involving flat spectrum radio quasars (FSRQs). Detection of FSRQs above about 20GeV pose a challenge to very-high-energy (VHE) astrophysics, because at those energies the ultraviolet emission from their broad line region should prevent photons produced by the central engine to leave the source. Although a few astrophysical explanations have been put forward, they are totally ad hoc. We show that a natural explanation instead arises within the conventional models of FSRQs provided that photon-ALP oscillations occur inside the source. Our analysis takes the FSRQ PKR 1222+206 as an example, and it looks tantalizing that basically the same choice of the free model parameters adopted in this case is consistent with those that provide the other three hints of the existence of ALPs
The gamma-ray brightest days of the blazar 3C 454.3
In the first week of December 2009, the blazar 3C 454.3 became the brightest
high energy source in the sky. Its photon flux reached and surpassed the level
of 1e-5 ph/cm2/s above 100 MeV. The Swift satellite observed the source several
times during the period of high gamma-ray flux, and we can construct really
simultaneous spectral energy distributions (SED) before, during, and after the
luminosity peak. Our main findings are: i) the optical, X-ray and gamma-ray
fluxes correlate; ii) the gamma-ray flux varies quadratically (or even more)
with the optical flux; iii) a simple one-zone synchrotron inverse Compton model
can account for all the considered SED; iv) in this framework the gamma-ray vs
optical flux correlation can be explained if the magnetic field is slightly
fainter when the overall jet luminosity is stronger; v) the power that the jet
spent to produce the peak gamma-ray luminosity is of the same order, or larger,
than the accretion disk luminosity. During the flare, the total jet power
surely surpassed the accretion power.Comment: 13 pages, 9 figures, revised version accepted for publication in
MNRA
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