977 research outputs found
Neutrino emission from BL Lac objects: the role of radiatively inefficient accretion flows
The origin of the astrophysical high-energy neutrinos discovered by IceCube
is currently a major mystery. The recent detection of IceCube-170922A, a
300 TeV neutrino potentially correlated with the flaring -ray
source TXS 0506+056, directs attention toward BL Lac objects (BL Lacs), the
subclass of blazars with weak emission lines. While high-energy neutrinos can
be produced via photohadronic interactions between protons accelerated in their
jets and ambient low-energy photons, the density of the latter in such objects
had generally been thought to be too low for efficient neutrino emission. Here
we consider the role of radiatively inefficient accretion flows (RIAFs), which
can plausibly exist in the nuclei of BL Lacs, as the source of target photons
for neutrino production. Based on simple model prescriptions for the spectra of
RIAFs at different accretion rates, we find that they can be sufficienly
intense to allow appreciable neutrino emission for the class of
low-synchrotron-peak BL Lacs such as TXS 0506+056. In constrast, for
high-synchrotron-peak BL Lacs including Mkn 421 and Mkn 501, the contribution
of RIAFs is subdominant and their neutrino production efficiency can remain
low, consistent with their non-detection by IceCube to date.Comment: 5 pages, 4 figures, accepted to MNRAS as Lette
Are GRB 980425 and GRB 031203 real outliers or twins of GRB 060218?
GRB 980425 and GRB 031203 are apparently two outliers with respect to the
correlation between the isotropic equivalent energy E_iso emitted in the prompt
radiation phase and the peak frequency E_peak of the spectrum in a vF(v)
representation (the so-called Amati relation). We discuss if these two bursts
are really different from the others or if their location in the E_iso-E_peak
plane is the result of other effects, such as viewing them off-axis, or through
a scattering screen, or a misinterpretation of their spectral properties. The
latter case seems particularly interesting after GRB 060218, that, unlike GRB
031203 and GRB 980425, had a prompt emission detected both in hard and soft
X-rays which lasted ~2800 seconds. This allowed to determine its E_peak and
total emitted energy. Although it shares with GRB 031203 the total energetics,
it is not an outlier with respect to the Amati correlation. We then investigate
if a hard-to-soft spectral evolution in GRB 031203 and GRB 980425, consistent
with all the observed properties, can give rise to a time integrated spectrum
with an E_peak consistent with the Amati relation.Comment: 13 pages, 9 figures, 2 tables. Accepted for publication in MNRA
Evidence for an axion-like particle from PKS 1222+216?
The surprising discovery by MAGIC of an intense, rapidly varying emission in
the energy range 70 - 400 GeV from the flat spectrum radio quasar PKS 1222+216
represents a challenge for all interpretative scenarios. Indeed, in order to
avoid absorption of \gamma rays in the dense ultraviolet radiation field of the
broad line region (BLR), one is forced to invoke some unconventional
astrophysical picture, like for instance the existence of a very compact (r\sim
10^{14} cm) emitting blob at a large distance (R \sim10^{18} cm) from the jet
base. We offer the investigation of a scenario based on the standard blazar
model for PKS 1222+216 where \gamma rays are produced close to the central
engine, but we add the new assumption that inside the source photons can
oscillate into axion-like particles (ALPs), which are a generic prediction of
several extensions of the Standard Model of elementary particle interactions.
As a result, a considerable fraction of very-high-energy photons can escape
absorption from the BLR through the mechanism of photon-ALP oscillations much
in the same way as they largely avoid absorption from extragalactic background
light when propagating over cosmic distances in the presence of large-scale
magnetic fields in the nG range. In addition we show that the above MAGIC
observations and the simultaneous Fermi/LAT observations in the energy range
0.3 - 3 GeV can both be explained by a standard spectral energy distribution
for experimentally allowed values of the model parameters. In particular, we
need a very light ALP just like in the case of photon-ALP oscillations in
cosmic space. Moreover, we find it quite tantalizing that the most favorable
value of the photon-ALP coupling happens to be the same in both situations.
Although our ALPs cannot contribute to the cold dark matter, they are a viable
candidate for the quintessential dark energy. [abridged]Comment: 32 pages, 10 figures, accepted for publication in Physical Review
Constraints on the Physical Parameters of TeV Blazars
We consider the constraints on the physical parameters of a homogeneous SSC
model that can be derived from the spectral shape and variability of TeV
blazars. Assuming that the relativistic electron spectrum is a broken power
law, where the break energy is a free parameter, we write the
analytical formulae that allow to connect the physical parameters of the model
to observable quantities. The constraints can be summarized in a plane where
the coordinates are the Doppler factor and the magnetic field. The consistency
between the break energy and the balance between cooling and escape and the
interpretation of the soft photon lags measured in some sources as radiative
cooling times are treated as additional independent constraints.
We apply themethod to the case of three well known blazars, PKS 2155-304, Mrk
421 and Mrk 501.Comment: 36 pages, incl. 6 figures in PS format, AAS LaTeX, to be published in
ApJ, Dec 199
Powerful high energy emission of the remarkable BL Lac object S5 0716+714
BL Lac objects of the intermediate subclass (IBLs) are known to emit a
substantial fraction of their power in the energy range 0.1--10 GeV. Detecting
gamma-ray emission from such sources provides therefore a direct probe of the
emission mechanisms and of the underlying powerhouse.
The AGILE gamma-ray satellite detected the remarkable IBL S5 0716+714 (z
\simeq 0.3) during a high state in the period from 2007 September - October,
marked by two very intense flares reaching peak fluxes of 200\times10^{-8} ph /
cm^2 s above 100 MeV, with simultaneous optical and X-ray observations. We
present here a theoretical model for the two major flares and discuss the
overall energetics of the source.
We conclude that 0716+714 is among the brightest BL Lac's ever detected at
gamma-ray energies. Because of its high power and lack of signs for ongoing
accretion or surrounding gas, the source is an ideal candidate to test the
maximal power extractable from a rotating supermassive black hole via the pure
Blandford-Znajek (BZ) mechanism. We find that during the 2007 gamma-ray flares
our source approached or just exceeded the upper limit set by BZ for a black
hole of mass 10^9 M_sunComment: 12 pages, 3 figure
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