101 research outputs found
On the Nature of MeV-blazars
Broad-band spectra of the FSRQ (flat-spectrum-radio quasars) detected in the
high energy gamma-ray band imply that there may be two types of such objects:
those with steep gamma-ray spectra, hereafter called MeV-blazars, and those
with flat gamma-ray spectra, GeV-blazars. We demonstrate that this difference
can be explained in the context of the ERC (external-radiation-Compton) model
using the same electron injection function. A satisfactory unification is
reachable, provided that: (a) spectra of GeV-blazars are produced by internal
shocks formed at the distances where cooling of relativistic electrons in a jet
is dominated by Comptonization of broad emission lines, whereas spectra of
MeV-blazars are produced at the distances where cooling of relativistic
electrons is dominated by Comptonization of near-IR radiation from hot dust;
(b) electrons are accelerated via a two step process and their injection
function takes the form of a double power-law, with the break corresponding to
the threshold energy for the diffusive shock acceleration. Direct predictions
of our model are that, on average, variability time scales of the MeV-blazars
should be longer than variability time scales of the GeV-blazars, and that both
types of the blazar phenomenon can appear in the same object.Comment: Accepted for publication in the Astrophysical Journa
Comptonization of Infrared Radiation from Hot Dust by Relativistic Jets in Quasars
We demonstrate the importance of near-infrared radiation from hot dust for
Compton cooling of electrons/positrons in quasar jets. In our model, we assume
that the non-thermal radiation spectra observed in OVV quasars are produced by
relativistic electrons/positrons accelerated in thin shells which propagate
down the jet with relativistic speeds. We show that the Comptonization of the
near-IR flux is likely to dominate the radiative output of OVV quasars in the
energy range from tens of keV up to hundreds of MeV, where it exceeds that
produced by Comptonization of the UV radiation reprocessed and rescattered in
the Broad Emission Line region. The main reason for this lies in the fact that
the jet encounters the ambient IR radiation over a relatively large distance as
compared to the distance where the energy density of the broad emission line
light peaks. In the soft - to mid energy X-ray band, the spectral component
resulting from Comptonization of the near-IR radiation joins smoothly with the
synchrotron-self-Compton component, which may be responsible for the soft X-ray
flux. At the highest observed gamma-ray energies, in the GeV range,
Comptonization of broad emission lines dominates over other components.Comment: 23 pages, including 5 Postscript figures and 3 tables, uses aastex.
Astrophysical Journal, accepted for publication in the December 20, 2000
issu
Implications on the blazar sequence and inverse Compton models from Fermi bright blazars
In this paper, we use the quasi-simultaneous spectra of Fermi bright blazars
and Fermi detected narrow line Seyfert 1 (NLS1) to study the blazar sequence
and inverse Compton (IC) models. I. The synchrotron peak luminosities (L_{s})
significantly inverse correlate with the synchrotron peak frequencies
(\nu_{s}), L_{s}\propto\nu_{s}^{-0.44}, which is consistent with the blazar
sequence. In addition to the correlation, there are some blazars showing low
\nu_{s} and low L_{s}. To study the relation between these low \nu_{s} low
L_{s} blazars and the blazar sequence, we present correlations of the parameter
L_{s}\nu_{s}^{1/4} with the ratio of Compton to synchrotron peak frequencies
(r_{Cs}\equiv\nu_{C}/\nu_{s}) and with the ratio of Compton to synchrotron
luminosities (CD\equiv L_{C}/L_{s}). The results indicate that both
correlations are significant with a Pearson's probability for null correlation
of p=0.0218 and p=0.0286 respectively. This does not support the idea that the
low \nu_{s} low L_{s} blazars are sources with less beaming. Another
possibility, as suggested by Ghisellini & Tavecchio, is that these blazars have
relative lower black hole masses. To test this, we collect the black hole
masses of 30 blazars from archives, and find that the hole mass correlates with
the parameter L_{s}\nu_{s}^{0.44} (p=0.0344). Therefore, the black hole masses
of low \nu_{s} low L_{s} blazars are statistically small. The NLS1s are thought
to have lower black hole masses. We find that the four NLS1s detected by Fermi
have low \nu_{s} and low L_{s}. This supports the above result. II. The ratio
r_{Cs} correlates with CD significantly (p=0.00375). The external Compton (EC)
model can naturally explain this correlation, while synchrotron self Compton
(SSC) model can not. This agrees with the findings of many authors that the EC
process dominates the gamma-ray emission of Flat Spectrum Radio Quasars.Comment: 9 figures, accepted by Ap
Multiwavelength study of TeV Blazar Mrk421 during giant flare
Context: The nearby (z=0.031) TeV blazar Mrk421 was reported to be in a high
state of flux activity since November, 2009. Aims: To investigate possible
changes in the physical parameters of Mrk421 during its high state of activity
using multiwavelength data. Methods: We have observed this source in bright
state using High Altitude GAmma Ray (HAGAR) telescope array at energies above
250 GeV during February 13 - 19, 2010. Optical, X-ray and gamma-ray archival
data are also used to obtain the SEDs and light curves. Results: Mrk421 was
found to undergo one of its brightest flaring episodes on February 17, 2010 by
various observations in X-rays and gamma-rays. HAGAR observations during
February 13 - 19, 2010 at the energies above 250 GeV show an enhancement in the
flux level, with a maximum flux of ~ 7 Crab units being detected on February
17, 2010. We present the spectral energy distributions during this flaring
episode and investigate the correlation of the variability in X-ray and
gamma-ray bands. Conclusions: Our multiwavelength study suggests that the flare
detected during February 16 and 17, 2010 could arise due to a passing shock in
the jet.Comment: 9 pages, 10 figures, Accepted for publication in Astronomy and
Astrophysics (A & A
A Multi-wavelength View of the TeV Blazar Markarian 421: Correlated Variability, Flaring, and Spectral Evolution
We report results from a multi-wavelength monitoring campaign on Mrk 421 over
the period of 2003-2004. The source was observed simultaneously at TeV and
X-ray energies, with supporting observations frequently carried out at optical
and radio wavelengths. The large amount of simultaneous data has allowed us to
examine the variability of Mrk 421 in detail. The variabilities are generally
correlated between the X-ray and gamma-ray bands, although the correlation
appears to be fairly loose. The light curves show the presence of flares with
varying amplitudes on a wide range of timescales both at X-ray and TeV
energies. Of particular interest is the presence of TeV flares that have no
coincident counterparts at longer wavelengths, because the phenomenon seems
difficult to understand in the context of the proposed emission models for TeV
blazars. We have also found that the TeV flux reached its peak days before the
X-ray flux during a giant flare in 2004. Such a difference in the development
of the flare presents a further challenge to the emission models. Mrk 421
varied much less at optical and radio wavelengths. Surprisingly, the normalized
variability amplitude in optical seems to be comparable to that in radio,
perhaps suggesting the presence of different populations of emitting electrons
in the jet. The spectral energy distribution (SED) of Mrk 421 is seen to vary
with flux, with the two characteristic peaks moving toward higher energies at
higher fluxes. We have failed to fit the measured SEDs with a one-zone SSC
model; introducing additional zones greatly improves the fits. We have derived
constraints on the physical properties of the X-ray/gamma-ray flaring regions
from the observed variability (and SED) of the source. The implications of the
results are discussed. (Abridged)Comment: 32 pages, 12 figures, to appear in Ap
Suzaku Observations of Luminous Quasars: Revealing the Nature of High-Energy Blazar Emission in Quiescent States
We present the results from the Suzaku X-ray observations of five
flat-spectrum radio quasars (FSRQs), namely PKS0208-512, Q0827+243,
PKS1127-145, PKS1510-089 and 3C 454.3. All these sources were additionally
monitored simultaneously or quasi-simultaneously by the Fermi satellite in
gamma-rays and the Swift UVOT in the UV and optical bands, respectively. We
constructed their broad-band spectra covering the frequency range from 10^14 Hz
up to 10^25 Hz, and those reveal the nature of high-energy emission of luminous
blazars in their low-activity states. The analyzed X-ray spectra are well
fitted by a power-law model with photoelectric absorption. In the case of
PKS0208-512, PKS1127-145, and 3C 454.3, the X-ray continuum showed indication
of hard-ening at low-energies. Moreover, when compared with the previous X-ray
observations, we see a significantly increasing contribution of low-energy
photons to the total X-ray fluxes when the sources are getting fainter. The
same behavior can be noted in the Suzaku data alone. A likely explanation
involves a variable, flat-spectrum component produced via inverse-Compton (IC)
emission, plus an additional, possibly steady soft X-ray component prominent
when the source gets fainter. This soft X-ray excess is represented either by a
steep powerlaw (photon indices Gamma ~ 3 - 5) or a blackbody-type emission with
temperatures kT ~ 0.1-0.2 keV. We model the broad-band spectra spectra of the
five observed FSRQs using synchrotron self-Compton (SSC) and/or
external-Compton radiation (ECR) models. Our modeling suggests that the
difference between the low and high-activity states in luminous blazars is due
to the different total kinetic power of the jet, most likely related to varying
bulk Lorentz factor of the outflow within the blazar emission zone.Comment: 36 pages, 8 figures, 11 tables. Accepted for publication in the
Astrophysical Journal
Radio-Loud Narrow-Line Seyfert 1 as a New Class of Gamma-Ray AGN
We report the discovery with Fermi/LAT of gamma-ray emission from three
radio-loud narrow-line Seyfert 1 galaxies: PKS 1502+036 (z=0.409), 1H 0323+342
(z=0.061) and PKS 2004-447 (z=0.24). In addition to PMN J0948+0022 (z=0.585),
the first source of this type to be detected in gamma rays, they may form an
emerging new class of gamma-ray active galactic nuclei (AGN). These findings
can have strong implications on our knowledge about relativistic jets and the
unified model of AGN.Comment: 16 pages, 1 figure. Accepted for publication on ApJ Letters.
Corresponding author: Luigi Foschin
Fermi/LAT discovery of gamma-ray emission from a relativistic jet in the narrow-line quasar PMN J0948+0022
We report the discovery by the Large Area Telescope (LAT) onboard the Fermi
Gamma-ray Space Telescope of high-energy gamma-ray emission from the peculiar
quasar PMN J0948+0022 (z=0.5846). The optical spectrum of this object exhibits
rather narrow Hbeta (FWHM(Hbeta) ~ 1500 km s^-1), weak forbidden lines and is
therefore classified as a narrow-line type I quasar. This class of objects is
thought to have relatively small black hole mass and to accrete at high
Eddington ratio. The radio loudness and variability of the compact radio core
indicates the presence of a relativistic jet. Quasi simultaneous
radio-optical-X-ray and gamma-ray observations are presented. Both radio and
gamma-ray emission (observed over 5-months) are strongly variable. The
simultaneous optical and X-ray data from Swift show a blue continuum attributed
to the accretion disk and a hard X-ray spectrum attributed to the jet. The
resulting broad band spectral energy distribution (SED) and, in particular, the
gamma-ray spectrum measured by Fermi are similar to those of more powerful
FSRQ. A comparison of the radio and gamma-ray characteristics of PMN J0948+0022
with the other blazars detected by LAT shows that this source has a relatively
low radio and gamma-ray power, with respect to other FSRQ. The physical
parameters obtained from modelling the SED also fall at the low power end of
the FSRQ parameter region discussed in Celotti & Ghisellini (2008). We suggest
that the similarity of the SED of PMN J0948+0022 to that of more massive and
more powerful quasars can be understood in a scenario in which the SED
properties depend on the Eddington ratio rather than on the absolute power.Comment: 10 pages, 5 figures, accepted for publication on ApJ Main Journal.
Corresponding author: L. Foschin
Correlation between Fermi/LAT gamma-ray and 37 GHz radio properties of northern AGN averaged over 11 months
Although the Fermi mission has increased our knowledge of gamma-ray AGN, many
questions remain, such as the site of gamma-ray production, the emission
mechanism, and the factors that govern the strength of the emission. Using data
from a high radio band, 37 GHz, uncontaminated by other radiation components
besides the jet emission, we study these questions with averaged flux densities
over the the first year of Fermi operations. We look for possible correlations
between the 100 MeV - 100 GeV band used by the Fermi satellite and 37 GHz radio
band observed at the Aalto University Metsahovi Radio Telescope, as well as for
differences between the gamma-ray emission of different AGN subsamples. We use
data averaged over the 1FGL period. Our sample includes 249 northern AGN,
including a complete sample of 68 northern AGN with a measured average flux
density exceeding 1 Jy. We find significant correlation between both the flux
densities and luminosities in gamma and radio bands. The Fermi luminosity is
inversely correlated with the peak frequency of the synchrotron component of
the AGN spectral energy distributions. We also calculate the gamma dominances,
defined as the ratio between the gamma and radio flux densities, and find an
indication that high-energy blazars are more gamma-dominated than low-energy
blazars. After studying the distributions of gamma and radio luminosities, it
is clear that BL Lacertae objects are different from quasars, with
significantly lower luminosities. It is unclear whether this is an intrinsic
difference, an effect of variable relativistic boosting across the synchrotron
peak frequency range, or the result of Fermi being more sensitive to hard
spectrum sources like BL Lacertae objects. Our results suggest that the gamma
radiation is produced co-spatially with the 37 GHz emission, i.e., in the jet.Comment: 8+5 pages, 9 figures and 2 tables. To be published in A&
The connection between gamma-ray emission and millimeter flares in Fermi/LAT blazars
We compare the gamma-ray photon flux variability of northern blazars in the
Fermi/LAT First Source Catalog with 37 GHz radio flux density curves from the
Metsahovi quasar monitoring program. We find that the relationship between
simultaneous millimeter (mm) flux density and gamma-ray photon flux is
different for different types of blazars. The flux relation between the two
bands is positively correlated for quasars and does not exist for BLLacs.
Furthermore, we find that the levels of gamma-ray emission in high states
depend on the phase of the high frequency radio flare, with the brightest
gamma-ray events coinciding with the initial stages of a mm flare. The mean
observed delay from the beginning of a mm flare to the peak of the gamma-ray
emission is about 70 days, which places the average location of the gamma-ray
production at or downstream of the radio core. We discuss alternative scenarios
for the production of gamma-rays at distances of parsecs along the length the
jet.Comment: 11 pages, 7 figures, accepted for publication in A&A, new version
contains changes suggested by refere
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