100 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
Possible Detection of Apparent Superluminal inward motion in Markarian 421 after the Giant X-ray flare in February, 2010
We report on the VLBI follow-up observations using the Japanese VLBI Network
(JVN) array at 22 GHz for the largest X-ray flare of TeV blazar Mrk 421 that
occurred in mid-February, 2010. The total of five epochs of observations were
performed at intervals of about 20 days between March 7 and May 31, 2010. No
new-born component associated with the flare was seen directly in the total
intensity images obtained by our multi-epoch VLBI observations. However, one
jet component located at ~1 mas north-west from the core was able to be
identified, and its proper motion can be measured as -1.66+/-0.46 mas yr^-1,
which corresponds to an apparent velocity of -3.48+/-0.97 c. Here, this
negative velocity indicates that the jet component was apparently moving toward
the core. As the most plausible explanation, we discuss that the apparent
negative velocity was possibly caused by the ejection of a new component, which
could not be resolved with our observations. In this case, the obtained Doppler
factor of the new component is around 10 to 20, which is consistent with the
ones typically estimated by model fittings of spectral energy distribution for
this source.Comment: 9 pages, 6 figures, 3 tables, accepted for publication in Ap
A 22-year Southern Sky Survey for Transient and Variable Radio Sources using the Molonglo Observatory Synthesis Telescope
We describe a 22-year survey for variable and transient radio sources,
performed with archival images taken with the Molonglo Observatory Synthesis
Telescope (MOST). This survey covers 2775 \unit{deg^2} of the sky south of
at an observing frequency of 843 MHz, an angular
resolution of 45 \times 45 \csc | \delta| \unit{arcsec^2} and a sensitivity
of 5 \sigma \geq 14 \unit{mJy beam^{-1}}. We describe a technique to
compensate for image gain error, along with statistical techniques to check and
classify variability in a population of light curves, with applicability to any
image-based radio variability survey. Among radio light curves for almost 30000
sources, we present 53 highly variable sources and 15 transient sources. Only 3
of the transient sources, and none of the variable sources have been previously
identified as transient or variable. Many of our variable sources are suspected
scintillating Active Galactic Nuclei. We have identified three variable sources
and one transient source that are likely to be associated with star forming
galaxies at , but whose implied luminosity is higher than the
most luminous known radio supernova (SN1979C) by an order of magnitude. We also
find a class of variable and transient source with no optical counterparts.Comment: Accepted for publication in MNRAS. 34 pages, 30 figures, 7 table
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
Synoptic studies of seventeen blazars detected in very high-energy gamma-rays
Since 2002, the number of detected blazars at gamma-ray energies above 100
GeV has more than doubled. I study 17 blazars currently known to emit E>100 GeV
gamma rays. Their intrinsic energy spectra are reconstructed by removing
extragalactic background light attenuation effects. Luminosity and spectral
slope in the E>100 GeV region are then compared and correlated among each
other, with X-ray, optical and radio data, and with the estimated black hole
(BH) masses of the respective host galaxies.
According to expectations from synchrotron self-Compton emission models, a
correlation on the 3.6-sigma significance level between gamma-ray and X-ray
fluxes is found, while correlations between gamma-ray and optical/radio fluxes
are less pronounced. Further, a general hardening of the E>100 GeV spectra with
increasing gamma-ray luminosity is observed. This goes in line with a
correlation of the gamma-ray luminosity and the synchrotron peak frequency,
which is also seen. Tests for possible selection effects reveal a hardening of
the spectra with increasing redshift. The gamma-ray emission might depend on
the mass of the central BH. The studied blazars show no correlation of the BH
masses with the spectral index and the luminosity in the E>100 GeV region.
Also temporal properties of the X-ray and E>100 GeV gamma-ray flux are
considered. No general trends are found, except that the blazars with the most
massive BHs do not show particularly high duty cycles. In general, VHE flare
time-scales are not found to scale with the BH mass.
As a specific application of the luminosity study, a constraint for the still
undetermined redshift of the blazar PG 1553+113 is discussed.Comment: 17 pages, 18 figures, MNRAS in press. Abstract extended; minor
modifications in sect. 3, 5.5; Fig. 7c corrected; references update
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
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