119 research outputs found
An Exceptional Radio Flare in Markarian 421
In September 2012, the high-synchrotron-peaked (HSP) blazar Markarian 421
underwent a rapid wideband radio flare, reaching nearly twice the brightest
level observed in the centimeter band in over three decades of monitoring. In
response to this event we carried out a five epoch centimeter- to
millimeter-band multifrequency Very Long Baseline Array (VLBA) campaign to
investigate the aftermath of this emission event. Rapid radio variations are
unprecedented in this object and are surprising in an HSP BL Lac object. In
this flare, the 15 GHz flux density increased with an exponential doubling time
of about 9 days, then faded to its prior level at a similar rate. This is
comparable with the fastest large-amplitude centimeter-band radio variability
observed in any blazar. Similar flux density increases were detected up to
millimeter bands. This radio flare followed about two months after a similarly
unprecedented GeV gamma-ray flare (reaching a daily E>100 MeV flux of (1.2 +/-
0.7)x10^(-6) ph cm^(-2) s^(-1)) reported by the Fermi Large Area Telescope
(LAT) collaboration, with a simultaneous tentative TeV detection by ARGO-YBJ. A
cross-correlation analysis of long-term 15 GHz and LAT gamma-ray light curves
finds a statistically significant correlation with the radio lagging ~40 days
behind, suggesting that the gamma-ray emission originates upstream of the radio
emission. Preliminary results from our VLBA observations show brightening in
the unresolved core region and no evidence for apparent superluminal motions or
substantial flux variations downstream.Comment: 5 pages, 8 figures. Contributed talk at the meeting "The Innermost
Regions of Relativistic Jets and Their Magnetic Fields", Granada, Spain.
Updated to correct author list and reference
Si and Mn Abundances in Damped Lya Systems with Low Dust Content
We have measured the abundances of Zn, Si, Mn, Cr, Fe, and Ni in three damped
Lyman alpha systems at redshifts z < 1 from high resolution echelle spectra of
QSOs recorded with the Keck I telescope. In all three cases the abundances of
Cr, Fe, and Ni relative to Zn indicate low levels of dust depletions. We
propose that when the proportion of refractory elements locked up in dust
grains is less than about 50 percent, it is plausible to assume an
approximately uniform level of depletion for all grain constituents and, by
applying a small dust correction, recover the intrisic abundances of Si and Mn.
We use this approach on a small sample of damped systems for which it is
appropriate, with the aim of comparing the metallicity dependence of the ratios
[Si/Fe] and [Mn/Fe] with analogous measurements in Milky Way stars. The main
conclusion is that the relative abundances of both elements in distant galaxies
are broadly in line with expectations based on Galactic data. Si displays a
mild enhancement at low metallicities, as expected for an alpha-capture
element, but there are also examples of near-solar [Si/Fe] at [Fe/H] < -1. The
underabundance of Mn at low metallicities is possibly even more pronounced than
that in metal-poor stars, and no absorption system has yet been found where
[Mn/Fe] is solar. The heterogeneous chemical properties of damped Lyman alpha
systems, evident even from this limited set of measurements, provide further
support for the conclusion from imaging studies that a varied population of
galaxies gives rise to this class of QSO absorbers.Comment: 29 pages, LaTex, 7 Postscript Figures. Accepted for Publication in
the Astrophysical Journa
15 GHz Monitoring of Gamma-ray Blazars with the OVRO 40 Meter Telescope in Support of Fermi
We present results from the first two years of our fast-cadence 15 GHz
gamma-ray blazar monitoring program, part of the F-GAMMA radio monitoring
project. Our sample includes the 1158 blazars north of -20 degrees declination
from the Candidate Gamma-Ray Blazar Survey (CGRaBS), which encompasses a
significant fraction of the extragalactic sources detected by the Fermi
Gamma-ray Space Telescope. We introduce a novel likelihood analysis for
computing a time series variability amplitude statistic that separates
intrinsic variability from measurement noise and produces a quantitative error
estimate. We use this method to characterize our radio light curves. We also
present results indicating a statistically significant correlation between
simultaneous average 15 GHz radio flux density and gamma-ray photon flux.Comment: 5 pages, 7 figures; 2009 Fermi Symposium; eConf Proceedings C09112
The F-GAMMA program: Multi-frequency study of Active Galactic Nuclei in the Fermi era. Program description and the first 2.5 years of monitoring
To fully exploit the scientific potential of the Fermi mission, we initiated
the F-GAMMA program. Between 2007 and 2015 it was the prime provider of
complementary multi-frequency monitoring in the radio regime. We quantify the
radio variability of gamma-ray blazars. We investigate its dependence on source
class and examine whether the radio variability is related to the gamma-ray
loudness. Finally, we assess the validity of a putative correlation between the
two bands. The F-GAMMA monitored monthly a sample of about 60 sources at up to
twelve radio frequencies between 2.64 and 228.39 GHz. We perform a time series
analysis on the first 2.5-year dataset to obtain variability parameters. A
maximum likelihood analysis is used to assess the significance of a correlation
between radio and gamma-ray fluxes. We present light curves and spectra
(coherent within ten days) obtained with the Effelsberg 100-m and IRAM 30-m
telescopes. All sources are variable across all frequency bands with amplitudes
increasing with frequency up to rest frame frequencies of around 60 - 80 GHz as
expected by shock-in-jet models. Compared to FSRQs, BL Lacs show systematically
lower variability amplitudes, brightness temperatures and Doppler factors at
lower frequencies, while the difference vanishes towards higher ones. The time
scales appear similar for the two classes. The distribution of spectral indices
appears flatter or more inverted at higher frequencies for BL Lacs. Evolving
synchrotron self-absorbed components can naturally account for the observed
spectral variability. We find that the Fermi-detected sources show larger
variability amplitudes as well as brightness temperatures and Doppler factors,
than non-detected ones. Flux densities at 86.2 and 142.3 GHz correlate with 1
GeV fluxes at a significance level better than 3sigma, implying that gamma rays
are produced very close to the mm-band emission region.Comment: Accepted for publication in section 4. Extragalactic astronomy of
Astronomy and Astrophysics (18 pages, 9 figures
Blazars in the Fermi Era: The OVRO 40-m Telescope Monitoring Program
The Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope
provides an unprecedented opportunity to study gamma-ray blazars. To capitalize
on this opportunity, beginning in late 2007, about a year before the start of
LAT science operations, we began a large-scale, fast-cadence 15 GHz radio
monitoring program with the 40-m telescope at the Owens Valley Radio
Observatory (OVRO). This program began with the 1158 northern (declination>-20
deg) sources from the Candidate Gamma-ray Blazar Survey (CGRaBS) and now
encompasses over 1500 sources, each observed twice per week with a ~4 mJy
(minimum) and 3% (typical) uncertainty. Here, we describe this monitoring
program and our methods, and present radio light curves from the first two
years (2008 and 2009). As a first application, we combine these data with a
novel measure of light curve variability amplitude, the intrinsic modulation
index, through a likelihood analysis to examine the variability properties of
subpopulations of our sample. We demonstrate that, with high significance
(7-sigma), gamma-ray-loud blazars detected by the LAT during its first 11
months of operation vary with about a factor of two greater amplitude than do
the gamma-ray-quiet blazars in our sample. We also find a significant (3-sigma)
difference between variability amplitude in BL Lacertae objects and
flat-spectrum radio quasars (FSRQs), with the former exhibiting larger
variability amplitudes. Finally, low-redshift (z<1) FSRQs are found to vary
more strongly than high-redshift FSRQs, with 3-sigma significance. These
findings represent an important step toward understanding why some blazars emit
gamma-rays while others, with apparently similar properties, remain silent.Comment: 23 pages, 24 figures. Submitted to ApJ
The July 2010 outburst of the NLS1 PMN J0948+0022
We report about the multiwavelength campaign on the Narrow-Line Seyfert 1
(NLS1) Galaxy PMN J0948+0022 (z = 0.5846) performed in 2010 July-September and
triggered by high activity as measured by Fermi/LAT. The peak luminosity in the
0.1-100 GeV energy band exceeded, for the first time in this type of source,
the value of 10^48 erg/s, a level comparable to the most powerful blazars. The
comparison of the spectral energy distribution of the NLS1 PMN J0948+0022 with
that of a typical blazar - like 3C 273 - shows that the power emitted at gamma
rays is extreme.Comment: 2011 Fermi Symposium proceedings - eConf C11050
Radio and gamma-ray follow-up of the exceptionally high activity state of PKS 1510-089 in 2011
We investigate the radio and gamma-ray variability of the flat spectrum radio
quasar PKS 1510-089 in the time range between 2010 November and 2012 January.
In this period the source showed an intense activity, with two major gamma-ray
flares detected in 2011 July and October. During the latter episode both the
gamma-ray and the radio flux density reached their historical peak.
Multiwavelength analysis shows a rotation of about 380 deg of the optical
polarization angle close in time with the rapid and strong gamma-ray flare in
2011 July. An enhancement of the optical emission and an increase of the
fractional polarization both in the optical and in radio bands is observed
about three weeks later, close in time with another gamma-ray outburst. On the
other hand, after 2011 September a huge radio outburst has been detected, first
in the millimeter regime followed with some time delay at centimeter down to
decimeter wavelengths. This radio flare is characterized by a rising and a
decaying stage, in agreement with the formation of a shock and its evolution,
as a consequence of expansion and radiative cooling. If the gamma-ray flare
observed in 2011 October is related to this radio outburst, then this strongly
indicates that the region responsible for the gamma-ray variability is not
within the broad line, but a few parsecs downstream along the jet.Comment: 14 pages, 12 figures, accepted for publication in MNRA
Detection of significant cm to sub-mm band radio and Îł-ray correlated variability in Fermi bright blazars
The exact location of the Îł-ray emitting region in blazars is still controversial. In order to attack this problem we present first results of a cross-correlation analysis between radio (11âcm to 0.8âmm wavelength, F-GAMMA programme) and Îł-ray (0.1â300âGeV) âŒ3.5âyr light curves of 54 Fermi-bright blazars. We perform a source stacking analysis and estimate significances and chance correlations using mixed source correlations. Our results reveal: (i) the first highly significant multiband radio and Îł-ray correlations (radio lagging Îł rays) when averaging over the whole sample, (ii) average time delays (source frame: 76 ± 23 to 7 ± 9âd), systematically decreasing from cm to mm/sub-mm bands with a frequency dependence Ï_(r, Îł)(Îœ) â Îœ^(â1), in good agreement with jet opacity dominated by synchrotron self-absorption, (iii) a bulk Îł-ray production region typically located within/upstream of the 3âmm core region (Ï_(3mm, Îł) = 12 ± 8âd), (iv) mean distances between the region of Îł-ray peak emission and the radio âÏ = 1 photosphereâ decreasing from 9.8 ± 3.0âpc (11âcm) to 0.9 ± 1.1âpc (2âmm) and 1.4 ± 0.8âpc (0.8âmm), (v) 3âmm/Îł-ray correlations in nine individual sources at a significance level where one is expected by chance (probability: 4 Ă 10^(â6)), (vi) opacity and âtime lag core shiftâ estimates for quasar 3C 454.3 providing a lower limit for the distance of the bulk Îł-ray production region from the supermassive black hole (SMBH) of âŒ0.8â1.6âpc, i.e. at the outer edge of the broad-line region (BLR) or beyond. A 3âmm Ï = 1 surface at âŒ2â3âpc from the jet base (i.e. well outside the âcanonical BLRâ) finally suggests that BLR material extends to several parsec distances from the SMBH
The first gamma-ray outburst of a Narrow-Line Seyfert 1 Galaxy: the case of PMN J0948+0022 in July 2010
We report on a multiwavelength campaign on the radio-loud Narrow-Line Seyfert
1 (NLS1) Galaxy PMN J0948+0022 (z=0.5846) performed in 2010 July-September and
triggered by a high-energy gamma-ray outburst observed by the Large Area
Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope. The peak flux in
the 0.1-100 GeV energy band exceeded, for the first time in this type of
source, the value of 10^-6 ph cm^-2 s^-1, corresponding to an observed
luminosity of 10^48 erg s^-1. Although the source was too close to the Sun
position to organize a densely sampled follow-up, it was possible to gather
some multiwavelength data that confirmed the state of high activity across the
sampled electromagnetic spectrum. The comparison of the spectral energy
distribution of the NLS1 PMN J0948+0022 with that of a typical blazar - like 3C
273 - shows that the power emitted at gamma rays is extreme.Comment: 8 pages, 7 figures, 2 tables. Accepted for the publication on MNRAS
Main Journal. Typo in bibliography correcte
The first gamma-ray outburst of a narrow-line Seyfert 1 galaxy: the case of PMN J0948+0022 in 2010 July
We report on a multiwavelength campaign for the radio-loud narrow-line Seyfert 1 (NLS1) galaxy PMN J0948+0022 (z= 0.5846) performed in 2010 July-September and triggered by a high-energy Îł-ray outburst observed by the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope. The peak flux in the 0.1-100 GeV energy band exceeded, for the first time in this type of source, the value of âŒ10â6 photon cmâ2 sâ1, corresponding to an observed luminosity of âŒ1048 erg sâ1. Although the source was too close to the Sun position to organize a densely sampled follow-up, it was possible to gather some multiwavelength data that confirmed the state of high activity across the sampled electromagnetic spectrum. The comparison of the spectral energy distribution of the NLS1 PMN J0948+0022 with that of a typical blazar - such as 3C 273 - shows that the power emitted at Îł-rays is extrem
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