47 research outputs found
Scheduling and calibration strategy for continuous radio monitoring of 1700 sources every three days
The Owens Valley Radio Observatory 40 meter telescope is currently monitoring
a sample of about 1700 blazars every three days at 15 GHz, with the main
scientific goal of determining the relation between the variability of blazars
at radio and gamma-rays as observed with the Fermi Gamma-ray Space Telescope.
The time domain relation between radio and gamma-ray emission, in particular
its correlation and time lag, can help us determine the location of the
high-energy emission site in blazars, a current open question in blazar
research. To achieve this goal, continuous observation of a large sample of
blazars in a time scale of less than a week is indispensable. Since we only
look at bright targets, the time available for target observations is mostly
limited by source observability, calibration requirements and slewing of the
telescope. Here I describe the implementation of a practical solution to this
scheduling, calibration, and slewing time minimization problem. This solution
combines ideas from optimization, in particular the traveling salesman problem,
with astronomical and instrumental constraints. A heuristic solution using well
stablished optimization techniques and astronomical insights particular to this
situation, allow us to observe all the sources in the required three days
cadence while obtaining reliable calibration of the radio flux densities.
Problems of this nature will only be more common in the future and the ideas
presented here can be relevant for other observing programs.Comment: Published in Proc. SPIE. 9149, Observatory Operations: Strategies,
Processes, and Systems V, 91492
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
Constraining the Limiting Brightness Temperature and Doppler Factors for the Largest Sample of Radio-bright Blazars
Relativistic effects dominate the emission of blazar jets complicating our understanding of their intrinsic properties. Although many methods have been proposed to account for them, the variability Doppler factor method has been shown to describe the blazar populations best. We use a Bayesian hierarchical code called {\it Magnetron} to model the light curves of 1029 sources observed by the Owens Valley Radio Observatory's 40-m telescope as a series of flares with an exponential rise and decay, and estimate their variability brightness temperature. Our analysis allows us to place the most stringent constraints on the equipartition brightness temperature i.e., the maximum achieved intrinsic brightness temperature in beamed sources which we found to be ⟨T_(eq)⟩=2.78 × 10^(11) K ± 26%. Using our findings we estimated the variability Doppler factor for the largest sample of blazars increasing the number of available estimates in the literature by almost an order of magnitude. Our results clearly show that γ-ray loud sources have faster and higher amplitude flares than γ-ray quiet sources. As a consequence they show higher variability brightness temperatures and thus are more relativistically beamed, with all of the above suggesting a strong connection between the radio flaring properties of the jet and γ-ray emission
Intrinsic brightness temperatures of blazar jets at 15 GHz
We have developed a new Bayesian Markov Chain Monte Carlo method to deconvolve light curves of blazars into individual flares, including proper estimation of the fit errors. We use the method to fit 15GHz light curves obtained within the OVRO 40-m blazar monitoring program where a large number of AGN have been monitored since 2008 in support of the Fermi Gamma-Ray Space Telescope mission. The time scales obtained from the fitted models are used to calculate the variability brightness temperature of the sources. Additionally, we have calculated brightness temperatures of a sample of these objects using Very Long Baseline Array data from the MOJAVE survey. Combining these two data sets enables us to study the intrinsic brightness temperature distribution in these blazars at 15 GHz. Our preliminary results indicate that the mean intrinsic brightness temperature in a sample of 14 sources is near the equipartition brightness temperature of ~ 10^(11)K
Spectroscopy of the Largest Ever γ-Ray-selected BL Lac Sample
We report on spectroscopic observations covering most of the 475 BL Lacs in the second Fermi Large Area Telescope (LAT) catalog of active galactic nuclei (AGNs). Including archival measurements (correcting several erroneous literature values) we now have spectroscopic redshifts for 44% of the BL Lacs. We establish firm lower redshift limits via intervening absorption systems and statistical lower limits via searches for host galaxies for an additional 51% of the sample leaving only 5% of the BL Lacs unconstrained. The new redshifts raise the median spectroscopic z from 0.23 to 0.33 and include redshifts as large as z = 2.471. Spectroscopic redshift minima from intervening absorbers have z = 0.70, showing a substantial fraction at large z and arguing against strong negative evolution. We find that detected BL Lac hosts are bright ellipticals with black hole masses M_• ~ 10^(8.5) – 10^9, substantially larger than the mean of optical AGNs and LAT Flat Spectrum Radio Quasar samples. A slow increase in M_• with z may be due to selection bias. We find that the power-law dominance of the optical spectrum extends to extreme values, but this does not strongly correlate with the γ-ray properties, suggesting that strong beaming is the primary cause of the range in continuum dominance
Spectroscopy of Broad Line Blazars from 1LAC
We report on optical spectroscopy of 165 Flat Spectrum Radio Quasars (FSRQs)
in the Fermi 1LAC sample, which have helped allow a nearly complete study of
this population. Fermi FSRQ show significant evidence for non-thermal emission
even in the optical; the degree depends on the gamma-ray hardness. They also
have smaller virial estimates of hole mass than the optical quasar sample. This
appears to be largely due to a preferred (axial) view of the gamma-ray FSRQ and
non-isotropic (H/R ~ 0.4) distribution of broad-line velocities. Even after
correction for this bias, the Fermi FSRQ show higher mean Eddington ratios than
the optical population. A comparison of optical spectral properties with Owens
Valley Radio Observatory radio flare activity shows no strong correlation.Comment: Accepted for publication in Ap
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