Blazar Times - No. 66 - February 2005
The Blazar Times
A Research Newsletter Dedicated to the BL Lac and Blazar Phenomena
No. 66 - February 2005 Editor: Travis A. Rector (blazar@uaa.alaska.edu)

TABLE OF CONTENTS

Journal Abstracts 1
Abstract Guidelines 4

Journal Abstracts
X-ray Flares from Markarian 501
Yongquan Xue1 and Wei Cui1
1 Dept. of Physics, Purdue University, 1396 Physics Building, West Lafayette, IN 47907 USA
Motivated by the recent finding of hierarchical X-ray flaring phenomenon in Mrk 421, we conducted a systematic search for X-ray flares from Mrk 501, another well-known TeV blazar, by making use of the rich RXTE archival database. We detected flares over a wide range of timescales, from months down to minutes, as in the case of Mrk 421. However, the flares do not seem to occur nearly as frequently in Mrk 501 as in Mrk 421 on any of the timescales. The flaring hierarchy also seems apparent in Mrk 501, suggesting that it might be common among TeV blazars. The results seem to imply a scale-invariant physical origin of the flares (large or small). The X-ray spectrum of the source shows a general trend of hardening toward the peak of long-duration flares, with indication of spectral hysteresis, which is often seen in TeV blazars. However, the data are not of sufficient quality to allow us to draw definitive conclusions about spectral variability associated with more rapid but weaker flares. We critically examine a reported sub-hour X-ray flare from Mrk 501, in light of intense background flaring activity at the time of the observation, and concluded that the flare is likely an artifact. On the other hand, we did identify a rapid X-ray flare that appears to be real. It lasted only for about 15 minutes, during which the flux of the source varied by about 30%. Sub-structures are apparent in its profile, implying variabilities on even shorter timescales. Such rapid variabilities of Mrk 501 place severe constraints on the physical properties of the flaring region in the jet, which have serious implications on the emission models proposed for TeV blazars.
Accepted by ApJ
For preprints contact: xuey@physics.purdue.edu or cui@physics.purdue.edu
For preprints via ftp or WWW: ttp://arxiv.org/abs/astro-ph/0412226/
VLBA Polarization Observations of Markarian 421 After a Gamma-Ray High State
B. Glenn Piner1,2, Philip G. Edwards3
1 Department of Physics and Astronomy, Whittier College, 13406 E. Philadelphia Street, Whittier, CA 90608 USA
2 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109 USA
3 Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan
We present four high dynamic range, dual-circular polarization, Very Long Baseline Array (VLBA) observations at 22 GHz of Markarian 421, taken throughout the year following the source's unprecedented gamma-ray high state in early 2001. These four new VLBA observations are combined with data from our earlier 1999 paper and archival VLBA data-sets that have become available since 1999 to produce a combined 28 epoch VLBA data-set on Mrk 421 spanning the years 1994 to 2002. No new component associated with the 2001 flares was seen on the total intensity images, but the combined data-set allowed precise measurements of the apparent speeds of the existing components. The peak measured apparent speed was for component C5, which has an apparent speed of 0.10.02 c (H0=71 km s-1 Mpc-1, Wm=0.27, and WL=0.73). No counterjet is seen with a limit on the jet to counterjet brightness ratio J > ~ 100. These observed VLBI properties of Markarian 421 are consistent with a jet with a bulk Lorentz factor G ~ 2 and an angle to the line-of-sight q ~ 1, suggesting a jet that decelerates between the gamma-ray producing region and the parsec scale. Although a limb-brightened structure is seen in some transverse slices across the jet, it is not seen consistently, inhibiting interpretation in terms of the fast-spine/slow-layer model that has been invoked for other sources. The VLBI core and inner jet (component C7) have fractional polarizations of ~ 5%, and an electric vector position angle (EVPA) aligned with the jet axis. Component C5 (at 1.5 mas from the core) has a higher fractional polarization of ~ 15%, and an EVPA nearly orthogonal to the jet axis. Significant variability is detected in the EVPA of component C6, which at two of the four epochs shows an EVPA aligned with the jet axis, possibly a sign of propagating disturbances that are only visible on the polarization images. If these propagating disturbances are linked to the 2001 gamma-ray high state, then their inferred apparent speed is between 1 and 3 c.
Accepted by ApJ
For preprints contact: gpiner@whittier.edu
For preprints via ftp or WWW: http://xxx.lanl.gov/abs/astro-ph/0412383/
Optical follow-up of BL Lac candidates in the 2BL sample
R. Nesci1, S. Sclavi1  and E. Massaro1
1 Dept. of Physics, Univ. La Sapienza, Ple. A. Moro 2, I-00185, Rome, ITALY
We investigate the nature of the BL Lac candidates in the northern part of the 2BL sample ([]) searching for optical variability by means of RC band photometry with the Asiago 1.8 and Loiano 1.5 meter telescopes during years 2002, 2003 and 2004. We also made aperture photometry of the candidates on the plates available on the web from the Palomar and UK Schmidt telescopes. No significant variability was detected for the majority of the objects without detected radio and/or X-ray emissions, while clear variability was found for the few sources of the sample with strong radio and/or X-ray emission, which is a constant feature of the "classical" BL Lacs. Some considerations on the nature of the 2BL sources are made, taking into account their most recent data available in proper motions catalogues (SuperCOSMOS, USNO-B1) and in the Sloan Digital Sky Survey.
Accepted by Astron. Astrophys.
For preprints contact: roberto.nesci@uniroma1.it

Linear Accelerating Superluminal Motion Model
J.F. Zhou1, C. Zheng2, T.P. Li1, Y. Su3  and T. Venturi4
1 Department of Engineering Physics and Center for Astrophysics, Tsinghua University, Beijing 100084,China
2 KIPAC, Stanford University, Standford, CA 94309, U.S.A.
3 National Astronomical Observatories, Chinese Academy of Sciences, Chaoyang District, Datun Road 20A, Beijing, 100012, China
4 Istituto di Radioastronomia del CNR, Via Gobetti 101, I-40129 Bologna, Italy
Accelerating superluminal motions were detected recently by multi-epoch Very Long Baseline Interferometry (VLBI) observations. Here, a Linear Accelerating Superluminal Motion (LASM) model is proposed to interpret the observed phenomena. The model provides a direct and accurate way to estimate the viewing angle of a relativistic jet. It also predicts that both Doppler boosting and deboosting effects may take place in an accelerating forward jet. The LASM model is applied to the data of the quasar 3C 273, and the initial velocity, acceleration and viewing angle of its three components are derived through modelfits. The variations of the viewing angle suggest that a supermassive black holebinary system may exist in the center of 3C273. The gap between the inner and outer jet in some radio loud AGNs my be explained in terms of Doppler deboosting effects when the components accelerate to ultra-relativistic speed.
ApJ 616L 95Z
For preprints contact: zhoujf@tsinghua.edu.cn
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