* Astronomy

Blobrana · L

RE: Markarian 501

Blobrana · L

Title: The jet of Markarian 501 from millions of Schwarzschild radii down to a few hundreds
Authors: M. Giroletti (1), G. Giovannini (1,2), W.D. Cotton (3), G.B. Taylor (4), M.A. Perez-Torres (5), M. Chiaberge (1,6), P.G. Edwards (7) ((1) Inaf Ira, (2) Unibo, (3) Nrao, (4) Unm, (5) Iaa-Csic, (6) Stsci, (7), Csiro)

Aims: The TeV BL Lac object Markarian 501 is a complex, core dominated radio source, with a one sided, twisting jet on parsec scales. In the present work, we attempt to extend our understanding of the source physics to regions of the radio jet which have not been accessed before.
Methods: We present new observations of Mrk 501 at 1.4 and 86 GHz. The 1.4 GHz data were obtained using the Very Large Array (VLA) and High Sensitivity Array (HSA) in November 2004, in full polarisation, with a final r.m.s. noise of 25 microJy/beam in the HSA total intensity image; the 86 GHz observations were performed in October 2005 with the Global Millimetre VLBI Array (GMVA), providing an angular resolution as good as 110 x 40 microarcseconds.
Results: The sensitivity and resolution provided by the HSA make it possible to detect the jet up to ~700 milliarcseconds (corresponding to a projected linear size of ~500 pc) from its base, while the superior resolution of the 86 GHz GMVA observations probes the innermost regions of the jet down to ~200 Schwarzschild radii. The brightness temperature at the jet base is in excess of 6e10 K. We find evidence of limb brightening on physical scales from <1 pc to ~40 pc. Polarisation images and fits to the trend of jet width and brightness vs. distance from the core reveal a magnetic field parallel to the jet axis.

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Blobrana · L

Title: A possible disk mechanism for the 23d QPO in Mkn~501
Authors: J.H. Fan, F.M. Rieger, T.X. Hua, U. C. Joshi, J. Li, Y.X. Wang, J.L. Zhou, Y.H. Yuan, J.B. Su, Y.W. Zhang

Optically thin two-temperature accretion flows may be thermally and viscously stable, but acoustically unstable. Here we propose that the O-mode instability of a cooling-dominated optically thin two-temperature inner disk may explain the 23- day quasi-periodic oscillation (QPO) period observed in the TeV and X-ray light curves of Mkn 501 during its 1997 high state. In our model the relativistic jet electrons Compton upscatter the disk soft X-ray photons to TeV energies, so that the instability-driven X-ray periodicity will lead to a corresponding quasi-periodicity in the TeV light curve and produce correlated variability. We analyse the dependence of the instability-driven quasi-periodicity on the mass (M) of the central black hole, the accretion rate ( M) and the viscous parameter (alpha) of the inner disk. We show that in the case of Mkn 501 the first two parameters are constrained by various observational results, so that for the instability occurring within a two-temperature disk where \alpha = 0.05 1.0, the quasi-period is expected to lie within the range of 8 to 100 days, as indeed the case. In particular, for the observed 23-day QPO period our model implies a viscosity coefficient alpha 0.28, a sub-Eddington accretion rate M ~= 0.02 MEdd and a transition radius to the outer standard disk of r0 ~ 60 rg, and predicts a period variation delta P/P ~ 0.23 due to the motion of the instability region.

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Blobrana · L

Title: The jet of Markarian 501 from the sub-parsec to the kpc scale
Authors: G. Giovannini, M. Giroletti, G.B. Taylor

We have observed the BL Lac object Markarian 501 at 1.4 GHz with the High Sensitivity Array and at 86 GHz with the global VLBI mm array. Thanks to the great resolution and sensitivity provided by these instruments, we probe regions of the radio jets never accessed before. The new data at 1.4 GHz allow us to map the one-sided jet at large distances from the core, and to constrain jet properties thanks to the high jet to counter-jet brightness ratio. The 86 GHz data give us a high resolution image of the nuclear region. Putting together these new results and available published data we discuss the properties of this source from sub-parsec to kiloparsec scales.

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Blobrana · L

Title: Observations of Mrk 421 and Mrk 501 in Spring 2006 with VERITAS
Authors: S.J. Fegan (for the VERITAS Collaboration)

VERITAS, the Very Energetic Radiation Telescope Imaging Array System, is an array of four imaging Atmospheric Cherenkov telescopes in southern Arizona. It is sensitive to gamma rays at energies above 100 GeV. Here, we discuss the results of observations of two well known VHE blazars, Markarian 421 and Markarian 501, during Spring 2006 which were made with the first two telescopes during the commissioning phase of VERITAS. During most of this time Mrk 421 was in an unusually active state while Mrk 501 was in a much lower flux state. As such, these observations provided an opportunity to test the sensitivity of the instrument to strong and weak sources. We discuss implications of these observations on our understanding of these objects.

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The BL Lacertae object Markarian 501 was identified as a source of gamma-ray emission at the Whipple Observatory in March 1995.
Markarian 501 is more than 300 million light years distant from the earth. It has an active nucleus and is the energy source for the jet of fast-moving particles.

R.A. = 16:53:52.2, Dec. = 39:45:36 (J2000)
V = 14mag

Markarian 501.kmz
Google Sky file

-- Edited by Blobrana at 15:46, 2008-01-12