VERITAS Observations of M 87 in 2011/2012

The giant radio galaxy M 87 is located at a distance of 16.7 Mpc and harbors a super-massive black hole (6 billion solar masses) in its center. M 87 is one of just three radio galaxies known to emit TeV gamma-rays. The structure of its relativistic plasma jet, which is not pointing towards our line of sight, is spatially resolved in X-ray (Chandra), optical and radio (VLA/VLBA) observations. The mechanism and location of the TeV emitting region is one of the least understood aspects of AGN. In spring 2008 and 2010, the three TeV observatories VERITAS, MAGIC and H.E.S.S. detected two major TeV flares in coordinated observations. Simultaneous high-resolution observations at other wavelengths - radio (2008) and X-rays (2008/2010) - gave evidence that one of the TeV flares was related to an event in the core region; however, no common/repeated patterns could be identified so far. VERITAS continued to monitor M 87 in 2011/2012. The results of these observations are presented.Comment: 4 pages, 3 figures; conference proceedings of the 5th International Symposium on High-Energy Gamma-Ray Astronomy (Gamma2012

Analyzing the Data from X-ray Polarimeters with Stokes Parameters

X-ray polarimetry promises to deliver unique information about the geometry of the inner accretion flow of astrophysical black holes and the nature of matter and electromagnetism in and around neutron stars. In this paper, we discuss the possibility to use Stokes parameters - a commonly used tool in radio, infrared, and optical polarimetry - to analyze the data from X-ray polarimeters such as scattering polarimeters and photoelectric effect polarimeters, which measure the linear polarization of the detected X-rays. Based on the azimuthal scattering angle (in the case of a scattering polarimeter) or the azimuthal component of the angle of the electron ejection (in the case of a photoelectric effect polarimeter), the Stokes parameters can be calculated for each event recorded in the detector. Owing to the additive nature of Stokes parameters, the analysis reduces to adding the Stokes parameters of the individual events and subtracting the Stokes parameters characterizing the background (if present). The main strength of this kind of analysis is that the errors on the Stokes parameters can be computed easily and are well behaved - in stark contrast of the errors on the polarization fraction and polarization direction. We demonstrate the power of the Stokes analysis by deriving several useful formulae, e.g. the expected error on the polarization fraction and polarization direction for a detection of $N_S$ signal and $N_{BG}$ background events, the optimal observation times of the signal and background regions in the presence of non-negligible background contamination of the signal, and the minimum detectable polarization (MDP) that can be achieved when following this prescription.Comment: 9 pages, 2 figures, accepted for publication in Astropart. Phy

e-EVN monitoring of M87

M87 is a privileged laboratory for a detailed study of the properties of jets, owing to its proximity (D=16.7 Mpc, 1 mas = 0.080 pc), its massive black hole (~6.0 x 10^9M) and its conspicuous emission at radio wavelengths and above. We started on November 2009 a monitoring program with the e-EVN at 5 GHz, in correspondence of the season of Very High Energy (VHE) observations. Indeed, two episodes of VHE activity have been reported in February and April 2010. We present here the main results of these multi-epoch observations: the inner jet and HST-1 are both detected and resolved in our datasets. We study the apparent velocity of HST-1, which seems to be increasing since 2005, and the flux density variability in the inner jet. All in all, the radio counterpart to this year’s VHE event seems to be different from the ones in 2005 and 2008, opening new scenario for the radio-high energy connection

The 2010 M 87 VHE flare and its origin: the multi-wavelength picture

The giant radio galaxy M 87, with its proximity (16 Mpc) and its very massive black hole ((3 - 6) \times 10^9 M_solar), provides a unique laboratory to investigate very high energy (E>100 GeV; VHE) gamma-ray emission from active galactic nuclei and, thereby, probe particle acceleration to relativistic energies near supermassive black holes (SMBH) and in relativistic jets. M 87 has been established as a VHE gamma-ray emitter since 2005. The VHE gamma-ray emission displays strong variability on timescales as short as a day. In 2008, a rise in the 43 GHz Very Long Baseline Array (VLBA) radio emission of the innermost region (core; extension of < 100 Rs ; Schwarzschild radii) was found to coincide with a flaring activity at VHE. This had been interpreted as a strong indication that the VHE emission is produced in the direct vicinity of the SMBH. In 2010 a flare at VHE was again detected triggering further multi-wavelength (MWL) observations with the VLBA, Chandra, and other instruments. At the same time, M 87 was also observed with the Fermi-LAT telescope at MeV/GeV energies, the European VLBI Network (EVN), and the Liverpool Telescope (LT). Here, preliminary results from the 2010 campaign will be reported.Comment: 6 pages, 2 figures; Procceedings of the workshop "High Energy Phenomena in Relativistic Outflows III" (HEPRO III), Barcelona, June 27 - July 1, 201