M87 - VERITAS 2008-2009 monitoring of the variable gamma-ray source M87 PDF Print



Upper panel: VERITAS night-by-night VHE gamma-ray flux and Chandra X-ray flux from the core and from the HST-1 knot of M87 in 2008 and 2009. Lower panel: Chandra X-ray flux fractional change per year.



VERITAS 2007-2008 monitoring of the variable gamma-ray source M87



Reference:  V. A. Acciari et al. (The VERITAS Collaboration), Astrophysical Journal 716: 819-824, 2010

Full text version here

ArXiv version: ArXiV:1005.0367v1

Contact person: Michelle Hui


M87 is a nearby radio galaxy located at a distance of 16.7Mpc, near the center of the Virgo cluster. The jet of M87 is constrained to be <30o from the line-of-sight by radio and optical observations.  This misalignment, combined with its proximity, have enabled detailed morphological studies and extensive modeling at radio, optical, and X-ray energies.  Flaring activity from M87 was first observed in VHE gamma rays by HESS in 2005 and subsequently by MAGIC and VERITAS in 2008.  Ground-based gamma-ray observations, unlike radio, optical, and X-ray observations, cannot resolve any jet structures of M87.  Therefore simultaneous multi-wavelength data is vital to determine the emission origin of gamma rays from M87.

In the 2008 observing season, VERITAS observed a VHE flare from M87 occurring over a 4-day timespan in February, with peak nightly flux above 250GeV at 7.7% of the Crab Nebula flux during a joint multi-wavelength monitoring campaign conducted by three major VHE experiments along with the Chandra X-ray Observatory.  Shortly after the VHE flare seen by VERITAS, the Chandra X-ray Observatory detected the flux from the core of M87 at a historical maximum, while the flux from the nearby knot HST-1 (located at 0.86' away from the core) remained quiescent.  Contemporaneous VHE gamma-ray, Chandra X-ray, and VLBA radio observations in 2008 suggest the core as the most likely source of VHE emission, in contrast to the 2005 VHE flare that was simultaneous with an X-ray flare in the HST-1 knot.  In 2009, VERITAS continued its monitoring of M87 and marginally detected M87 at a flux of ~1% of the Crab Nebula.  No VHE flaring activity was observed in 2009.  X-ray time scale analysis showed smaller fractional change per year (fpy) for the core during 2009 than in 2008. This may be an indication of correlation between large X-ray flux changes and flaring activities in VHE gamma rays.

Even with M87 day-scale VHE flux variability and additional constraints from radio and X-ray observations, there remain several plausible models which may explain how particles are accelerated to very high energies near the black hole and how the consequent radiation is able to reach us.  Multi-wavelength monitoring work is being continued, and is essential to address these questions.



Figures from paper (click to get full size image):


Figure 3: Photon index versus differential flux as reported by HESS in 2004 and 2005, by MAGIC in 2008 (overall, flare, and non-flare), and by VERITAS in 2007 and 2008 (overall, flare, and pre-flare). All spectra are compatible within their statistical errors. A linear coorelation fit between photon index and flux has the same probability as a constant photon index fit.
Figure 4: M87 yearly VHE gamma-ray and X-ray fluxes. The VHE gamma-ray flux is for energy >730GeV due to the original flux scale used in the HEGRA paper. This yearly flux plot is first presented in Acciari et al. 2008 and is now up dated with 2008 and 2009 data. Grey areas represent the range of variable VHE fluxes observed that year to give a more accurate picture of the flux level of M87.
Figure 5: Upper panel: VERITAS night-by-night VHE gamma-ray flux and Chandra X-ray flux from the core and from the HST-1 knot of M87 in 2008 and 2009. Lower panel: Chandra X-ray flux fractional change per year, see article for definition.


Last Updated on Tuesday, 21 June 2011 08:24

Current Weather