Veritas Observations of Day-Scale Flaring of M 87 in 2010 April

Abstract

VERITAS has been monitoring the very-high-energy (VHE; > 100 GeV) gamma-ray activity of the radio galaxy M 87 since 2007. During 2008, flaring activity on a timescale of a few days was observed with a peak flux of (0.70 ± 0.16) × 10 −11 cm −2 s −1 at energies above 350 GeV. In 2010 April, VERITAS detected a flare from M 87 with peak flux of (2.71 ± 0.68) × 10 −11 cm −2 s −1 for E > 350 GeV. The source was observed for six consecutive nights during the flare, resulting in a total of 21 hr of good-quality data. The most rapid flux variation occurred on the trailing edge of the flare with an exponential flux decay time of 0.90 +0.22 −0.15 days. The shortest detected exponential rise time is three times as long, at 2.87 +1.65 −0.99 days. The quality of the data sample is such that spectral analysis can be performed for three periods: rising flux, peak flux, and falling flux. The spectra obtained are consistent with power-law forms. The spectral index at the peak of the flare is equal to 2.19 ± 0.07. There is some indication that the spectrum is softer in the falling phase of the flare than the peak phase, with a confidence level corresponding to 3.6 standard deviations. We discuss the implications of these results for the acceleration and cooling rates of VHE electrons in M 87 and the constraints they provide on the physical size of the emitting region.

INTERPRETATION OF THE FLARES OF M87 AT TeV ENERGIES IN THE CLOUD-JET INTERACTION SCENARIO

“…It is worth noting that γγ absorption softens the VHE spectrum in Fig. 6 (middle), in good agreement with observations in the decay epoch (Aliu et al 2012). Note also that the non-absorbed VHE lightcurve peak is broad enough to accommodate the two days separation between the VHE maximum and the X-ray observation.…”

“…X-ray Chandra observations taken approximately 2 − 3 days after the peak of the VHE gamma-ray emission revealed an enhanced flux from the core by a factor of ∼ 2 (L X ∼ 10 41 erg s −1 ), with a variability timescale of < 2 days (Abramowski et al 2012;Harris et al 2011). VERITAS obtained VHE spectra consistent with a power-law for three flare phases: rising flux, peak flux, and falling flux (Aliu et al 2012). At the peak of the flare, the photon index was ≈ 2.2, and there is indication at a few σ level that the spectrum is somewhat softer in the rising and falling phases.…”

Magnetospheric Gamma-Ray Emission in Active Galactic Nuclei

“…Given its proximity, M87 has been a prime target to probe scenarios for the formation of relativistic jets with high-resolution radio observations exploring scales down to some tens of r g and much effort has been recently dedicated into this direction (e.g., Acciari et al 2009;Doeleman et al 2012;Hada et al 2014Hada et al , 2016Kino et al 2015;Akiyama et al 2015Akiyama et al , 2017. At VHE energies, M87 has revealed at least three active γ-ray episodes during which day-scale flux variability (i.e., h = c∆τ obs ∼ r g ) has been observed (Aharonian et al 2006;Albert et al 2008;Acciari et al 2009;Abramowski et al 2012;Aliu et al 2012). The VHE spectrum is compatible with a relatively hard powerlaw (photon index ∼ 2.2) extending from 300 GeV to beyond 10 TeV, while the corresponding TeV output is relatively moderate, with an isotropic equivalent luminosity of L V HE (3 − 10) × 10 40 erg s −1 .…”

Gamma-rays from non-blazar AGN

“…At VHE energies, M87 is known for its particularly interesting characteristics, including rapid day-scale variability (on flux doubling time scales ∆t obs ∼ 1 d) detected during active source states, and a relatively hard spectrum compatible with a power law of photons index Γ VHE 2.2 and extending from ∼ 300 GeV to beyond 10 TeV, e.g. [65,66,67,68]. Both this hard VHE spectrum and the observed rapid variability are remarkable features for a misaligned AGN, and reminiscent of those seen in IC 310.…”