Spectrum and variability of the Galactic center VHE γ-ray source HESS J1745–290

Abstract

Aims. A detailed study of the spectrum and variability of the source HESS J1745−290 in the Galactic Center (GC) region using new data from the H.E.S.S. array of Cherenkov telescopes is presented. Flaring activity and quasi periodic oscillations (QPO) of HESS J1745−290 are investigated. Methods. The image analysis is performed with a combination of a semi-analytical shower model and the statistical moment-based Hillas technique. The spectrum and lightcurves of HESS J1745−290 are derived with a likelihood method based on a spectral shape hypothesis. Rayleigh tests and Fourier analysis of the H.E.S.S. GC signal are used to study the periodicity of the source. Results. With a three-fold increase in statistics compared to previous work, a deviation from a simple power law spectrum is detected for the first time. The measured energy spectrum over the three years 2004, 2005 and 2006 of data taking is compatible with both a power law spectrum with an exponential cut-off and a broken power law spectrum. The curvature of the energy spectrum is likely to be intrinsic to the photon source, as opposed to effects of interstellar absorption. The power law spectrum with an exponential cut-off is characterized by a photon index of 2.10 ± 0.04 stat ± 0.10 syst and a cut-off energy at 15.7 ± 3.4 stat ± 2.5 syst TeV. The broken power law spectrum exhibits spectral indices of 2.02 ± 0.08 stat ± 0.10 syst and 2.63 ± 0.14 stat ± 0.10 syst with a break energy at 2.57 ± 0.19 stat ± 0.44 syst TeV. No significant flux variation is found. Increases in the γ-ray flux of HESS J1745−290 by at least a factor of two would be required for a 3σ detection of a flare with time scales of an hour. Investigation of possible QPO activity at periods claimed to be detected in X-rays does not show any periodicities in the H.E.S.S. signal.

Low-luminosity AGNs

“…For Sgr A*, we have L VHE = 3 × 10 34 erg/s (Aharonian et al 2009a) and L s = L IR = 10 36 erg/s (Yuan et al 2003), M = 3.6 × 10 6 M , and for M 87 L VHE = 3 × 10 40 erg/s and L s = L IR = 10 39 erg/s (Perlman et al 2007), M = 3 × 10 9 M . Substituting these values in to Eq.…”

“…For the region of Sgr A*, the origin of the VHE emission is not quite clear (HESS collaboration 2010). One argument in favor of a disk origin of the VHE radiation is that the observed spectral index and the cutoff energy of VHE radiation (Aharonian et al 2009a), β −2.1 and E c 16 TeV, are close to the values that follow from the observation of IR radiation from the disk, β −2.2, γ m 10 4 , and E m 10 TeV. The argument of Aharonian et al (2008) that VHE radiation from Sgr A* is likely from the jet rather than the disk is based on the assumptions that the X-ray emission originates in the disk and there is no time variability in the VHE flux during the X-ray flare.…”

Reliability of Monte Carlo event generators for gamma-ray dark matter searches

“…If DM particles annihilate or decay into SM particles, the signature of the final products of such processes may be detected up to some uncertainty in the astrophysical background component. In order to set constraints on the diverse DM models and get a better understanding of the astrophysical factor associated with the distribution of this kind of matter, numerous signals detected in gamma-rays, neutrinos, positrons, antiprotons and other particles have been studied in the available literature [10][11][12][13][14][15][16][17][18]. Most of these analysis make use of Monte Carlo event generator packages, that allow to predict the spectra of final-state particles generated by DM annihilation and decays into SM particles.…”

A new method of reconstructing VHEγ-ray spectra: the Template Background Spectrum

“…For this, H.E.S.S. data on the Galactic centre (HESS J1745-290, Aharonian et al 2009a) were used (the analysis of this source will be discussed later).…”

“…HESS J1745-290: the Galactic centre source HESS J1745-290 (Aharonian et al 2009a) is a strong point-like source with E/B ≈ 1 and is located in a crowded FoV with many nearby sources and a complex diffuse γ-ray emission region. In addition, there are bright stars in the FoV.…”