Time-dependent absorption of very high-energy gamma-rays from the Galactic center by pair-production

Very high energy (VHE) gamma-rays have been detected from the direction of the Galactic center. The H.E.S.S. Cherenkov telescopes have located this gamma-ray source with a preliminary position uncertainty of 8.5" per axis (6" statistic + 6" sytematic per axis). Within the uncertainty region several possible counterpart candidates exist: the Super Massive Black Hole Sgr A*, the Pulsar Wind Nebula candidate G359.95-0.04, the Low Mass X-Ray Binary-system J174540.0-290031, the stellar cluster IRS 13, as well as self-annihilating dark matter. It is experimentally very challenging to further improve the positional accuracy in this energy range and therefore, it may not be possible to clearly associate one of the counterpart candidates with the VHE-source. Here, we present a new method to investigate a possible link of the VHE-source with the near environment of Sgr A* (within approximately 1000 Schwarzschild radii). This method uses the time- and energy-dependent effect of absorption of gamma-rays by pair-production (in the following named pair-eclipse) with low-energy photons of stars closely orbiting the SMBH Sgr A*.Comment: 4 pages, 6 figures, Published in Proceedings of the 4th International Meeting on High Energy Gamma-Ray Astronomy (Gamma 08), Heidelber

The HI absorption distance of HESS J1943+213 favours its extragalactic nature

The H.E.S.S. collaboration (Abramowski et al. 2011) dicovered a new TeV point-like source HESS J1943+213 in the Galactic plane and suggested three possible low-energy-band counterparts: a $\gamma$-ray binary, a pulsar wind nebula (PWN), or a BL Lacertae object. We measure the distance to the radio counterpart G57.76-1.29 of HESS J1943+213. We analyze Very Large Array observations to obtain a reliable HI absorption spectrum.The resulting distance limit is $\ge$ 16 kpc. This distance strongly supports that HESS J1943+213 is an extragalactic source, consistent with the preferred counterpart of the HESS collaboration.Comment: 3 figures, 2 pages, A&A accepte

Very-high energy gamma-ray astronomy: A 23-year success story in high-energy astroparticle physics

Very-high energy (VHE) gamma quanta contribute only a minuscule fraction - below one per million - to the flux of cosmic rays. Nevertheless, being neutral particles they are currently the best "messengers" of processes from the relativistic/ultra-relativistic Universe because they can be extrapolated back to their origin. The window of VHE gamma rays was opened only in 1989 by the Whipple collaboration, reporting the observation of TeV gamma rays from the Crab nebula. After a slow start, this new field of research is now rapidly expanding with the discovery of more than 150 VHE gamma-ray emitting sources. Progress is intimately related with the steady improvement of detectors and rapidly increasing computing power. We give an overview of the early attempts before and around 1989 and the progress after the pioneering work of the Whipple collaboration. The main focus of this article is on the development of experimental techniques for Earth-bound gamma-ray detectors; consequently, more emphasis is given to those experiments that made an initial breakthrough rather than to the successors which often had and have a similar (sometimes even higher) scientific output as the pioneering experiments. The considered energy threshold is about 30 GeV. At lower energies, observations can presently only be performed with balloon or satellite-borne detectors. Irrespective of the stormy experimental progress, the success story could not have been called a success story without a broad scientific output. Therefore we conclude this article with a summary of the scientific rationales and main results achieved over the last two decades.Comment: 45 pages, 38 figures, review prepared for EPJ-H special issue "Cosmic rays, gamma rays and neutrinos: A survey of 100 years of research

Interpretation of the flares of M87 at TeV energies in the cloud-jet interaction scenario

Active galactic nuclei with misaligned jets have been recently established as a class of high-energy gamma-ray sources. M87, a nearby representative of this class, shows fast TeV variability on timescales less than one day. We present calculations performed in the framework of the scenario in which gamma-ray flares in non-blazar active galactic nuclei are produced by a red giant or a gas cloud interacting with the jet. We show that both the light curve and energy spectrum of the spectacular April 2010 flare can be reproduced by this model, assuming that a relatively massive cloud of approx 1.e29 g penetrates into the jet at few tens of Schwarzschild radii from the super-massive black hole.Comment: 8 pages, 8 figures, accepted by Ap

Galactic Structure Toward the Carina Tangent

This investigation presents a photometric study of the Galactic structure toward the Carina arm tangent. The field is located between 280 deg and 286 deg galactic longitude and -4 deg to 4 deg galactic latitude. All currently available uvbybeta data is used to obtain homogeneous color excesses and distances for more than 260 stars of spectral types O to G. We present revised distances and average extinction for the open clusters and cluster candidates NGC 3293, NGC 3114, Loden 46 and Loden 112. The cluster candidate Loden 112 appears to be a very compact group at a true distance modulus of 11.06 +\- 0.11 (s.e.) (1629 +84,-80 pc), significantly closer than previous estimates. We found other OB stars at that same distance and, based on their proper motions, suggest a new OB association at coordinates 282 deg < l < 285 deg, -2 deg < b < 2 deg. Utilizing BV photometry and spectral classification of the known O-type stars in the very young open cluster Wd 2 we provide a new distance estimate of 14.13 +\-0.16 (s.e.) (6698 +512,-475 pc), in excellent agreement with recent distance determinations to the giant molecular structures in this direction. We also discuss a possible connection between the HII region RCW 45 and the highly-reddened B+ star CPD -55 3036 and provide a revised distance for the luminous blue variable HR Car.Comment: accepted to PAS

Dark Matter and Fundamental Physics with the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) is a project for a next-generation observatory for very high energy (GeV-TeV) ground-based gamma-ray astronomy, currently in its design phase, and foreseen to be operative a few years from now. Several tens of telescopes of 2-3 different sizes, distributed over a large area, will allow for a sensitivity about a factor 10 better than current instruments such as H.E.S.S, MAGIC and VERITAS, an energy coverage from a few tens of GeV to several tens of TeV, and a field of view of up to 10 deg. In the following study, we investigate the prospects for CTA to study several science questions that influence our current knowledge of fundamental physics. Based on conservative assumptions for the performance of the different CTA telescope configurations, we employ a Monte Carlo based approach to evaluate the prospects for detection. First, we discuss CTA prospects for cold dark matter searches, following different observational strategies: in dwarf satellite galaxies of the Milky Way, in the region close to the Galactic Centre, and in clusters of galaxies. The possible search for spatial signatures, facilitated by the larger field of view of CTA, is also discussed. Next we consider searches for axion-like particles which, besides being possible candidates for dark matter may also explain the unexpectedly low absorption by extragalactic background light of gamma rays from very distant blazars. Simulated light-curves of flaring sources are also used to determine the sensitivity to violations of Lorentz Invariance by detection of the possible delay between the arrival times of photons at different energies. Finally, we mention searches for other exotic physics with CTA.Comment: (31 pages, Accepted for publication in Astroparticle Physics

XMM-Newton evidence of shocked ISM in SN 1006: indications of hadronic acceleration

Shock fronts in young supernova remnants are the best candidates for being sites of cosmic ray acceleration up to a few PeV, though conclusive experimental evidence is still lacking. Hadron acceleration is expected to increase the shock compression ratio, providing higher postshock densities, but X-ray emission from shocked ambient medium has not firmly been detected yet in remnants where particle acceleration is at work. We exploited the deep observations of the XMM-Newton Large Program on SN 1006 to verify this prediction. We performed spatially resolved spectral analysis of a set of regions covering the southeastern rim of SN 1006. We studied the spatial distribution of the thermodynamic properties of the ambient medium and carefully verified the robustness of the result with respect to the analysis method. We detected the contribution of the shocked ambient medium. We also found that the postshock density of the interstellar medium significantly increases in regions where particle acceleration is efficient. Under the assumption of uniform preshock density, we found that the shock compression ratio reaches a value of ~6 in regions near the nonthermal limbs. Our results support the predictions of shock modification theory and indicate that effects of acceleration of cosmic ray hadrons on the postshock plasma can be observed in supernova remnants.Comment: Accepted for publication in A&

Locating the VHE source in the Galactic Centre with milli-arcsecond accuracy

Very high-energy gamma-rays (VHE; E>100 GeV) have been detected from the direction of the Galactic Centre up to energies E>10 TeV. Up to now, the origin of this emission is unknown due to the limited positional accuracy of the observing instruments. One of the counterpart candidates is the super-massive black hole (SMBH) Sgr A*. If the VHE emission is produced within ~10^{15} cm ~1000 r_G (r_G=G M/c^2 is the Schwarzschild radius) of the SMBH, a decrease of the VHE photon flux in the energy range 100--300 GeV is expected whenever an early type or giant star approaches the line of sight within ~ milli-arcseconds (mas). The dimming of the flux is due to absorption by pair-production of the VHE photons in the soft photon field of the star, an effect we refer to as pair-production eclipse (PPE). Based upon the currently known orbits of stars in the inner arcsecond of the Galaxy we find that PPEs lead to a systematic dimming in the 100--300 GeV band at the level of a few per cent and lasts for several weeks. Since the PPE affects only a narrow energy band and is well correlated with the passage of the star, it can be clearly discriminated against other systematic or even source-intrinsic effects. While the effect is too small to be observable with the current generation of VHE detectors, upcoming high count-rate experiments like the Cherenkov telescope array (CTA) will be sufficiently sensitive. Measuring the temporal signature of the PPE bears the potential to locate the position and size of the VHE emitting region within the inner 1000 r_G or in the case of a non-detection exclude the immediate environment of the SMBH as the site of gamma-ray production altogether.Comment: 7 pages, published in MNRAS 402, pg. 1342-134

GAMMA-RAY OBSERVATIONS OF GAMMA-RAY BINARIES

Gamma-ray binaries are binary systems that emit most of their radiative output above 1 MeV. Following the detection of five such systems in the past decade, they have been clearly established as a population of galactic GeV and TeV sources. In this review I discuss their recent gamma-ray observational results from Cherenkov telescopes and the Fermi satellite. A common trend has emerged in the high-energy spectra of several of these sources, with the detection of two separate components at GeV and TeV energies that cannot be explained as being emitted from a single region, and here I discuss a possible scenario giving rise to two separate acceleration locations in gamma-ray binaries