IAR   05382
INSTITUTO ARGENTINO DE RADIOASTRONOMIA
Unidad Ejecutora - UE
artículos
Título:
Search for very-high-energy γ-ray emission from Galactic globular clusters with H.E.S.S.
Autor/es:
H.E.S.S. COLLABORATION; MEDINA, M. C.
Revista:
ASTRONOMY AND ASTROPHYSICS
Editorial:
EDP SCIENCES S A
Referencias:
Lugar: Paris; Año: 2013 vol. 551 p. 26 - 34
ISSN:
0004-6361
Resumen:
Context. Globular clusters (GCs) are established emitters of high-energy (HE, 100 MeV < E < 100 GeV) γ-ray
radiation which could originate from the cumulative emission of the
numerous millisecond pulsars (msPSRs) in the clusters cores or from
inverse Compton (IC) scattering of relativistic leptons accelerated in
the GC environment. These stellar clusters could also constitute a new
class of sources in the very-high-energy (VHE, E > 100 GeV) γ-ray regime, judging from the recent detection of a signal from the direction of Terzan 5 with the H.E.S.S. telescope array.
Aims. To search for VHE γ-ray sources
associated with other GCs, and to put constraints on leptonic emission
models, we systematically analyzed the observations towards 15 GCs taken
with the H.E.S.S. array of imaging atmospheric Cherenkov telescopes.
Methods. We searched for point-like and extended VHE γ-ray
emission from each GC in our sample and also performed a stacking
analysis combining the data from all GCs to investigate the hypothesis
of a population of faint emitters. Assuming IC emission as the origin of
the VHE γ-ray signal from the direction of Terzan 5, we calculated the expected γ-ray
flux from each of the 15 GCs, based on their number of millisecond
pulsars, their optical brightness and the energy density of background
photon fields.
Results. We did not detect significant VHE γ-ray
emission from any of the 15 GCs in either of the two analyses. Given
the uncertainties related to the parameter determinations, the obtained
flux upper limits allow to rule out the simple IC/msPSR scaling model
for NGC 6388 and NGC 7078. The upper limits derived from the stacking
analyses are factors between 2 and 50 below the flux predicted by the
simple leptonic scaling model, depending on the assumed source extent
and the dominant target photon fields. Therefore, Terzan 5 still remains
exceptional among all GCs, as the VHE γ-ray emission either
arises from extra-ordinarily efficient leptonic processes, or from a
recent catastrophic event, or is even unrelated to the GC itself.