Globular clusters are old stellar systems which exhibit very-high stellar densities in their cores. The globular cluster Terzan 5 is characterized by a high stellar encounter rate and hosts the largest detected population of millisecond pulsars. It also features bright GeV gamma-ray emission and extended X-ray radiation. However, no globular clusters have been detected in very-high-energy gamma rays (VHE, E> 100 GeV) so far. In order to investigate this possibility Terzan 5 has been observed with the H.E.S.S. telescope array in this energy band. The discovery of a source of VHE gamma rays from the direction of this globular cluster will be reported. The results of the VHE analysis and a multi-wavelength view of Terzan 5 will be presented in this contribution. No counterpart or model can fully explain the observed morphology of the detected VHE gamma-ray source.
Several types of galactic VHE gamma-ray sources like pulsar wind nebulae (PWNe) and supernova remnants (SNRs) have been detected to date but none so far in the vicinity of a globular cluster. Globular clusters are old stellar systems which exhibit very-high stellar densities in their cores. This leads to numerous stellar encounters [1] and to the prolific formation of millisecond pulsars (msPSRs) [2]. Globular clusters are predicted VHE gamma-ray sources. In these models the gamma-ray emission is produced by inverse Compton (IC) up-scattering of stellar radiation fields and the cosmic microwave background by relativistic electrons originating either from the msPSRs themselves or their PWNe [3,4,5].
Here the discovery of VHE gamma-ray emission from the direction of the Galactic globular cluster Terzan 5 is reported. This globular cluster is located at a distance of 5.9 kpc [6] at RA(J2000) 17 h 48 m 04 s .85 and Dec -24 • 46 ′ 44 ′′ .6 (Galactic coordinates: l = 3.84 • , b = 1.69 • ) and exhibits a core radius r c = 0 ′ .15, a half-mass radius r h = 0 ′ .52 and a tidal radius r t = 4 ′ .6 [7]. Terzan 5 hosts the largest population of detected msPSRs (33) [2] and it has been discovered by Fermi-LAT in the GeV range [8,9]. So far only upper limits have been reported from globular clusters in the VHE gamma-ray range (e.g. [10,11,12]).
The observations presented here have been undertaken with the H.E.S.S. array which is an array of four Imaging Atmospheric Cherenkov Telescopes located in the Khomas highlands in Namibia. Stereoscopic trigger and analysis method allow efficient cosmic ray background rejection and accurate energy and arrival direction reconstruction for gammarays in the energy range 100 GeV -100 TeV. Terzan 5 has been observed for 90 hours of good life-time with 3-and 4 telescopes with an average zenith angle of 20.4 • and a mean pointing direction off-set of 0.95 • . These observations resulted in the detection of a source of VHE gammarays in the vicinity of Terzan 5. Using hard cuts [13] a significance of 5.3 σ at the position of Terzan 5 is found with a nearby peak significance of 7.5 σ. This is confirmed by an independent calibration and analysis chain [14]. The source appears to extend beyond the tidal radius of the globular cluster. A 2-dimensional Gaussian fit results in a best-fit position of RA(J2000) 17 h 47 m 49 s ± 1 m .8 stat ± 1 s .3 sys and Dec -24 • 48 ′ 30 ′′ ± 36 ′′ stat ± 20 ′′ sys , offset by 4 ′ .0 from the GC center. Therefore the source is named HESS J1747 -248. The size of the source is given by the Gaussian widths 9 ′ .6±2 ′ .4 and 1 ′ .8±1 ′ .2, for the major and minor axes respectively, oriented 92 • ±6 • westwards from North. For spectral analysis a more restrictive data selection has been applied to improve the energy reconstruction which resulted in 62 hours of data. For a power-law spectral
, the flux normalization k at E 0 = 1 TeV is (5.2±1.1)×10 -13 cm -2 s -1 TeV -1 and the spectral index Γ = 2.5 ± 0.3 stat ± 0.2 sys . This corresponds to an integral flux of (1.2±0.3)×10 -12 cm -2 s -1 , or 1.5% of the Crab flux, in the 440 GeV -24 TeV range. There are not enough excess events to discuss a more complex spectral model.
Several interesting structures have been found in the surroundings of HESS J1747 -248 in archival multiwavelength data. In the X-ray regime diffuse emission extending beyond r h has been reported [17]. This diffuse emission is centered on the core of Terzan 5, exhibits an unabsorbed flux of (5.5 ± 0.8) × 10 -13 erg cm -2 s -1 in the 1 -7 keV band and is most likely of non-thermal origin with a hard spectrum with photon index 0.9 ± 0.5. Also diffuse radio emission has been found which extends to the north-west of Terzan 5 but does not show a taletelling morphology like e.g. a SNR shell [18]. The origin of the diffuse X-ray emission as well as the diffuse radio emission is ambiguous but could be connected to the large population of msPSRs in Terzan 5 [17,18]. Several scenarios for VHE gamma-ray emission would predict multi-wavelength counterparts, however, the relation between HESS J1747 -248 and the diffuse X-ray and radio sources remains unclear at the moment.
Available multi-wavelength data on the one hand do not show any typical VHE gamma-ray emitter (like PWN or SNR) in the vicinity of the detected source. On the other hand the properties of the source, namely the extension and the off-set (2σ level) are unexpected for a globular cluster. Therefore the results of the H.E.S.S. observations are difficult to interpret. Here scenarios excluding and including the globular cluster as the origin of the VHE gamma-ray emission are briefly discussed.
The positional concurrence between the globular cluster and the VHE gamma-ray source could in principle be just a chance coincidence of physically unrelated objects. Notably the source parameters (extension, photon spectrum) of HESS J1747 -248 are compatible with the properties of VHE gamma ray detected PW
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