We report on the latest discovery of an extended Very High Energy (VHE) gamma-ray source near the 67 ms pulsar PSR J1831-0952 during the H.E.S.S. Galactic Plane Survey (GPS). The dispersion measure distance of the pulsar (4.3 kpc) would imply that less than 1% of its spin-down energy is required to provide the observed VHE luminosity of the source. No other plausible counterparts have yet been found through preliminary multi- wavelength searches. The most likely scenario is that the VHE emission originates from the -yet unseen at other wavelengths-wind nebula of PSR J1831-0952. If so this would constitute another case of a gamma-ray discovered pulsar wind nebula.
The H.E.S.S. (High Energy Stereoscopic System, an array of four imaging atmospheric Cherenkov telescopes located in the Khomas Highland in Namibia) has revealed more than 60 sources of very-high-energy (VHE) γ-rays through its Galactic Plane Survey (GPS) since 2004. Thanks to the use of advanced multivariate analysis techniques and the accumulation of exposure, H.E.S.S. has achieved a sensitivity of better than 2% of Crab in the core region of the GPS (i.e. l = 282 • to 60 • ) [1]. Pulsar Wind Nebulae (PWNe) constitute by far the dominating source population as compared to that of young shell-type Supernova Remnants (SNRs), or to that of older and/or interacting remnants. About one third of H.E.S.S. sources have still either no known counterparts in other wavelengths, or lack any clear emission scenario. The data on HESS J1831-098 consist of observations either dedicated to nearby sources such as SNR 21.5-0.9/HESS J1833-105, or from the extension of H.E.S.S GPS near l = 21 • . These data were taken in 2004 (May-Oct.), 2005 (June and July), 2007 (Apr. and July), 2008 (Sept.) and 2009 (Apr.-July), for a total observation time on HESS J1831-098 of ∼ 52 hours. After application of the H.E.S.S. standard data quality criteria [2] based on hardware and weather conditions, the data set for HESS J1831-098 amounts to a total live-time of 40 hours with average zenith angle and average offset (to the FoV center) of 22.8 • , and 1.30 • , respectively. The mean offset is rather large because observations were not specifically targeted at this source.
The standard Hillas H.E.S.S. event reconstruction scheme was applied to the data after calibration [3]. The rejection of cosmic-ray showers was done by application of a recently developed multivariate analysis [4]. The sky maps were produced with an image size cut of 80 photo-electrons (p.e.) and using the Ring Background method [5] where the background at each test position on the sky is derived from a ring surrounding it with a mean radius of 0.7 • . To derive the spectrum, the same cut on image size was applied together with the a Reflected-Region procedure to estimate the Background, followed by the application of a forward-folding method [6].
The excess count map of the 0.4 • × 0.4 • region around HESS J1831-098 is shown on Figure 1. The map is smoothed with a Gaussian of σ ∼ 0.12 • . An extended γ-ray emission to the south-east of PSR J1831-0952 is observed with a peak pre-trials significance of 7.9σ for the standard integration radius of θ = 0.22 • , used for generation of the GPS maps when searching for extended sources. The significance level of the source after a conservative correction for trials is 5.8σ. The fit of the excess map with a two dimensional symmetrical Gaussian function, convolved with the H.E.S.S Point-Spread Function (PSF), results in a source centroid position of α ∼ 18 h 31 m 25 s , δ ∼ -9 • 54 , with a width of σ ∼ 0. Note that the fitted centroid does not coincide with the emission peak due to its departure from a Gaussian shape.
A circular region of 0.3 • radius, chosen as a compromise between optimal signal-to-noise ratio and independence of source morphology, was used to determine the energy spectrum. The differential spectrum, based on the 484 excess events in the circular region, is shown in Fig. 2. The best fitted shape is a power-law (dφ/dE = φ 0 (E/1TeV) -Γ ) with no significant indication of a cut-off up within the fitting energy range which extends up to 30 TeV. The fitted flux (uncorrected for events falling outside the integration disk) is φ 0 = (1.1 ± 0.1) × 10 -12 TeV -1 cm -2 s -1 and the photon index Γ = 2.1 ± 0.1. The integrated flux for E > 1 TeV corresponds to about 4% of the flux of the Crab nebula in the same energy range [2].
A preliminary multi-wavelength search for energetic counterparts to HESS J1831-098 resulted in the sole pulsar PSR J1831-0952 [7], which lies at a small angular offset of ∼ 0.05 • from the H.E.S.S. source’s best fit position (see Fig. 1). Some X-ray data from Chandra and XMM-Newton are available, but the observations have been done at large offsets with respect to PSR J1831-0952 and hence are not very useful. The search in the GeV energy was carried out using ∼ 33 months of public data from the Large Area Telescope (LAT) on board of Fermi (collected from 2008 August 4 (MJD 54682) to 2011 April 10 (MJD 55661)). A region of interest (ROI) of 6 • was used to select events in the [10-100] GeV range. The ROI radius was chosen such as to be large enough to get a reliable value for the normalization of the diffuse model and to be several times greater than the LAT PSF in the selected energy range. Events were analysed by applying the standard Fermi Science Tools to events of class 4 which is recommended for studies of faint eConf C110509 diffuse sources and which go beyond 20 GeV (in order to to minimize the non-photon background contamination). The corresponding instrument response function used for this
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