Are GeV and TeV spectra connected? the case of Galactic gamma-ray sources

More than 100 sources are now known at VHE γray energies and 1451 sources are listed in the first Fermi LAT catalog, comparing with ∼10 VHE γray sources and ∼300 HE γ-ray sources around the turn of the century. Given the large number of sources, we follow previous studies [2,4] and compare the HE and VHE source positions, as an important step to identify a group of sources emitting both in the HE and VHE bands.

We cross-correlated the 1FGL source centroid positions with VHE γ-ray source centroid positions. Only sources that are not associated with an extragalactic source were considered. Using the same manner as described in [2], the VHE source extent and 95% uncertainty in the 1FGL source centroids are taken into account. All first Fermi/LAT catalog sources are assumed to be point sources as in [3]. Those sources with an ending ‘c’ should be regarded with caution given the imperfect knowledge of the diffuse γ-ray background [3]. In total we identified 31 1FGL sources that are spatially coincident with one VHE source. In addition, the VHE source in the Westerlund 1 region, which are ∼0.6 • extended, is found to be spatially coincident with three 1FGL sources. HESS J1809-193 is coincident with two 1FGL sources. The list of these 1FGL-VHE source pairs are presented in Table I.

Based on pulsar timing information and dedicated efforts described in the corresponding literature, as well as spatial coincidences, the 1FGL sources in the list of coincidences include several classes: 2 HMXBs (LS I +61 • 303 and LS 5039), 8 PSRs, 4 SNRs (IC 443, W28, W49B, W51C), 2 PSR/PWN (Crab and Vela), 6 SNR/PWN candidates, one H II region, and 13 unassiciated sources.

The GeV spectral points are taken from the 1FGL catalog where point source analysis was used, while the VHE spectra shown are the best-fit power law taken from the respective literature.

We identify several cases of which the 0.1-100 GeV spectra and the VHE spectra cannot be described by a single spectral components, as shown in Figs 12345. The flux in the five energy bands in [3] are plotted together with the best-fit power law in the VHE range. In several other cases, the GeV emission come from a γ-ray pulsar, i.e., those 1FGL source identified as a pulsar, that shows cutoff at several GeV and VHE emission mostly likely come from the associated VHE γ-ray emitting PWN. We only present cases where the 1FGL source is not identified as a pulsar.

We found five VHE sources that are spatially coincident with a 1FGL source but the GeV-TeV spectra are incompatible with a single spectral component: HESS J0852-463, HESS J1614-518, HESS J1702-420, HESS J1809-193, and HESS J1848-018.

The cases presented here might represent a group of GeV/TeV sources where the spectral mis-matches indicate different radiations working at different energies or that radiation comes from different parts of a γ-ray source. Further studies of these spectral mis-match GeV/TeV spatially coincident cases are encouraged.

In this study, it is found that a significant number of VHE sources are spatially coincident with a counterpart in the first Fermi/LAT catalog, establishing a population of sources that emit both in the HE and VHE energy bands. This confirms our previous as-sessment using the Fermi bright source list [2].

However, the GeV-TeV spectra of some of these spatially coincident sources cannot be described by a single spectral component. While some of these cases are γ-ray pulsars accompanied by VHE γ-ray emitting nebulae, we highlight five cases where the 100 MeV to multi-TeV spectra of coincident 1FGL/VHE source pairs do not seem to be well fit by a single spectral component.

Notes added in proof : The second Fermi catalog has been released after the conference. We note that one of the coincidence pairs, 1FGL J1702.4-4147c, does not have a 2FGL counterpart.