The High-Altitude Water Cherenkov Gamma-Ray Observatory

Extended TeV Sources

Many of the TeV gamma ray sources detected with the HESS imaging air Cherenkov telescope (IACT) in the Galactic plane are extended, i.e., larger than the point response of the telescope of about 0.1°. Instead of appearing as a point source, like a far away star, our instruments are able to detect these sources as spatially extended. This could be due to the interactions of cosmic rays with material in the extended vicinity of their acceleration sites. For example, HESS measurements of TeV emission from the W28 supernova remnant indicate that the molecular clouds surrounding the remnant also emit TeV gamma rays. This could be due to the interaction of cosmic rays with the material inside the clouds.

Geminga excess
Observations of extended TeV emission in the region around the Geminga pulsar with Milagro. The location of the pulsar is shown as the white dot in the center of the image; the color scale indicates the signficance of the excess flux above background in units of σ.

Unfortunately, while facilities such as HESS are quite sensitive, it is difficult for such narrow-field instruments to observe the full extent of large sources. When the size of a source is much larger than the point spread function of an imaging air Cherenkov telescope, the sensitivity of the detector worsens because the background increases. Current observations of extended sources with HESS are clustered around the limit of their sensitivity, suggesting the presence of even more extended sources than currently claimed. In particular, nearby sources may have a larger angular extent than currently reported by IACTs.

One such nearby source is Geminga, a supernova remnant that is located several hundred parsecs from Earth. The Milagro collaboration has reported evidence of TeV emission from a 2.9°-wide region around Geminga, which corresponds to a physical size of about 8 pc. Given a measurement time comparable to Milagro, HAWC will be able to observe the emission from Geminga with a significance of >50σ.

With such high statistics, it should be possible to estimate the energy spectrum as a function of angular distance to the center of the pulsar. The highest energy electrons should lose energy quickly as they propagate away from the source; this is not true for protons. Therefore, if Geminga is an electron accelerator we will see a clear change in the spectral index of the resultant gamma rays the farther from the source we look.

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