Abstract
The very high energy (VHE; >100 GeV) source HESS J0632+057 has been recently confirmed as a γ-ray binary, a subclass of the high-mass X-ray binary population, through the detection of an orbital period of 321 d. We performed a deep search for the emission of HESS J0632+057 in the GeV energy range using data from the Fermi Large Area Telescope (LAT). The analysis was challenging due to the source being located in close proximity to the bright γ-ray pulsar PSR J0633+0632 and lying in a crowded region of the Galactic plane where there is prominent diffuse emission. We formulated a Bayesian block algorithm adapted to work with weighted photon counts, in order to define the off-pulse phases of PSR J0633+0632. A detailed spectral-spatial model of a 5° circular region centred on the known location of HESS J0632+057 was generated to accurately model the LAT data. No significant emission from the location of HESS J0632+057 was detected in the 0.1–100 GeV energy range integrating over ∼3.5 yr of data, with a 95 per cent flux upper limit of F0.1-100 GeV < 3 × 10− 8 ph cm−2 s−1. A search for emission over different phases of the orbit also yielded no significant detection. A search for source emission on shorter time-scales (days–months) did not yield any significant detections. We also report the results of a search for radio pulsations using the 100-m Green Bank Telescope. No periodic signals or individual dispersed bursts of a likely astronomical origin were detected. We estimated the flux density limit of < 90/40 μJy at 2/9 GHz. The LAT flux upper limits combined with the detection of HESS J0632+057 in the 136–400 TeV energy band by the MAGIC collaboration imply that the VHE spectrum must turn over at energies <136 GeV placing constraints on any theoretical models invoked to explain the γ-ray emission.
Highlights
The Large Area Telescope (LAT) flux upper limits combined with the detection of HESS J0632+057 in the 136–400 TeV energy band by the MAGIC collaboration imply that the very high energy (VHE) spectrum must turn over at energies
The population of γ -ray binaries is a subclass of the high-mass X-ray binary (HMXB) population; they are binary systems comprised of a high-mass (>10 M ) star and a compact object whose spectral energy distribution (SED) peaks above 1 MeV
The 95 per cent LAT flux upper limit for the 0.1–100 GeV energy band of
Summary
The population of γ -ray binaries is a subclass of the high-mass X-ray binary (HMXB) population; they are binary systems comprised of a high-mass (>10 M ) star and a compact object whose spectral energy distribution (SED) peaks above 1 MeV. It was noted that a potential optical counterpart to the source was a massive emissionline star, MWC 148, of spectral-type B0pe These facts led to the hypothesis that the source may be a new γ -ray binary, a nature that was confirmed by the discovery of an orbital period of 321 ± 5 d detected in the X-ray counterpart by Bongiorno et al (2011) using data from a long-term Swift-XRT monitoring campaign. Rea & Torres (2011) conducted a Chandra observing campaign during the high state of X-ray and TeV activity of HESS J0632+057 in 2011 February Their timing analysis did not find pulsations in the X-ray data and they derived 3σ upper limits on the X-ray pulsed fraction of ∼30 per cent, similar to those derived for LS I + 61◦303 and LS 5039 (Rea et al 2010, 2011). A search for radio pulsations using the 100-m Green Bank Telescope (GBT) is reported
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