Light curve above 1 TeV from HESS J0632+057 is shown assuming a spectral shape of dn/dE ~ E^-Gamma with Gamma = 2.5. The downward pointing arrows show the 99% confidence limits derived here from the VERITAS data. The HESS fluxes are taken from Aharonian et al.(2007). The X-ray fluxes measured by XMM-Newton and Swift are indicated by open symbols.

HESS J0632+057 - A possible TeV binary system?

Reference:  V. A. Acciari et al. (The VERITAS Collaboration), The Astrophysical Journal, 698: L94, 2009

Full text version

ArXiv version: ArXiV:0905.3139

Contact person: Gernot Maier

HESS J0632+057 is one of only two unidentified very-high-energy gamma-ray sources which appear to be point-like within experimental resolution. It is possibly associated with the massive Be star MWC 148 and has been suggested to resemble known TeV binary systems like LS I +61 303 or LS 5039 .

HESS J0632+057 was observed by VERITAS for 31 hours in 2006, 2008 and 2009. During these observations, no significant signal in gamma rays with energies above 1 TeV was detected from the direction of HESS J0632+057. A flux upper limit corresponding to 1.1% of the flux of the Crab Nebula has been derived from the VERITAS data. The non-detection by VERITAS excludes with a probability of 99.993% that HESS J0632+057 is a steady gamma-ray emitter. Contemporaneous X-ray observations with Swift XRT reveal a factor of 1.8 ± 0.4 higher flux in the 1-10 keV range than earlier X-ray observations of HESS J0632+057.

Point-like gamma-ray sources stand out among the many galactic VHE objects with spatially extended gamma-ray emission. The latter are usually associated with either pulsar wind nebulae or supernova remnants. High-mass X-ray binaries constitute the only known class of galactic objects with variable point-like VHE emission (see e.g., the VERITAS results on the TeV binary LS I +61 303). TeV binaries show variable emission of gamma rays, likely connected to changes in physical parameters associated with the orbital movement. While there has been no compact companion discovered for MWC 148, the point-like nature of the VHE emission combined with the variable X-ray emission can easily be explained by a production scenario similar to those in TeV binaries. Future multiwavelength observations combined with results from ground-based and space-based gamma-ray observatories will provide a deeper understanding of the true nature of HESS J0632+057.

Figures from paper (click to get full size image): 

Figure 1: VERITAS significance map of the region around HESS J0632+057 for the whole data set and an energy threshold of 720 GeV. The background is estimated using the ring background method. The location of HESS J0632+057 is indicated by a black triangle. Also shown are the 95% and 99% confidence regions of the EGRET sources 3EGJ0634+0521 (white lines) and 3EGJ0631+0642 (GeV J0633+0645) (black lines). The Fermi source 0FGLJ0633.5+0634 is indicated with a “x” sign; the blue circle denotes the 95% confidence region (Abdo et al. 2009). The circle at the bottom right indicates the angular resolution of the VERITAS observations.

Figure 2 (shown above): Light curve above 1 TeV from HESS J0632+057 is shown assuming a spectral shape of dn/dE ~ E^-Gamma with Gamma = 2.5. The downward pointing arrows show the 99% confidence limits derived here from the VERITAS data. The HESS fluxes are taken from Aharonian et al.(2007). The X-ray fluxes measured by XMM-Newton and Swift are indicated by open symbols.

Figure 3: X-ray spectrum of HESS J0632+057 from XMM-Newton EPIC data in 2007 September (Hinton et al. 2009) and Swift/XRT observations in 2009 January.