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Register a new userA hydrodynamics-informed, radiation model for HESS J0632$+$057 from radio to gamma rays
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Relativistic hydrodynamical simulations of the eccentric gamma-ray binary HESS J0632$+$057, show that the energy of a putative pulsar wind should accumulate in the binary surroundings between periastron and apastron, being released by fast advection close to apastron. To assess whether this could lead to a maximum of the non-thermal emission before apastron, we derive simple prescriptions for the non-thermal energy content, the radiation efficiency, and the impact of energy losses on non-thermal particles, in the simulated hydrodynamical flow. These prescriptions are used to estimate the non-thermal emission in radio, X-rays, GeV, and TeV, from the shocked pulsar wind in a binary system simulated using a simplified 3-dimensional scheme for several orbital cycles. Lightcurves at different wavelengths are derived, together with synthetic radio images for different orbital phases. The dominant peak in the computed lightcurves is broad and appears close to, but before, apastron. This peak is followed by a quasi-plateau shape, and a minor peak only in gamma rays right after periastron. The radio maps show ejection of radio blobs before apastron in the periastron-apastron direction. The results show that a scenario with a highly eccentric high-mass binary hosting a young pulsar can explain the general phenomenology of HESS J0632$+$057: despite its simplicity, the adopted approach yields predictions that are robust at a semi-quantitative level and consistent with multiwavelength observations.
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HESS J0632+057 is an eccentric gamma-ray Be binary that produces non-thermal radio, X-rays, GeV, and very high-energy gamma rays. The non-thermal emission of HESS J0632+057 is modulated with the orbital period, with a dominant maximum before apastron passage. The nature of the compact object in HESS J0632+057 is not known, although it has been proposed to be a young pulsar as in PSR B1259-63, the only gamma-ray emitting high-mass binary known to host a non-accreting pulsar. In this Letter, we present hydrodynamical simulations of HESS J0632+057 in the context of a pulsar and a stellar wind interacting in an eccentric binary, and propose a scenario for the non-thermal phenomenology of the source. In this scenario, the non-thermal activity before and around apastron is linked to the accumulation of non-thermal particles in the vicinity of the binary, and the sudden drop of the emission before apastron is produced by the disruption of the two-wind interaction structure, allowing these particles to efficiently escape. In addition to providing a framework to explain the non-thermal phenomenology of the source, this scenario predicts extended, moving X-ray emitting structures similar to those observed in PSR B1259-63.
Context. After the detection of a 321-days periodicity in X-rays, HESS J0632+057 can be robustly considered a new member of the selected group of gamma-ray binaries. These sources are known to show extended radio structure at scales of milliarcseconds (mas). Aims. We present the expected extended radio emission on mas scales from HESS J0632+057. Methods. We observed HESS J0632+057 with the European VLBI Network (EVN) at 1.6 GHz in two epochs: during the January/February 2011 X-ray outburst and 30 days later. Results. The VLBI image obtained during the outburst shows a compact ~0.4 mJy radio source, whereas 30 days later the source has faded and appears extended, with a projected size of ~75 AU. The peak of the emission is displaced between runs 21+/-5 AU, which is bigger than the orbit size. The position of the radio source is compatible with the Be star MWC 148, which sets the proper motion of the binary system below 3 mas yr^-1 in each coordinate. The brightness temperature of the source is above 2 times 10^6 K. We compare the multiwavelength properties of HESS J0632+057 with those of the previously known gamma-ray binaries. Conclusions. HESS J0632+057 displays extended and variable non-thermal radio emission. Its morphology, size, and displacement at AU scales are similar to those found in the other gamma-ray binaries, PSR B1259-63, LS 5039 and LS I +61 303, supporting a similar nature for HESS J0632+057.
HESS J0632+057 is one of only two unidentified high energy gamma-ray sources which appear to be point-like in nature. It is possibly associated with the massive star MWC 148 and has been suggested to resemble known TeV binary systems like LS I +61 303 or LS 5039. These binaries are rare and extreme (only three TeV binaries are known to date), and their gamma-ray emission mechanism has not been understood. HESS J0632+057 was observed by VERITAS, an array of four 12 m imaging atmospheric Cherenkov telescopes, in 2006, 2008 and 2009. Based on these observations we present evidence for variability in the high energy gamma-ray emission from HESS J0632+057.
We study changes in the $gamma$--ray intensity at very high energies observed from the $gamma$--ray binary HESS J0632+057. Publicly available data collected by Cherenkov telescopes were examined by means of a simple method utilizing solely the number of source and background events. Our results point to time variability in signal from the selected object consistent with periodic modulation of the source intensity.
The High Energy Stereoscopic System (HESS) survey of the Galactic plane has established the existence of a substantial number (~40) of Galactic TeV gamma-ray sources, a large fraction of which remain unidentified. HESS J0632+057 is one of a small fraction of these objects which is point-like in nature(<2 rms), and is one of only two point-like sources that remain unidentified. Follow-up observations of this object with XMM-Newton have revealed an X-ray source coincident with the TeV source and with the massive star MWC 148, of the spectral type B0pe. This source exhibits a hard spectrum, consistent with an absorbed power law with Gamma = 1.26 +/- 0.04, and shows significant variability on hour timescales. We discuss this spatial coincidence and the implied spectral energy distribution of this object and argue that it is likely a new gamma-ray binary system with a close resemblance to the three known members of this class, and in particular to LS I +61 303. Further X-ray, radio and optical observations of this system are needed to firmly establish HESS J0632+057 as a new member of this rare class of Galactic objects.
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