Abstract
Abstract The results of gamma-ray observations of the binary system HESS J0632 + 057 collected during 450 hr over 15 yr, between 2004 and 2019, are presented. Data taken with the atmospheric Cherenkov telescopes H.E.S.S., MAGIC, and VERITAS at energies above 350 GeV were used together with observations at X-ray energies obtained with Swift-XRT, Chandra, XMM-Newton, NuSTAR, and Suzaku. Some of these observations were accompanied by measurements of the Hα emission line. A significant detection of the modulation of the very high-energy gamma-ray fluxes with a period of 316.7 ± 4.4 days is reported, consistent with the period of 317.3 ± 0.7 days obtained with a refined analysis of X-ray data. The analysis of data from four orbital cycles with dense observational coverage reveals short-timescale variability, with flux-decay timescales of less than 20 days at very high energies. Flux variations observed over a timescale of several years indicate orbit-to-orbit variability. The analysis confirms the previously reported correlation of X-ray and gamma-ray emission from the system at very high significance, but cannot find any correlation of optical Hα parameters with fluxes at X-ray or gamma-ray energies in simultaneous observations. The key finding is that the emission of HESS J0632 + 057 in the X-ray and gamma-ray energy bands is highly variable on different timescales. The ratio of gamma-ray to X-ray flux shows the equality or even dominance of the gamma-ray energy range. This wealth of new data is interpreted taking into account the insufficient knowledge of the ephemeris of the system, and discussed in the context of results reported on other gamma-ray binary systems.
Highlights
The most common definition of gamma-ray binaries is based on their composition and their energy output: a gamma-ray binary consists of a compact object orbiting a star, with periodic releases of large amounts of nonthermal emission at energies >1 MeV (Dubus 2013)
The applied methods are: peak fitting (Bongiorno et al 2011), Z-transformed discrete correlation functions (Alexander 1997, ZDCF), phase dispersion minimization (Stellingwerf 1978, PDM), discrete correlation functions (Edelson & Krolik 1988, DCF), and correlation analysis comparing the light curves with a binned-average light curve (Malyshev et al 2019, PCC)
The lack ofcorrelation between optical and high-energy emission (X- and gamma-rays) observed in HESS J0632 + 057 can be explained by the difference in the integration times required at these frequencies, which is in agreement with the results found by Ahnen et al (2016) for the case of LS I +61° 303
Summary
The most common definition of gamma-ray binaries is based on their composition and their energy output: a gamma-ray binary consists of a compact object orbiting a star, with periodic releases of large amounts of nonthermal emission at energies >1 MeV (Dubus 2013). The class of gamma-ray binary sources consists of fewer than ten members, and every member shows different characteristics. Exceptions are the gamma-ray pulsar binaries PSR B1259-63/LS 2883 and PSR J2032 + 4127/MT91 213, each of which consists of a neutron star and a Be-type star. For the binary system HESS J0632 + 057, which hosts a Be star (MWC 148), the observational characteristics point toward a neutron star as the compact object, interacting with the circumstellar disk of the Be star. A microquasar scenario, in which particles are accelerated in the jets of a black hole, cannot be ruled out (see Bosch-Ramon & Khangulyan 2009 for a review)
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