HESS J0632+057: from discovery to detailed exploration

December 2021

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The majority of stars around us are gravitationally bound in stellar binary systems. If the two stars are close enough, in rare cases we see variable emission from the outflowing matter from one of the stellar objects interacting with its companion. A well-known example is the recurrent nova RS Ophiuchi, which recently attracted some attention because its latest outburst could not only be seen by the naked eye in visible light, but was also detectable at up to TeV photon energies [1].

Looking at the Galactic plane in X-rays, almost all bright sources are identified as stellar binaries. These objects constitute the class of X-ray binaries (XRBs); a few hundred of these objects are known to date. In these binaries, one of the two objects is either a neutron star or a black hole, rather than a normal star. In most XRB systems, the X-ray emission stems from the matter that currently is or has recently been accreted towards the surface of the compact object.

So, what about gamma rays? Stellar-class binary systems are also detected with gamma-ray telescopes, though not in large numbers. Interestingly, for many of these objects, their discovery as a gamma-ray source triggered detailed studies and resulted in their identification as binary stars. The binary systems with detectable gamma-ray emission are called gamma-ray binaries; in total, only about ten such objects are known to date.

Fig. 1: This optical image of the outer regions of the Rosette Nebula (taken from the Digital Sky Survey DSS) depicts the Be star MWC 148 and its surroundings. This massive blue star is part of the gamma-ray binary star HESS J0632+057. (Image credit: esa sky)

One of these gamma-ray binaries is HESS J0632+057. This gamma-ray source was discovered with the H.E.S.S. telescopes back in 2007 ([2], H.E.S.S. SoM Feb. 2007). The source is located on the sky near the Monoceros Loop and the Rosette Nebula (figure 1). The fact that one normally refers to the object as HESS J0632+057 indicates that the discovery of TeV gamma-ray emission from the object indeed drove the investigations that finally led to the classification as a binary system.

Soon after the discovery of the gamma-ray source, it was suspected that HESS J0632+057 indeed represents a binary system [3], given its point-source nature, its positional coincidence with the massive star MWC 148, and its broad-band emission spectrum (including data from X-ray follow-up observations) which resembled that of other gamma-ray binaries. However, it took more than four years to establish a binary period of 321 +- 5 days using continued X-ray observations [4]) and joint H.E.S.S. and VERITAS efforts to firmly establish TeV variability in conjunction with the orbital period of the system ([5], [6], [7], H.E.S.S. SoM Sept. 2011). With this, the gamma-ray binary nature of HESS J0632+057 was successfully demonstrated.

However, the nature of the compact object (pulsar or black hole) in HESS J0632+057 is still unknown. It is believed that the high-energy emission from many (if not all) gamma-ray binaries does not stem from an accretion process but from the interaction of two outflows, which originate from the stellar and compact object, respectively. The binary period of HESS J0632+057 is amongst the longest of all gamma-ray binaries, only superseded by the PSR B1259-63/SS 2883 (3.4 years) and the PSR J2032+4127/MT91 213 (50 years) systems. These are actually the only two gamma-ray binaries for which the nature of the compact object is firmly established, from radio pulsations. Contrary to all other gamma-ray binaries, the gamma-ray emission from HESS J0632+057 in the GeV band is very faint; the first indications only came as recently as 2016 with the detection of two GeV photons using Fermi-LAT data [8].

Since 2011, the three TeV collaborations H.E.S.S., MAGIC, and VERITAS have used their telescope systems to continue exploring the gamma-ray emission from HESS J0632+057, which luckily is observable both from the Southern and Northern Hemispheres. The three collaborations have now joined forces to assemble the most comprehensive collection of data about the object, comprising all information from 2004 to 2019, not only from the gamma-ray band but also from X-ray and optical wavelengths. Ten years after the last SoM issue about the object, the paper has now been accepted for publication in the Astrophysical Journal [9].

Fig. 2: TeV (left) and X-ray (right) orbital light curves of HESS J0632+057. The flux points were folded with an orbital period of 317.3 days. (Image credit: [9])

The wealth of presented data is, e.g., illustrated by the fact that the orbital period of the system could be derived from the TeV fluxes alone; so far, this has only been possible for the much brighter gamma-ray binary LS 5039. As expected, the measured orbital period of 316.7 +- 4.4 days is well compatible with the most recent value from X-rays (317.3 +- 0.7 days). Figure 2 displays the period-folded light-curves in the TeV gamma-ray and X-ray bands, which reveal very similar patterns. Despite the large flux variation, the TeV spectrum is remarkably consistent with a power law with photon index 2.6 at all times. Moreover, no correlation between variability of the TeV and optical emission could be detected.

Neither the TeV nor the X-ray data exhibit strong orbit-to-orbit variability. However, two TeV outbursts were discovered (in Jan. 2011 and Jan. 2018) thanks to the extremely extensive observations conducted with the three TeV facilities. Contemporaneous H-alpha data taken in 2018 indicate that the size of the circumstellar disk had increased during the days of outburst, an observation that might permit a connection between the state of the disk and the high-energy flux level to be established.

The wealth of the data presented in [9] is available online. The analysis reported in the paper clearly reveals that, at this stage, significant progress in understanding the nature of the physics processes at work in HESS J0632+057 can most likely only be reached with multiwavelength (optical X-ray TeV) observations of this gamma-ray binary, using current or forthcoming instruments.


[1] Wagner, S.J., for the H.E.S.S. collaboration: ATEL 14844 (2021) Detection of VHE gamma-ray emission from the recurrent nova RS Ophiuchi with H.E.S.S.

[2] Aharonian, F. A., Akhperjanian, A. G., Bazer-Bachi, et al. (HESS Collaboration) 2007, Discovery of a point-like very-high-energy gamma-ray source in Monoceros, A&A, 469, L1

[3] Hinton, J. A., Skilton, J. L., Funk, S., et al. 2009, HESS J0632+057: A New Gamma-Ray Binary?, ApJ, 690, L101

[4] Bongiorno, S. D., Falcone, A. D., Stroh, M., et al. 2011, A New TeV Binary: The Discovery of an Orbital Period in HESS J0632+057, ApJL, 737, L11

[5] Ong, R., for the VERITAS collaboration: ATEL 3153 (2011) VERITAS Reports Increased Activity of HESS J0632+057 in Very High Energy Gamma Rays

[6] Mariotti, M., for the MAGIC collaboration: ATEL 3161 (2011) MAGIC confirms VHE gamma-ray emission from HESS J0632+057 between 7-9 Feb 2011

[7] Maier G., for the VERITAS collaboration: Observation of binary systems at very high energies with VERITAS, 5th International Meeting on High Energy Gamma-Ray Astronomy. AIP Conference Proceedings, Dec. 2012

[8] Malyshev, D. & Chernyakova, M., 2016, Constraints on the spectrum of HESS J0632+057 from Fermi-LAT data, MNRAS, 463, 3074

[9] Adams, C. B., Benbow, W., Brill, A., et al. (VERITAS, MAGIC, HESS Collaboration) 2021, Observation of the gamma-ray binary HESS J0632+057 with the H.E.S.S., MAGIC, and VERITAS telescopes, arXiv: 2109.11894