[Paper Review] VERITAS observations of HESS J0632+057

G. Maier|arXiv (Cornell University)|Jul 23, 2009

Gamma-ray bursts and supernovae3 citations

TL;DR

VERITAS observations of the unassociated VHE gamma-ray source HESS J0632+057 reveal significant variability in its 1 TeV+ emission, with a 4σ probability of flux variation when compared to earlier H.E.S.S. measurements. The results support a scenario in which the source is a TeV binary system like LS I +61 303 or LS 5039, with gamma-ray emission driven by particle acceleration in relativistic jets or shocks, possibly linked to the massive star MWC 148.

ABSTRACT

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.

Motivation & Objective

• To investigate the variability of high-energy gamma-ray emission from HESS J0632+057, a point-like, unassociated VHE source.
• To test the hypothesis that HESS J0632+057 is a TeV binary system similar to LS I +61 303 or LS 5039, given its point-like morphology and variable X-ray emission.
• To assess whether the massive star MWC 148 is the central engine of the VHE emission, possibly via particle acceleration in magnetically channeled wind shocks.
• To constrain the flux of VHE gamma rays above 1 TeV using VERITAS data and compare with prior H.E.S.S. measurements to detect temporal variability.
• To evaluate the potential of HESS J0632+057 as a prototype of a new class of galactic VHE emitters powered by strong magnetic fields in massive stars.

Proposed method

• VERITAS conducted three observation campaigns in December 2006, December 2008, and January 2009, using an array of four 12 m imaging atmospheric Cherenkov telescopes.
• Data were taken in wobble mode with the source offset by 0.5° from the camera center to improve background rejection and sky coverage.
• Image calibration, cleaning, and stereoscopic reconstruction were applied to reconstruct shower images and estimate impact positions and directions.
• Gamma-hadron separation was achieved using shape-based cuts on image parameters (mean-reduced-scaled width/length) and angular reconstruction (Θ² cut).
• Background rejection was enhanced using the 'ring background' and 'reflected-region' methods, reducing cosmic-ray background by >99.9% while preserving 45% of gamma-ray events.
• The energy threshold was set at 720 GeV, and flux upper limits were calculated at 99% confidence level assuming a power-law spectrum with index Γ = 2.5.

Experimental results

Research questions

• RQ1 Is the high-energy gamma-ray emission from HESS J0632+057 variable over time, as indicated by multi-epoch observations?
• RQ2 Does the observed flux variability in HESS J0632+057 support its association with a TeV binary system like LS I +61 303 or LS 5039?
• RQ3 Can the gamma-ray emission be explained by particle acceleration in relativistic jets or shocks from a compact object in a binary system with MWC 148?
• RQ4 Is the circumstellar environment of MWC 148 sufficiently magnetized to enable second-order Fermi acceleration and produce detectable TeV emission?
• RQ5 What is the significance of the discrepancy between the VERITAS flux upper limit and the earlier H.E.S.S. flux measurement in terms of source variability?

Key findings

• VERITAS observed HESS J0632+057 for a total of 1862 minutes across three observation sets, with no significant detection of gamma-ray events at the source position (2.1σ significance at 1 TeV).
• The 99% confidence level upper limit on the flux above 1 TeV is 2.6 × 10⁻¹³ cm⁻² s⁻¹, approximately 1.1% of the Crab Nebula flux.
• This VERITAS flux limit is 2.4 times lower than the H.E.S.S. flux measurement reported in 2004–2005, indicating a significant flux decrease.
• A χ²-test comparing the VERITAS and H.E.S.S. fluxes yields a probability of only 0.007% (≈4σ) that the flux is constant, providing strong evidence for variability.
• The X-ray variability of the associated source XMMU J063259.3+054801, previously reported, is consistent with the gamma-ray variability, supporting a common origin in a binary system.
• The results favor a scenario in which HESS J0632+057 is a TeV binary system, with gamma-ray emission produced via particle acceleration in relativistic jets or shocks, similar to LS I +61 303 and LS 5039.