[Paper Review] Looking for Gamma-ray Emission from TeV Binary Candidates with HAWC

S. BenZvi|arXiv (Cornell University)|Aug 14, 2015

Astrophysics and Cosmic Phenomena4 citations

TL;DR

This paper presents the first unbiased survey of TeV binary candidates in the Northern Hemisphere using the High-Altitude Water Cherenkov (HAWC) Observatory, leveraging its high uptime and wide field of view to detect periodic and flaring gamma-ray emission. Despite limited sensitivity for high-declination sources, HAWC successfully monitored LS I +61° 303 during a flare, though no significant excess was detected due to poor pointing sensitivity, demonstrating HAWC's capability for long-term monitoring of variable gamma-ray binaries.

ABSTRACT

The Milky Way contains hundreds of binary systems which are known to emit in radio and X-rays, but only a handful of binaries have been observed to produce very high-energy gamma rays. In addition, the emission mechanisms which produce the gamma rays in the few known sources are not well understood. To improve the statistics of binary sources in the TeV band, the High-Altitude Water Cherenkov Gamma-ray Observatory, or HAWC, has begun to carry out a simultaneous survey of TeV binary candidates in the Northern Hemisphere between 100 GeV and 100 TeV. HAWC is a surface array that records air showers from cosmic rays and gamma rays with a high uptime and wide field of view, making it well-suited to observe time-dependent emission from objects such as TeV binaries. We describe the sensitivity of HAWC to periodic emission from Galactic sources of gamma rays and present data from the first year of observations with the partially constructed observatory.

Motivation & Objective

• To perform an unbiased, continuous survey of TeV binary candidates in the Northern Hemisphere using HAWC’s wide field of view and high uptime.
• To detect periodic and flaring gamma-ray emission from Galactic binary systems, especially those with unknown or weak TeV emission.
• To improve the statistical sample of known TeV binaries, which currently includes only five sources, to better understand emission mechanisms.
• To enable real-time follow-up of flares by triggering sensitive IACT observations through the HAWC Online Flare Monitor.
• To assess HAWC’s sensitivity to periodic signals from binaries like LS 5039 (3.9-day period) and HESS J0632+057 (315-day period).

Proposed method

• HAWC uses a surface array of 300 water Cherenkov detectors to record air showers from cosmic rays and gamma rays, with 1200 photomultiplier tubes per detector to capture Cherenkov light.
• Event reconstruction combines timing and spatial patterns of PMT hits to determine shower direction and distinguish gamma rays from hadronic cosmic rays using topological cuts.
• A cut-and-count analysis subtracts off-source background counts from on-source counts to identify time-dependent emission, with residual counts analyzed for periodicity.
• The Lomb-Scargle periodogram is used to search for periodic modulation in event rates, with simulated data used to estimate sensitivity to sources with 5% of the Crab flux.
• A parametric maximum-likelihood fit is applied to reconstruct integral flux light curves above 1 TeV for variable sources.
• The HAWC Online Flare Monitor continuously tracks 30 Galactic TeV binary candidates for flaring activity to trigger follow-up with IACTs.

Experimental results

Research questions

• RQ1 Can HAWC detect periodic gamma-ray emission from TeV binary candidates using its wide-field, high-up-time design?
• RQ2 What is the sensitivity of HAWC to flaring emission from gamma-ray binaries, particularly those with high-declination sources?
• RQ3 How well can HAWC distinguish periodic signals from known binaries like LS 5039 and HESS J0632+057?
• RQ4 Can HAWC identify new TeV binary systems through unbiased, continuous monitoring of candidate sources?
• RQ5 What is the expected detection efficiency for flares at 20% of the Crab flux within one week of observation?

Key findings

• HAWC detected no statistically significant excess or variability from LS I +61° 303 during a known flare in late 2014, primarily due to poor sensitivity at high declination.
• The sensitivity of HAWC-250 to high-declination sources is approximately two orders of magnitude lower than at the zenith, limiting detection of high-declination binaries.
• A source with 5% of the Crab flux and a 4-day period is expected to be detectable with 5σ significance within one year of observation, assuming full detector configuration.
• A flare at 20% of the Crab flux would be detected at 5σ within one week, indicating strong potential for flare detection with improved sensitivity.
• The full HAWC array is expected to detect periodic emission from LS 5039 (3.9-day period) within the first year of full operations.
• HESS J0632+057, with its favorable declination and hard spectrum, is expected to be detected within roughly one year, though its 315-day orbital period requires several years to fully characterize.