Exceptionally Bright Tev Flares from the Binary LS I +61° 303

The Astrophysical Journal, 2011

We present the results of observations of the TeV binary LS I +61 • 303 with the VERITAS telescope array between 2008 and 2010, at energies above 300 GeV. In the past, both ground-based gamma-ray telescopes VERITAS and MAGIC have reported detections of TeV emission near the apastron phases of the binary orbit. The observations presented here show no strong evidence for TeV emission during these orbital phases; however, during observations taken in late 2010, significant emission was detected from the source close to the phase of superior conjunction (much closer to periastron passage) at a 5.6 standard deviation ( 5.6 σ) post-trials significance. In total, between October 2008 and December 2010 a total exposure of 64.5 hours was accumulated with VERITAS on LS I +61 • 303, resulting in an excess at the 3.3σ significance level for constant emission over the entire integrated dataset. The flux upper limits derived for emission during the previously reliably active TeV phases (i.e. close to apastron) are less than 5% of the Crab Nebula flux in the same energy range. This result stands in apparent contrast to previous observations by both MAGIC and VERITAS which detected the source during these phases at 10% of the Crab Nebula flux. During the two year span of observations, a large amount of X-ray data were also accrued on LS I +61 • 303 by the Swift X-ray Telescope (XRT) and the Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA). We find no evidence for a correlation between emission in the X-ray and TeV regimes during 20 directly overlapping observations. We also comment on data obtained contemporaneously by the Fermi Large Area Telescope (LAT).

The Astrophysical Journal, 2013

18 25 1981), LS I +61 • 303 has been known historically for its energetic outbursts at radio, X-26 ray, GeV, and TeV wavelengths (Abdo et al.

The Astrophysical Journal, 2008

The Astrophysical Journal, 2014

In March 2013, a flaring episode from the Crab Nebula lasting ∼ 2 weeks was detected by the Fermi-LAT (Large Area Telescope on board the Fermi Gammaray Space Telescope). VERITAS provides simultaneous observations throughout this period. During the flare, the Fermi-LAT detected a 20-fold increase in flux above the average synchrotron flux > 100 MeV seen from the Crab Nebula. Simultaneous measurements with VERITAS are consistent with the non-variable long-term average Crab Nebula flux at TeV energies. Assuming a linear correlation between the very-high-energy flux change > 1 TeV and the flux change seen in the Fermi-LAT band > 100 MeV during the period of simultaneous observations, the linear correlation factor can be constrained to be at most 8.6 × 10 −3 with 95% confidence.

The Astrophysical …, 2009

We present results from a long-term monitoring campaign on the TeV binary LSI +61 • 303 with VERITAS at energies above 500 GeV, and in the 2-10 keV hard X-ray bands with RXTE and Swift, sampling nine 26.5 day orbital cycles between September 2006 and February 2008. The binary was observed by VERITAS to be variable, with all integrated observations resulting in a detection at the 8.8σ (2006/2007) and 7.3σ (2007/2008) significance level for emission above 500 GeV. The source was detected during active periods with flux values ranging from 5 to 20% of the Crab Nebula, varying over the course of a single orbital cycle. Additionally, the observations conducted in the 2007-2008 observing season show marginal evidence (at the 3.6σ significance level) for TeV emission outside of the apastron passage of the compact object around the Be star. Contemporaneous hard X-ray observations with RXTE and Swift show large variability with flux values typically varying between 0.5 and 3.0 ×10 −11 ergs cm −2 s −1 over a single orbital cycle. The contemporaneous X-ray and TeV data are examined and it is

The Astrophysical Journal, 2011

Science, 2011

A young and energetic pulsar powers the well-known Crab Nebula. Here we describe two separate gamma-ray (photon energy >100 MeV) flares from this source detected by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. The first flare occurred in February 2009 and lasted approximately 16 days. The second flare was detected in September 2010 and lasted approximately 4 days. During these outbursts the gamma-ray flux from the nebula increased by factors of four and six, respectively. The brevity of the flares implies that the gamma rays were emitted via synchrotron radiation from PeV (10^15 eV) electrons in a region smaller than 1.4 10^-2 pc. These are the highest energy particles that can be associated with a discrete astronomical source, and they pose challenges to particle acceleration theory.

The Astrophysical Journal, 2009

We present optical to very-high energy (VHE) gamma-ray observations of Mrk 421 between 2008 May 24 and June 23. A high-energy (HE) gamma-ray signal was detected by AGILE-GRID during June 9-15, brighter than the average flux observed by EGRET in Mrk 421 by a factor of ≈ 1.5. In 20-60 keV X-rays, a large-amplitude 5day flare (June 9-15) was resolved with a maximum flux of approx. 55 mCrab. SuperAGILE, RXTE/ASM and Swift/BAT data show a clearly correlated flaring structure between soft and hard X-rays, with a high flux/amplitude variability in hard X-rays. Hints of the same flaring behavior is also detected in the simultaneously recorded GASP-WEBT optical data. A target of opportunity observation by Swift near the flare maximum on June 12-13 revealed the highest 2-10 keV flux ever observed (>100 mCrab) and a peak synchrotron energy of approx. 3 keV, a large shift from typical values of 0.5-1 keV. Observations at VHE (E >200 GeV) gamma-rays during June 6-8 show the source flux peaking in a bright state, well correlated with the simultaneous peak in the X-rays. The gamma-ray flare can be interpreted within the framework of the Synchrotron Self Compton model in terms of a rapid acceleration of leptons in the jet.

Astronomy & Astrophysics, 2009

Context. A short duration burst reminiscent of a soft gamma-ray repeater/anomalous X-ray pulsar behaviour was detected in the direction of LS I +61 303 by the Swift satellite. While the association with this well known gamma-ray binary is likely, a different origin cannot be excluded. Aims. We explore the error box of this unexpected flaring event and establish the radio, near-infrared and X-ray sources in our search for any peculiar alternative counterpart. Methods. We carried out a combined analysis of archive Very Large Array radio data of LS I +61 303 sensitive to both compact and extended emission. We also reanalysed previous near infrared observations with the 3.5 m telescope of the Centro Astronómico Hispano Alemán and X-ray observations with the Chandra satellite. Results. Our deep radio maps of the LS I +61 303 environment represent a significant advancement on previous work and 16 compact radio sources in the LS I +61 303 vicinity are detected. For some detections, we also identify near infrared and X-ray counterparts. Extended emission features in the field are also detected and confirmed. The possible connection of some of these sources with the observed flaring event is considered. Based on these data, we are unable to claim a clear association between the Swift-BAT flare and any of the sources reported here. However, this study represents the most sophisticated attempt to determine possible alternative counterparts other than LS I +61 303.