Light curve above 1 TeV from HESS J0632+057 is shown assuming a spectral shape of dn/dE ~ E^-Gamma with Gamma = 2.5. The downward pointing arrows show the 99% confidence limits derived here from the VERITAS data. The HESS fluxes are taken from Aharonian et al.(2007). The X-ray fluxes measured by XMM-Newton and Swift are indicated by open symbols.

HESS J0632+057

HESS J0632+057 is one of only two unidentified very-high-energy gamma-ray sources which appear to be point-like within experimental resolution. It is possibly associated with the massive Be star MWC 148 and has been suggested to resemble known TeV binary systems like LS I +61 303 or LS 5039 .

HESS J0632+057 was observed by VERITAS for 31 hours in 2006, 2008 and 2009. During these observations, no significant signal in gamma rays with energies above 1 TeV was detected from the direction of HESS J0632+057. A flux upper limit corresponding to 1.1% of the flux of the Crab Nebula has been derived from the VERITAS data. The non-detection by VERITAS excludes with a probability of 99.993% that HESS J0632+057 is a steady gamma-ray emitter. Contemporaneous X-ray observations with Swift XRT reveal a factor of 1.8 ± 0.4 higher flux in the 1-10 keV range than earlier X-ray observations of HESS J0632+057.

Point-like gamma-ray sources stand out among the many galactic VHE objects with spatially extended gamma-ray emission. The latter are usually associated with either pulsar wind nebulae or supernova remnants. High-mass X-ray binaries constitute the only known class of galactic objects with variable point-like VHE emission (see e.g., the VERITAS results on the TeV binary LS I +61 303). TeV binaries show variable emission of gamma rays, likely connected to changes in physical parameters associated with the orbital movement. While there has been no compact companion discovered for MWC 148, the point-like nature of the VHE emission combined with the variable X-ray emission can easily be explained by a production scenario similar to those in TeV binaries. Future multiwavelength observations combined with results from ground-based and space-based gamma-ray observatories will provide a deeper understanding of the true nature of HESS J0632+057.

For more information on the science of this object please see V.A.Acciari et al, The Astrophysical Journal, 698, L94, 2009 (ArXiv:0905.3139).  You may also contact Gernot Maier.

Significance map of the region around LS I +61 303. Top: Observations during orbital phases 0.8 to 0.5. Bottom: Observations around apastron (orbital phases 0.5 to 0.8).

 The X-Ray Binary LS I +61 303

Gamma-ray binaries are a rare class of galactic very-high-energy (VHE) gamma-ray emitter.  LS I + 61 303 (distance 2 kpc, orbital period 26.5 days) is one of only a few high-mass X-ray binaries currently detected in VHE gamma-rays. This binary consists of a massive  Be-type star surrounded by a dense circumstellar disk and a stellar size compact object. The system is relatively small, extending over just a few astronomical units. This fact, combined with the high density of low-energy photons from the stellar companion and an orbital phase dependent geometry, results in a complex environment for the acceleration of high-energy charged particles and the production and absorption of gamma rays. The unknown nature of the compact object (neutron star or black hole) allows two very different scenarios to provide the necessary power for the production of VHE gamma rays. In the microquasar model, charged particles are accelerated in an accretion-driven relativistic jet, similar to the mechanism known from extragalactic blazars . The second scenario assumes that the compact object is a pulsar . Particles are accelerated in the shock created by the collision of the expanding pulsar wind with the equatorial disk or wind of the companion star.

LS I +61 303 was observed over several orbital cycles with VERITAS during the construction phase with a 2-telescope array. The binary has been detected as a source of VHE gamma-rays with energies above 300 GeV at high significance (8.4 sigma). The detected flux is measured to be strongly variable; interestingly, the maximum flux is found during most orbital cycles at apastron (see Figures, periastron takes place at phase 0.23, apastron is at phase 0.73). This indicates a strong dependence of particle acceleration and energy loss mechanism on the relative position. The production, variability and spectral energy distribution of the VHE gamma-rays can be explained by inverse Compton scattering of low-energy photons by electrons with energies in the TeV range. Future observations with the much more sensitive full VERITAS array together with contemporaneous measurements in other wavelengths will tackle the open question whether the high-energy electron are acceleration in accretion-driven jet or in a pulsar wind shock.

 For more information on the science of this object please see V.A.Acciari et al, The Astrophysical Journal, Volume 679, Issue 2, pp. 1427-1432 (arXiv:0802.2363).  You may also contact Gernot Maier .

Lower panel: The light curve of the integral photon flux above 200 GeV is shown. A spectral shape of dN/dE ~ E^-Gamma with Gamma = 3.8 is assumed. Each flux point corresponds to one observation period (defined by ~3 weeks of operation between two full-moon phases), with the exception of the flare around MJD 54538 (1) for which a night-by-night binning is used (see inlay for details). Upper panel: The X-ray flux as measured by Swift for the same time period. The vertical lines are shown for easier comparison.

 Discovery of the Blazar W Comae

VERITAS detected very high-energy (VHE) gamma-ray emission from the intermediate-frequency-peaked BL Lacertae object W Comae (z=0.102). The source was observed between January and April 2008.· A strong outburst of gamma-ray emission was measured in the middle of March, lasting for only four days (see ATel#1422). The energy spectrum measured during the two highest flare nights is fit by a power-law and is found to be very steep, with a differential photon spectral index of Γ = 3.81 ± 0.35_stat ± 0.34_syst. The integral photon flux above 200 GeV during those two nights corresponds to roughly 9% of the flux from the Crab Nebula. Quasi-simultaneous Swift observations at X-ray energies were triggered by the VERITAS observations. The spectral energy distribution of the flare data can be described by synchrotron-self-Compton (SSC) or external-Compton (EC) leptonic jet models, with the latter offering a more natural set of parameters to fit the data.

Beginning of June 2008, a second gamma-ray flare was observed by VERITAS from W Comae with fluxes reaching a level twice as high as the first flare (see ATel#1565). Those observations were taken under moon light conditions, work in progress.

W Comae is the first blazar of the IBL class to be detected at energies above 100 GeV. The extension of the VHE catalog to the FRSQ, LBL and IBL classes will play a major role in our understanding of blazar populations and blazar dynamics. The known VHE-emitting blazar 1ES 1218+304 is detected roughly 2deg North of W Comae. For the first time in VHE gamma-ray astronomy two extragalactic VHE gamma-ray sources are detected in the same field of view.

For more information on the science of this object, please see V. A. Acciari et. al., The Astrophysical Journal Letters, 684:L73–L77 (ArXiv:0808.0889). You may also contact Matthias Beilicke. 

The multiwavelenght data obtained by VERITAS (>500 GeV) and Swift/RXTE (2-10 keV) on LS I +61 303. Although there exist many instances of closely contemporaneous data in both TeV and X-ray, a clear picture of the relation between emission in these bands has yet to emerge.

 LS I +61 303

LS I +61 303 is one of only a handful of binary star systems which are known TeV emitters. It is known to be a pairing of a massive main sequence star and a compact object of unknown nature (most likely a neutron star , but the presence of a black hole has not been ruled out). The system is known to exhibit large outbursts of radio, X-ray, and TeV electromagnetic radiation; these bursts highly correlated with the ~26.5 day orbital system. The most distinct feature of the TeV gamma-ray emission from this system is that it only significantly appears at "apastron " passage when the two objects in the system are furthest apart. This feature of the system has fueled extensive modeling of the fundamental nature of LS I +61 303, a nature which is not clearly known at this point.

The two principal competing models are known as the "microquasar " and "binary pulsar " models which rely on particle acceleration around an acretion disk or particle acceleration at the interfact shock between a pulsar and stellar wind respectively. Although it is not clear which of these models best fits the observed data from LS I +61 303, each of these models to some degree makes predictions about correlations (or lack thereof) between the TeV and X-ray emission. Along these lines, VERITAS has completed an extensive, long-term monitoring campaign of the system in the TeV energy regime taken in conjunction with X-ray observations taken by the Swift and RXTE satellites.

Observations were taken 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 system was observed by VERITAS to be variable, with all integrated observations resulting in a detection at the 8.8 sigma (2006/2007) and 7.3 sigma (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 3.6 sigma 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 shown that the TeV sampling is not dense enough to detect a correlation between the two bands.

While this work does not shed any light on the fundamental nature of the system, it demonstrates the need for further multiwavelength campaigns of this kind to increase the sampling density in both bands. In addition, the marginal evidence for periastron TeV emission in the system is enticing and deserved to be followed up, as this emission may play a key role in our ability to distinguish between the models which seek to explain this curious and confusing galactic source.

For more information on the science of this object please see V.A.Acciari et al, The Astrophysical Journal, 700, 1034, 2009 (ArXiv:0904.4422 ).  You may also contact Andy Smith .