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The recently completed High Altitude Water Cherenkov (HAWC) gamma-ray observatory has been taking data with a partial array for more than one year and is now operating with >95% duty cycle in its full configuration. With an instantaneous field of view of 2 sr, two-thirds of the sky is surveyed every day at gamma-ray energies between approximately 100 GeV and 100 TeV. Any source location in the field of view can be monitored each day, with an exposure of up to $sim$ 6 hours. These unprecedented observational capabilities allow us to continuously scan the highly variable extra-galactic gamma-ray sky. By monitoring the flaring behavior of Active Galactic Nuclei we aim to significantly increase the observational data base for characterizing particle acceleration mechanisms in these sources and for studying cosmological properties like the extra-galactic background light. In this work we present first studies of data taken between June 2013 and July 2014 with a partial array configuration. Flux light curves, binned in week-long intervals, for the TeV-emitting blazars Markarian 421 and 501 are discussed with respect to indications of flaring states and we highlight coincident multi-wavelength observations. Results for both sources show indications of gamma-ray flare observations and demonstrate that a water Cherenkov detector can monitor TeV-scale variability of extra-galactic sources on weekly time scales. The analysis methods presented here can provide daily flux measurements with a minimum time interval of one transit and will be applied to new data from the completed HAWC array for monitoring of blazars and other transients.
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We present 10 years of R-band monitoring data of 31 northern blazars which were either detected at very high energy (VHE) gamma rays or listed as potential VHE gamma-ray emitters. The data comprise 11820 photometric data points in the R-band obtained in 2002-2012. We analyze the light curves by determining their power spectral density (PSD) slopes assuming a power-law dependence with a single slope $beta$ and a Gaussian probability density function (PDF). We use the multiple fragments variance function (MFVF) combined with a forward-casting approach and likelihood analysis to determine the slopes and perform extensive simulations to estimate the uncertainties of the derived slopes. We also look for periodic variations via Fourier analysis and quantify the false alarm probability through a large number of simulations. Comparing the obtained PSD slopes to values in the literature, we find the slopes in the radio band to be steeper than those in the optical and gamma rays. Our periodicity search yielded one target, Mrk 421, with a significant (p<5%) period. Finding one significant period among 31 targets is consistent with the expected false alarm rate, but the period found in Mrk~421 is very strong and deserves further consideration}.
Studies of radio galaxies at TeV energies are fascinating because their jets are misaligned concerning our sightline. Thus, it provides us with a unique opportunity to study the structure of their jets, the radiative processes, and the acceleration mechanisms involved in them. In addition, some radio galaxies have presented variability in their emission, like the giant radio galaxy M87, which has reported several activity periods. Due to its duty cycle >95% and instantaneous field of view of 2 sr, HAWC provides daily monitoring of variable sources visible from the Northern Hemisphere. In this work, we show the results of monitoring M87 between January 2015 and December 2018. HAWCs observations are consistent with the low activity state reported by other instruments (like H.E.S.S and MAGIC). However, after September 2017 (~MJD 58000), the HAWC measurements of M87 show hints of higher activity.
We present results from daily monitoring of gamma rays in the energy range $sim0.5$ to $sim100$ TeV with the first 17 months of data from the High Altitude Water Cherenkov (HAWC) Observatory. Its wide field of view of 2 steradians and duty cycle of $>95$% are unique features compared to other TeV observatories that allow us to observe every source that transits over HAWC for up to $sim6$ hours each sidereal day. This regular sampling yields unprecedented light curves from unbiased measurements that are independent of seasons or weather conditions. For the Crab Nebula as a reference source we find no variability in the TeV band. Our main focus is the study of the TeV blazars Markarian (Mrk) 421 and Mrk 501. A spectral fit for Mrk 421 yields a power law index $Gamma=2.21 pm0.14_{mathrm{stat}}pm0.20_{mathrm{sys}}$ and an exponential cut-off $E_0=5.4 pm 1.1_{mathrm{stat}}pm 1.0_{mathrm{sys}}$ TeV. For Mrk 501, we find an index $Gamma=1.60pm 0.30_{mathrm{stat}} pm 0.20_{mathrm{sys}}$ and exponential cut-off $E_0=5.7pm 1.6_{mathrm{stat}} pm 1.0_{mathrm{sys}}$ TeV. The light curves for both sources show clear variability and a Bayesian analysis is applied to identify changes between flux states. The highest per-transit fluxes observed from Mrk 421 exceed the Crab Nebula flux by a factor of approximately five. For Mrk 501, several transits show fluxes in excess of three times the Crab Nebula flux. In a comparison to lower energy gamma-ray and X-ray monitoring data with comparable sampling we cannot identify clear counterparts for the most significant flaring features observed by HAWC.
The High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory surveys the very high energy sky in the 300 GeV to $>100$ TeV energy range. HAWC has detected two blazars above $11sigma$, Markarian 421 (Mrk 421) and Markarian 501 (Mrk 501). The observations are comprised of data taken in the period between June 2015 and July 2018, resulting in a $sim 1038$ days of exposure. In this work we report the time-averaged spectral analysis for both sources above 0.5 TeV. Taking into account the flux attenuation due to the extragalactic background light (EBL), the intrinsic spectrum of Mrk 421 is described by a power law with an exponential energy cut-off with index $alpha=2.26pm(0.12)_{stat}(_{-0.2}^{+0.17})_{sys}$ and energy cut-off $E_c=5.1pm(1.6)_{stat}(_{-2.5}^{+1.4})_{sys}$ TeV, while the intrinsic spectrum of Mrk 501 is better described by a simple power law with index $alpha=2.61pm(0.11)_{stat}(_{-0.07}^{+0.01})_{sys}$. The maximum energies at which the Mrk 421 and Mrk 501 signals are detected are 9 and 12 TeV, respectively. This makes these some of the highest energy detections to date for spectra averaged over years-long timescales. Since the observation of gamma radiation from blazars provides information about the physical processes that take place in their relativistic jets, it is important to study the broad-band spectral energy distribution (SED) of these objects. To this purpose, contemporaneous data from the Large Area Telescope on board the {em Fermi} satellite and literature data, in the radio to X-ray range, were used to build time-averaged SEDs that were modeled within a synchrotron self-Compton leptonic scenario to derive the physical parameters that describe the nature of the respective jets.
Only five binary systems have been found to emit at TeV energies. Each of these systems is composed of a massive O or B type star and a compact object (black hole or a pulsar). The type of compact object and the origin of the gamma-ray emission is unknown for most of these systems. Extending spectral observations to higher energies can help disentangle the nature of the compact object as well as the particle acceleration mechanisms present. Interestingly, the TeV emission from these systems does not always coincide with their emission in GeV or X-ray, which is how many such systems have been originally discovered. Increased coverage of these systems may allow HAWC to see precisely when in the orbit the TeV emission begins and ends. The HAWC Observatory detects TeV gamma-rays with high sensitivity, covering over two-thirds of the overhead sky every day. Applying a stacking method to known TeV binary systems can help HAWC enhance the signal from TeV binaries above the steady background from other sources in the galaxy. We will present results from this stacking analysis using 760 days of HAWC data.
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