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L. Rizk

Publications and source records attributed to L. Rizk.

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VERITAS Observations Contemporaneous with the LHAASO Detection of NGC 4278

Significant gamma-ray emission between 1 TeV and 20 TeV from a point source, 1LHAASO J1219+2915, consistent with the location of the LINER/LLAGN galaxy NGC 4278 was recently reported by the LHAASO collaboration. These data were later split into active and quasi-quiet states, with most of the LHAASO significance coming from the active state (MJD 59449-59589). Subsequent analysis of Fermi-LAT and Swift-XRT observations have been used to explore the double-peaked broad-band emission. Models of the spectral energy distribution (SED) are currently unconstrained due to the lack of contemporaneous multi-wavelength data at either peak. Here we report serendipitous observations of NGC 4278 with VERITAS, made possible by the contemporaneous observations of the nearby blazars 1ES 1218+304, 1ES 1215+303, and W Comae, each of which are located within $2^\circ$ of NGC 4278. VERITAS did not detect any gamma-ray emission and a flux upper limit was calculated. The flux upper limits constrain the photon spectrum of the quasi-quiet period, and together with Fermi-LAT, indicate a peak in the SED between 100 GeV and 2 TeV. We present an interpretation of the broadband SED that is based on acceleration of protons in the corona of the AGN, followed by p-$\gamma$ interactions and optically thin $\gamma$-ray emission. Within this framework, the implied neutrino signal is slightly below the current sensitivity of IceCube.

astro-ph.HE

Multiwavelength observation of a candidate pulsar halo LHAASO J0621+3755 and the first X-ray detection of PSR J0622+3749

Pulsar halos are regions around middle-aged pulsars extending out to tens of parsecs. The large extent of the halos and well-defined central cosmic-ray accelerators make this new class of Galactic sources an ideal laboratory for studying cosmic-ray transport. LHAASO J0621+3755 is a candidate pulsar halo associated with the middle-aged gamma-ray pulsar PSR J0622+3749. We observed LHAASO J0621+3755 with VERITAS and XMM-Newton in the TeV and X-ray bands, respectively. For this work, we developed a novel background estimation technique for imaging atmospheric Cherenkov telescope observations of such extended sources. No halo emission was detected with VERITAS (0.3--10 TeV) or XMM-Newton (2--7 keV) within 1 degree and 10 arcmin around PSR J0622+3749, respectively. Combined with the LHAASO-KM2A and Fermi-LAT data, VERITAS flux upper limits establish a spectral break at ~1--10 TeV, a unique feature compared with Geminga, the most studied pulsar halo. We model the gamma-ray spectrum and LHAASO-KM2A surface brightness as inverse Compton emission and find suppressed diffusion around the pulsar, similar to Geminga. A smaller diffusion suppression zone and harder electron injection spectrum than Geminga are necessary to reproduce the spectral cutoff. A magnetic field <= 1 uG is required by our XMM-Newton observation and synchrotron spectral modeling, consistent with Geminga. Our findings support slower diffusion and lower magnetic field around pulsar halos than the Galactic averages, hinting at magnetohydrodynamic turbulence around pulsars. Additionally, we report the detection of an X-ray point source spatially coincident with PSR J0622+3749, whose periodicity is consistent with the gamma-ray spin period of 333.2 ms. The soft spectrum of this source suggests a thermal origin.

astro-ph.HE