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Register a new userA Search for Pulsed Very High-Energy Gamma Rays from Thirteen Young Pulsars in Archival VERITAS Data
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A. Archer
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We conduct a search for periodic emission in the very high-energy gamma-ray band (VHE; E > 100 GeV) from a total of 13 pulsars in an archival VERITAS data set with a total exposure of over 450 hours. The set of pulsars includes many of the brightest young gamma-ray pulsars visible in the Northern Hemisphere. The data analysis resulted in non-detections of pulsed VHE gamma rays from each pulsar. Upper limits on a potential VHE gamma-ray flux are derived at the 95% confidence level above three energy thresholds using two methods. These are the first such searches for pulsed VHE emission from each of the pulsars, and the obtained limits constrain a possible flux component manifesting at VHEs as is seen for the Crab pulsar.
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The binary millisecond radio pulsar PSR J1023+0038 exhibits many characteristics similar to the gamma-ray binary system PSR B1259--63/LS 2883, making it an ideal candidate for the study of high-energy non-thermal emission. It has been the subject of multi-wavelength campaigns following the disappearance of the pulsed radio emission in 2013 June, which revealed the appearance of an accretion disk around the neutron star. We present the results of very high-energy gamma-ray observations carried out by VERITAS before and after this change of state. Searches for steady and pulsed emission of both data sets yield no significant gamma-ray signal above 100 GeV, and upper limits are given for both a steady and pulsed gamma-ray flux. These upper limits are used to constrain the magnetic field strength in the shock region of the PSR J1023+0038 system. Assuming that very high-energy gamma rays are produced via an inverse-Compton mechanism in the shock region, we constrain the shock magnetic field to be greater than $sim$2 G before the disappearance of the radio pulsar and greater than $sim$10 G afterwards.
The recent discovery of electromagnetic signals in coincidence with neutron-star mergers has solidified the importance of multimessenger campaigns in studying the most energetic astrophysical events. Pioneering multimessenger observatories, such as LIGO/Virgo and IceCube, record many candidate signals below the detection significance threshold. These sub-threshold event candidates are promising targets for multimessenger studies, as the information provided by them may, when combined with contemporaneous gamma-ray observations, lead to significant detections. Here we describe a new method that uses such candidates to search for transient events using archival very-high-energy gamma-ray data from imaging atmospheric Cherenkov telescopes (IACTs). We demonstrate the application of this method to sub-threshold binary neutron star (BNS) merger candidates identified in Advanced LIGOs first observing run. We identify eight hours of archival VERITAS observations coincident with seven BNS merger candidates and search them for TeV emission. No gamma-ray emission is detected; we calculate upper limits on the integral flux and compare them to a short gamma-ray burst model. We anticipate this search method to serve as a starting point for IACT searches with future LIGO/Virgo data releases as well as in other sub-threshold studies for multimessenger transients, such as IceCube neutrinos. Furthermore, it can be deployed immediately with other current-generation IACTs, and has the potential for real-time use that places minimal burden on experimental operations. Lastly, this method may serve as a pilot for studies with the Cherenkov Telescope Array, which has the potential to observe even larger fields of view in its divergent pointing mode.
The vast majority of pulsars detected by the Fermi Large Area Telescope (LAT) display exponentially cutoff spectra with cutoffs falling in a narrow band around a few GeV. Early spectral modelling predicted spectral cutoffs at energies of up to 100 GeV, assuming curvature radiation. It was therefore not expected that pulsars would be visible in the very-high energy (VHE) regime (>100 GeV). The VERITAS announcement of the detection of pulsed emission from the Crab pulsar at energies up to 400 GeV (and now up to 1.5 TeV as detected by MAGIC) therefore raised important questions about our understanding of the electrodynamics and local environment of pulsars. H.E.S.S. has now detected pulsed emission from the Vela pulsar down to tens of GeV, making this the second pulsar detected by a ground-based Cherenkov telescope. Deep upper limits have also been obtained by VERITAS and MAGIC for the Geminga pulsar. We will review the latest developments in VHE pulsar science, including an overview of the latest observations, refinements, and extensions to radiation models and magnetic field structures, and the implementation of new radiation mechanisms. This will assist us in understanding the VHE emission detected from the Crab pulsar, and predicting the level of VHE emission expected from other pulsars, which is very important for the upcoming CTA.
We present a search of very high energy gamma-ray emission from the Northern $textit{Fermi}$ Bubble region using data collected with the High Altitude Water Cherenkov (HAWC) gamma-ray observatory. The size of the data set is 290 days. No significant excess is observed in the Northern $textit{Fermi}$ Bubble region, hence upper limits above $1,text{TeV}$ are calculated. The upper limits are between $3times 10^{-7},text{GeV}, text{cm}^{-2}, text{s}^{-1},text{sr}^{-1}$ and $4times 10^{-8},text{GeV},text{cm}^{-2},text{s}^{-1},text{sr}^{-1}$. The upper limits disfavor a proton injection spectrum that extends beyond $100,text{TeV}$ without being suppressed. They also disfavor a hadronic injection spectrum derived from neutrino measurements.
Geminga is a nearby (250 pc) middle-aged (spin-down time scale ~12,000 years) pulsar associated with a supernova remnant. Geminga has been a prime candidate for the origin of the unexpectedly high flux of cosmic-ray positrons above 10 GeV detected at Earth. Extended TeV gamma-ray emission from a 2-degree region around the Geminga pulsar was detected by the HAWC observatory, thus suggesting efficient, high-energy leptonic acceleration. Fermi-LAT observations show that the density of GeV leptons in the TeV nebula is lower than predicted by single zone and two zone diffusion models constrained with the HAWC measurements. However, the energy gap between Fermi-LAT and HAWC (~500 GeV to ~1 TeV) remains under-examined. The VERITAS gamma-ray observatory is sensitive in the energy range from 100 GeV to greater than 30 TeV, filling the gap between Fermi-LAT and HAWC. Therefore, VERITAS measurements potentially provide missing information. VERITAS has observed Geminga for 93 hours since 2009 including 28 hours in the 2018/2019 season. However, the standard VERITAS data analysis techniques have insufficient sensitivity to sources extended at the scale of the HAWC detection, due to difficulties with background estimation. We developed the Matched Runs Method (MRM) for VERITAS analysis of spatially extended sources. MRM has been demonstrated to be an effective technique by applying it to archival VERITAS data, and we are currently applying it to the Geminga observations. Here we present the summary of the MRM.
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