ABSTRACT Distant BL Lacertae objects emit γ-rays that interact with the extragalactic background light (EBL), creating electron-positron pairs, and reducing the flux measured by ground-based imaging ...atmospheric Cherenkov telescopes (IACTs) at very-high energies (VHE). These pairs can Compton-scatter the cosmic microwave background, creating a γ-ray signature at slightly lower energies that is observable by the Fermi Large Area Telescope (LAT). This signal is strongly dependent on the intergalactic magnetic field (IGMF) strength (B) and its coherence length (LB). We use IACT spectra taken from the literature for 5 VHE-detected BL Lac objects and combine them with LAT spectra for these sources to constrain these IGMF parameters. Low B values can be ruled out by the constraint that the cascade flux cannot exceed that observed by the LAT. High values of B can be ruled out from the constraint that the EBL-deabsorbed IACT spectrum cannot be greater than the LAT spectrum extrapolated into the VHE band, unless the cascade spectrum contributes a sizable fraction of the LAT flux. We rule out low B values (B 10−19 G for LB ≥ 1 Mpc) at >5 in all trials with different EBL models and data selection, except when using >1 GeV spectra and the lowest EBL models. We were not able to constrain high values of B.
In this work, we describe the optical properties of the single photoelectron (SPE) calibration system designed for NectarCAM, a camera proposed for the Medium Sized Telescopes (MST) of the Cherenkov ...Telescope Array (CTA). One of the goals of the SPE system, as integral part of the NectarCAM camera, consists in measuring with high accuracy the gain of its photo-detection chain. The SPE system is based on a white painted screen where light pulses are injected through a fishtail light guide from a dedicated flasher. The screen – placed 15 mm away from the focal plane – is mounted on an XY motorization that allows movements over all the camera plane. This allows in-situ measurements of the SPE spectra via a complete scan of the 1855 photo-multiplier tubes (PMTs) of NectarCAM. This calibration process will enable the reduction of the systematic uncertainties on the energy reconstruction of γ-rays coming from distant astronomical sources and detected by CTA.
We discuss the design of the screen used in the calibration system and we present its optical performances in terms of light homogeneity and timing of the signal.
Distant BL Lacertae objects emit γ-rays that interact with the extragalactic background light (EBL), creating electron–positron pairs, and reducing the flux measured by ground-based imaging ...atmospheric Cherenkov telescopes (IACTs) at very-high energies (VHE). These pairs can Compton-scatter the cosmic microwave background, creating a γ-ray signature at slightly lower energies that is observable by the Fermi Large Area Telescope (LAT). This signal is strongly dependent on the intergalactic magnetic field (IGMF) strength (B) and its coherence length (L{sub B}). We use IACT spectra taken from the literature for 5 VHE-detected BL Lac objects and combine them with LAT spectra for these sources to constrain these IGMF parameters. Low B values can be ruled out by the constraint that the cascade flux cannot exceed that observed by the LAT. High values of B can be ruled out from the constraint that the EBL-deabsorbed IACT spectrum cannot be greater than the LAT spectrum extrapolated into the VHE band, unless the cascade spectrum contributes a sizable fraction of the LAT flux. We rule out low B values (B ≲ 10{sup −19} G for L{sub B} ≥ 1 Mpc) at >5σ in all trials with different EBL models and data selection, except when using >1 GeV spectra and the lowest EBL models. We were not able to constrain high values of B.
Gamma-ray bursts (GRBs) are particle acceleration sites that can emit photons in the very high-energy (VHE) domain through non-thermal processes. From 2004 until 2018, the current generation of ...Imaging Atmospheric Cherenkov Telescopes (IACTs) did not detect any GRB in the VHE domain. However, from 2018 to 2020, five detections have been reported. In this work, we try to solve the case of the missing VHE GBRs prior to 2018. We aim to identify GRBs that might have eluded VHE detection in the past years by the H.E.S.S., MAGIC, and VERITAS IACTs. To do so, we study GRBs with known redshift detected by \emph{Swift} from 2004 until June 2022. We first identify all GRBs that could have been observed by these IACTs since 2004, considering observation conditions and visibility constraints. We assume a relation between the X-rays and the VHE gamma rays based on the VHE GRBs detected to date and combine this with the redshift measurements, instrument response information, and observation conditions to predict the observed VHE gamma-ray flux from the \emph{Swift}-XRT measurements. We report findings on 12 bright low-redshift GRBs that we identify as most likely to have been detected in the VHE domain by current IACTs. The rate of IACT-detectable GRBs with ideal observation conditions is \(<\)1 VHE GRB per year with the current configuration. With its lower energy threshold and higher sensitivity, this rate increases to \(\sim\)4 VHE GRBs per year with CTA.
Fast radio bursts (FRBs) are millisecond-duration radio signals from unknown cosmic origin. Many models associate FRBs with high-energy astrophysical objects such as magnetars. In this attempt to ...find counterparts to FRBs, we explore gamma-ray bursts (GRBs) from the Swift and Fermi missions. We first search for spatial correlations between FRB and GRB populations as a whole and then search for a one-by-one correlation between each of the FRBs and GRBs investigated. Temporal coincidences are not considered. To evaluate the significance of any correlation found, we generate background realizations that take into account instrumentally induced anisotropies in the distribution of the sources. Neither study yields any significant counterpart detection. We estimate that less than 4\% of the FRBs are associated with GRBs in the studied samples
Gravitational wave (GW) events, particularly those connected to the merger of compact objects such as neutron stars, are believed to be the primary source of short gamma-ray bursts. To explore the ...very high energy (VHE) component of the emission from these events, the H.E.S.S. collaboration has dedicated a substantial effort and observing time to follow up on these events. During the second and third GW observing runs, H.E.S.S. was the first ground-based instrument to observe the GW170817 binary neutron star merger. In addition, H.E.S.S. followed four binary black hole mergers. The data acquired by H.E.S.S. was used to constrain the VHE emission from these events for the first time. H.E.S.S. also monitored the GW170817 source for approximately 50 hours and obtained limits that constrained the magnetic field in the merger remnant to \(> 24 \mu G\). As the fourth GW observing run (O4) approaches, the H.E.S.S. collaboration has allocated significant observation time to the follow-up of GW events. This contribution provides an overview of the science results derived from the H.E.S.S. follow-up of GW events, a technical overview of the GW follow-up strategies for O4, and an update on H.E.S.S. activities during O4.
Muon ring images observed with Imaging Atmospheric Cherenkov Telescopes (IACTs) provide a powerful means to calibrate the optical throughput of IACTs and monitor their optical point spread function. ...We investigate whether muons ring images can be used as the primary optical throughput calibration method for the telescopes of the future Cherenkov Telescope Array (CTA) and find several additional systematic effects in comparison to previous works. To ensure that the method achieves the accuracy required by CTA, these systematic effects need to be taken into account and minor modifications to the hardware and analysis are necessary. We derive analytic estimates for the expected muon data rates to be used for optical throughput calibration, monitoring of the optical point spread function, with achievable statistical and systematic uncertainties, and explore the potential of muon ring images as a secondary method of camera pixel flat-fielding.
A survey of unidentified 100 MeV γ-ray sources is undertaken, with the Whipple 10m telescope, with the objective of detecting very high energy (>350 GeV) γ-ray emission. The survey consists of ...nineteen sets of observations of sources detected by the EGRET instrument on the Compton Gamma-Ray Observatory between 1991 and 1995. Results for 21 EGRET sources are reported; in some cases two EGRET sources are close enough to be viewed in a single observation. For each EGRET source, candidate associations are listed and the implications of each candidate for VHE emission discussed. Finally, a study of the performance of a next-generation ground based instrument, VERITAS, using simulations is presented. The implications of the increased sensitivity of such an instrument for suture gamma-ray surveys is briefly discussed.
A survey of unidentified 100 MeV γ-ray sources is undertaken, with the Whipple 10m telescope, with the objective of detecting very high energy (>350 GeV) γ-ray emission. The survey consists of ...nineteen sets of observations of sources detected by the EGRET instrument on the Compton Gamma-Ray Observatory between 1991 and 1995. Results for 21 EGRET sources are reported; in some cases two EGRET sources are close enough to be viewed in a single observation. For each EGRET source, candidate associations are listed and the implications of each candidate for VHE emission discussed. Finally, a study of the performance of a next-generation ground based instrument, VERITAS, using simulations is presented. The implications of the increased sensitivity of such an instrument for suture γ-ray surveys is briefly discussed.
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