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Frank Krennrich

Publications and source records attributed to Frank Krennrich.

18 recordsLinked to original sources
arXiv2025

VERITAS and multiwavelength observations of the Blazar B3 2247+381 in response to an IceCube neutrino alert

While the sources of the diffuse astrophysical neutrino flux detected by the IceCube Neutrino Observatory are still largely unknown, one of the promising methods used towards understanding this is investigating the potential temporal and spatial correlations between neutrino alerts and the electromagnetic radiation from blazars. We report on the multiwavelength target-of-opportunity observations of the blazar B3 2247+381, taken in response to an IceCube multiplet alert for a cluster of muon neutrino events compatible with the source location between May 20, 2022 and November 10, 2022. B3 2247+381 was not detected with VERITAS during this time period. The source was found to be in a low-flux state in the optical, ultraviolet and gamma-ray bands for the time interval corresponding to the neutrino event, but was detected in the hard X-ray band with NuSTAR during this period. We find the multiwavelength spectral energy distribution is well described using a simple one-zone leptonic synchrotron self-Compton radiation model. Moreover, assuming the neutrinos originate from hadronic processes within the jet, the neutrino flux would be accompanied by a photon flux from the cascade emission, and the integrated photon flux required in such a case would significantly exceed the total multiwavelength fluxes and the VERITAS upper limits presented here. The lack of flaring activity observed with VERITAS, combined with the low multiwavelength flux levels, and given the significance of the neutrino excess is at 3$σ$ level (uncorrected for trials), makes B3 2247+381 an unlikely source of the IceCube multiplet. We conclude that the neutrino excess is likely a background fluctuation.

astro-ph.HE↗
arXiv2025

An in-depth study of Gamma rays from the Starburst Galaxy M 82 with VERITAS

Assuming Galactic cosmic rays originate in supernovae and the winds of massive stars, starburst galaxies should produce very-high-energy (VHE; E$>$100 GeV) gamma-ray emission via the interaction of their copious quantities of cosmic rays with the large reservoirs of dense gas within the galaxies. Such VHE emission was detected by VERITAS from the starburst galaxy M 82 in 2008-09. An extensive, multi-year campaign followed these initial observations, yielding a total of 254 h of good quality VERITAS data on M 82. Leveraging modern analysis techniques and the larger exposure, these VERITAS data show a more statistically significant VHE signal ($\sim$6.5 standard deviations ($σ$)). The corresponding photon spectrum is well fit by a power law ($Γ= 2.3 \pm 0.3_{stat} \pm0.2_{sys}$) and the observed integral flux is F($>$450 GeV) = $(3.2 \pm0.6_{stat} \pm 0.6_{sys}) \times 10^{-13}~\mathrm{cm^{-2}~s}^{-1}$, or $\sim$0.4\% of the Crab Nebula flux above the same energy threshold. The improved VERITAS measurements, when combined with various multi-wavelength data, enable modeling of the underlying emission and transport processes. A purely leptonic scenario is found to be a poor representation of the gamma-ray spectral energy distribution (SED). A lepto-hadronic scenario with cosmic rays following a power-law spectrum in momentum (index $s\simeq 2.25$), and with significant bremsstrahlung below $1$~GeV, provides a good match to the observed SED. The synchrotron emission from the secondary electrons indicates that efficient non-radiative losses of cosmic-ray electrons may be related to advective escape from the starburst core.

astro-ph.HE↗
arXiv2023

VERITAS discovery of very high energy gamma-ray emission from S3 1227+25 and multiwavelength observations

We report the detection of very high energy gamma-ray emission from the blazar S3 1227+25 (VER J1230+253) with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). VERITAS observations of the source were triggered by the detection of a hard-spectrum GeV flare on May 15, 2015 with the Fermi-Large Area Telescope (LAT). A combined five-hour VERITAS exposure on May 16th and May 18th resulted in a strong 13$σ$ detection with a differential photon spectral index, $Γ$ = 3.8 $\pm$ 0.4, and a flux level at 9% of the Crab Nebula above 120 GeV. This also triggered target of opportunity observations with Swift, optical photometry, polarimetry and radio measurements, also presented in this work, in addition to the VERITAS and Fermi-LAT data. A temporal analysis of the gamma-ray flux during this period finds evidence of a shortest variability timescale of $τ_{obs}$ = 6.2 $\pm$ 0.9 hours, indicating emission from compact regions within the jet, and the combined gamma-ray spectrum shows no strong evidence of a spectral cut-off. An investigation into correlations between the multiwavelength observations found evidence of optical and gamma-ray correlations, suggesting a single-zone model of emission. Finally, the multiwavelength spectral energy distribution is well described by a simple one-zone leptonic synchrotron self-Compton radiation model.

astro-ph.HE↗
arXiv2022

Design and Performance of the Prototype Schwarzschild-Couder Telescope Camera

The prototype Schwarzschild-Couder Telescope (pSCT) is a candidate for a medium-sized telescope in the Cherenkov Telescope Array. The pSCT is based on a novel dual mirror optics design which reduces the plate scale and allows for the use of silicon photomultipliers as photodetectors. The prototype pSCT camera currently has only the central sector instrumented with 25 camera modules (1600 pixels), providing a 2.68$^{\circ}$ field of view (FoV). The camera electronics are based on custom TARGET (TeV array readout with GSa/s sampling and event trigger) application specific integrated circuits. Field programmable gate arrays sample incoming signals at a gigasample per second. A single backplane provides camera-wide triggers. An upgrade of the pSCT camera is in progress, which will fully populate the focal plane. This will increase the number of pixels to 11,328, the number of backplanes to 9, and the FoV to 8.04$^{\circ}$. Here we give a detailed description of the pSCT camera, including the basic concept, mechanical design, detectors, electronics, current status and first light.

astro-ph.IM↗
arXiv2019

Measurement of Beam-Correlated Background Neutrons from the Fermilab Booster Neutrino Beam in ANNIE Phase-I

The Accelerator Neutrino Neutron Interaction Experiment (ANNIE) aims to make a unique measurement of neutron yield from neutrino-nucleus interactions and to perform R&D for the next generation of water-based neutrino detectors. In this paper, we characterize beam-induced neutron backgrounds in the experimental hall at Fermi National Accelerator Laboratory. It is shown that the background levels are sufficiently low to allow the next stage of the experiment to proceed. These measurements are relevant to other Booster Neutrino Beam (BNB) experiments located adjacent to ANNIE Hall, where dirt neutrons and sky-shine could present similar backgrounds.

physics.ins-det↗
arXiv2018

An image-based array trigger for Imaging Atmospheric Cherenkov Telescope Arrays

It is anticipated that forthcoming, next generation, atmospheric Cherenkov telescope arrays will include a number of medium-sized telescopes that are constructed using a dual-mirror Schwarzschild-Couder configuration. These telescopes will sample a wide ($8^{\circ}$) field of view using a densely pixelated camera comprising over $10^{4}$ individual readout channels. A readout frequency congruent with the expected single-telescope trigger rates would result in substantial data rates. To ameliorate these data rates, a novel, hardware-level Distributed Intelligent Array Trigger (DIAT) is envisioned. A copy of the DIAT operates autonomously at each telescope and uses reduced resolution imaging data from a limited subset of nearby telescopes to veto events prior to camera readout {and any subsequent network transmission of camera data that is required for centralized storage or aggregation}. We present the results of Monte-Carlo simulations that evaluate the efficacy of a "Parallax width" discriminator that can be used by the DIAT to efficiently distinguish between genuine gamma-ray initiated events and unwanted background events that are initiated by hadronic cosmic rays.

astro-ph.IM↗
arXiv2015

Simulations of a Distributed Intelligent Array Trigger for the Cherenkov Telescope Array

It is anticipated that the forthcoming Cherenkov Telescope Array (CTA) will include a number of medium-sized telescopes that are constructed using a dual-mirror Schwarzschild-Couder configuration. These telescopes will sample a wide ($8^{\circ}$) field of view using a densely pixelated camera comprising over $10^{4}$ individual readout channels. A readout frequency congruent with the expected single-telescope trigger rates would result in substantial data rates. To ameliorate these data rates, a novel, hardware-level Distributed Intelligent Array Trigger (DIAT) is envisioned. A copy of the DIAT operates autonomously at each telescope and uses reduced metadata from a limited subset of nearby telescopes to veto events prior to camera readout. We present the results of Monte-Carlo simulations that evaluate the efficacy of a "Parallax width" discriminator that can be used by the DIAT to efficiently distinguish between genuine gamma-ray initiated events and unwanted background events that are initiated by hadronic cosmic rays.

astro-ph.IM↗
arXiv2013

The hunt for axionlike particles with the Cherenkov Telescope Array

Gamma-ray photons can convert into axion-like particles (ALPs) and vice versa when interacting with astrophysical magnetic fields. The conversions of ALPs may have important implications for astrophysics since they would imprint characteristic features on the spectra of gamma-ray sources. The features may be detected by the Fermi Gamma-ray Space Telescope and by current Imaging Atmospheric Cherenkov Telescopes (IACTs) or the future Cherenkov Telescope Array (CTA). Thus, gamma-ray observations can provide valuable constraints on the nature of ALPs, and represent an excellent complementary approach to laboratory searches for ALPs. CTA's twenty times larger collection area and improved sensitivity compared to current IACTs will make it an extraordinarily powerful instrument to search for signatures of ALPs imprinted in the spectra of gamma-ray sources in the energy range between a few tens of GeV up to a few dozen TeV. The US contribution of Medium Size Telescopes will be therefore particularly important for ALP searches, as it will improve the sensitivity of the core energy regime (100 GeV - 10 TeV) of CTA by a factor of 2-3. As photon/ALP conversions are used as the main vehicle in the search, rather than the usual self-annihilations for WIMP candidates, the unknown dark matter nature will be scrutinized from a different perspective. Indeed, CTA could probe a region of the ALP parameter space in which ALPs might account for the total cold dark matter content in the Universe.

astro-ph.HE↗
arXiv2012

The Extragalactic Background Light and the Gamma-ray Opacity of the Universe

The extragalactic background light (EBL) is one of the fundamental observational quantities in cosmology. All energy releases from resolved and unresolved extragalactic sources, and the light from any truly diffuse background, excluding the cosmic microwave background (CMB), contribute to its intensity and spectral energy distribution. It therefore plays a crucial role in cosmological tests for the formation and evolution of stellar objects and galaxies, and for setting limits on exotic energy releases in the universe. The EBL also plays an important role in the propagation of very high energy gamma-rays which are attenuated en route to Earth by pair producing gamma-gamma interactions with the EBL and CMB. The EBL affects the spectrum of the sources, predominantly blazars, in the ~10 GeV to 10 TeV energy regime. Knowledge of the EBL intensity and spectrum will allow the determination of the intrinsic blazar spectrum in a crucial energy regime that can be used to test particle acceleration mechanisms and VHE gamma-ray production models. Conversely, knowledge of the intrinsic gamma-ray spectrum and the detection of blazars at increasingly higher redshifts will set strong limits on the EBL and its evolution. This paper reviews the latest developments in the determination of the EBL and its impact on the current understanding of the origin and production mechanisms of gamma-rays in blazars, and on energy releases in the universe. The review concludes with a summary and future directions in Cherenkov Telescope Array techniques and in infrared ground-based and space observatories that will greatly improve our knowledge of the EBL and the origin and production of very high energy gamma-rays.

astro-ph.CO↗
arXiv2011

Strong New Constraints on the Extragalactic Background Light in the Near- to Mid-IR

Direct measurements of the extragalactic background light (EBL) in the near-IR to mid-IR waveband are extremely difficult due to an overwhelming foreground from the zodiacal light that outshines the faint cosmological diffuse radiation field by more than an order of magnitude. Indirect constraints on the EBL are provided by gamma-ray observations of AGN. Using the combination of the Fermi Gamma-Ray Space Telescope together with the current generation of ground-based air Cherenkov telescopes (H.E.S.S., MAGIC, and VERITAS) provides unprecedented sensitivity and spectral coverage for constraining the EBL in the near- to mid-IR. In this paper we present new limits on the EBL based on the analysis of the broad-band spectra of a select set of gamma-ray blazars covering 200 MeV to several TeV. The EBL intensity at 15 microns is constrained to be 1.36 +/- 0.58 nW m^-2 sr^-1. We find that the fast evolution and baseline EBL models of Stecker et al. (2006), as well as the model of Kneiske et al. (2004), predict significantly higher EBL intensities in the mid-IR (15 microns) than is allowed by the constraints derived here. In addition, the model of Franceschini et al. (2008) and the fiducial model of Dominguez et al. (2011) predict near- to mid-IR ratios smaller than that predicted by our analysis. Namely, their intensities in the near-IR are too low while their intensities in the mid-IR are marginally too high. All of the aforementioned models are inconsistent with our analysis at the >3 sigma level.

astro-ph.CO↗
arXiv2007

Detecting a unique EBL signature with TeV gamma rays

We discuss prospects for detecting a spectral break in gamma-ray spectra of blazars due to the extragalactic background light (EBL) density falling off between the near and mid-IR. A measurable spectral change in the TeV spectra at 1 TeV could arise from a rapid or slow drop in the EBL density above ~1 micron. This effect is mediated by the ratio of the near to mid-IR density of EBL. A detection of such a spectral feature could become a clear signature of EBL absorption. A non-detection would give a strong observational constraint to the shape of the EBL spectrum. We present calculations estimating the sensitivity of TeV telescopes for detecting such a break for blazar observations at different redshifts.

astro-ph↗
arXiv2005

The Near Infrared Background: Interplanetary Dust or Primordial Stars?

The intensity of the diffuse ~ 1 - 4 micron sky emission from which solar system and Galactic foregrounds have been subtracted is in excess of that expected from energy released by galaxies and stars that formed during the z < 5 redshift interval (Arendt & Dwek 2003, Matsumoto et al. 2005). The spectral signature of this excess near-infrared background light (NIRBL) component is almost identical to that of reflected sunlight from the interplanetary dust cloud, and could therefore be the result of the incomplete subtraction of this foreground emission component from the diffuse sky maps. Alternatively, this emission component could be extragalactic. Its spectral signature is consistent with that of redshifted continuum and recombination line emission from HII regions formed by the first generation of very massive stars. In this paper we analyze the implications of this spectral component for the formation rate of these Population III stars, the redshift interval during which they formed, the reionization of the universe and evolution of collapsed halo masses. We find that to reproduce the intensity and spectral shape of the NIRBL requires a peak star formation rate that is higher by about a factor of 4 to 10 compared to those derived from hierarchical models. Furthermore, an extragalactic origin for the NIRBL leads to physically unrealistic absorption-corrected spectra of distant TeV blazars. All these results suggest that Pop III stars contribute only a fraction of the NIRBL intensity with zodiacal light, star forming galaxies, and/or non-nuclear sources giving rise to the remaining fraction.

astro-ph↗
arXiv2005

Is There an Imprint of Primordial Stars in the Tev Gamma-Ray Spectrum of Blazars?

The 1 - 5 micron diffuse sky emission from which local foreground emission from the solar system and the Galaxy have been subtracted exceeds the brightness that can be attributed to normal star forming galaxies. The nature of this excess near-infrared background light (NIRBL) is still controversial. On one hand, it has been interpreted as a distinct spectral feature created by the redshifted emission from primordial (Population III) stars that have formed at redshifts > 8. On the other hand, the NIRBL spectrum is almost identical to that of the zodiacal cloud, raising the possibility that it is of local origin. Blazars can, in principle, offer a simple test for the nature and origin of the NIRBL. Very high energy gamma-ray photons emitted by these objects are attenuated on route to earth by photon-photon interactions with the extragalactic background light (EBL). This paper examines whether the extragalactic nature of the NIRBL can be determined from the analysis of the TeV spectra of blazars. (Abridged)

astro-ph↗
arXiv2004

Simultaneous Constraints on the Spectrum of the Extragalactic Background Light and the Intrinsic Tev Spectra of Mrk 421, Mrk 501, and H1426+428

Very high energy (~ TeV) $γ$-rays from blazars are attenuated by photons from the extragalactic background light (EBL). Observations of blazars can therefore provide an ideal opportunity for determining the EBL intensity if their intrinsic spectrum is known. Conversely, knowledge of the EBL intensity can be used to determine the intrinsic blazar spectrum. Unfortunately, neither the EBL intensity nor the intrinsic blazar spectrum is known with high enough precision to accurately derive one quantity from the other. In this paper we use the most recent data on the EBL to construct twelve different realizations representing all possible permutations between EBL limits and the detections in the different wavelength regions. We use these realizations to explore the effects of the EBL on the inferred spectra of blazars. In particular, we show that the frequently cited "IR background-TeV gamma-ray crisis" does not exist, and derive the intrinsic spectra and peak energies of the blazars Mrk 421, 501 and H1426+428 for EBL realizations that give rise to physically viable intrinsic blazar spectra. We also show that the intrinsic spectrum of Mrk~421 during a period of intense flaring activity has a peak energy that seems to shift to higher energies at higher flux states. Finally, we also explore the effect of the uncertainties in the absolute calibration of the gamma-ray energies on derived TeV opacities and the intrinsic blazar spectra.

astro-ph↗
arXiv2002

Toward Ultra Short Gamma Ray Burst Ground Based Detection, SGARFACE status

We present the status and motivation of the Short GAmma Ray Front Air Cherenkov Experiment (SGARFACE) which will be operated parallel to standard Very High Energy gamma-ray observations using the Whipple 10m telescope. SGARFACE is sensitive to 100MeV-10GeV gamma-ray bursts with durations ranging from 100ns to 100us providing a fluence sensitivity as low as few $\rm 1E-9 erg/cm^2. Preliminary data taking started in November 2002.

astro-ph↗
arXiv2001

TeV Gamma-Ray Astronomy in the new Millennium

The field of TeV gamma-ray astronomy is reviewed with emphasis on its relation to the origin of cosmic rays. The discovery of TeV photons from supernova remnants and active galaxies has provided the first direct observational link between specific astrophysical objects and particle production at the TeV scale. TeV gamma-ray observations constrain the high end of the electromagnetic spectrum, a regime most sensitive for testing particle acceleration and emission models. TeV telescopes have made important contributions to the understanding of blazars and supernova remnants, however, it will take the next generation atmospheric Cherenkov telescopes and satellite-based gamma-ray detectors to unravel the mystery of hadronic cosmic-ray sources. A short review of TeV observations is followed by a discussion of the capabilities and scientific potential of the next generation ground-based atmospheric Cherenkov telescopes.

astro-ph↗
arXiv1999

New Generation Atmospheric Cherenkov Detectors

High energy gamma-ray astronomy has been established during the last decade through the launch of the Compton Gamma Ray Observatory (CGRO) and the success of its ground-based counterpart, the imaging atmospheric Cherenkov technique. In the aftermath of their important and surprising scientific results a worldwide effort developing and designing new generation atmospheric Cherenkov detectors is underway. These novel instruments will have higher sensitivity at E > 250 GeV, but most importantly, will be able to close the unexplored energy gap between 20 GeV and 250 GeV. Several ground-based detectors are proposed or under construction. Aspects of the techniques used and sensitivity are discussed in this overview paper. The instruments cover largely complementary energy ranges and together are expected to explore the gamma-ray sky between 20 GeV and 100 TeV with unprecedented sensitivity.

astro-ph↗