arXiv subjects

Anthony M. Brown

Publications and source records attributed to Anthony M. Brown.

At least 37 records · Page 2Linked to original sources

arXiv2019

Trinity: An Air-Shower Imaging System for the Detection of Ultrahigh Energy Neutrinos

Efforts to detect ultrahigh energy neutrinos are driven by several objectives: What is the origin of astrophysical neutrinos detected with IceCube? What are the sources of ultrahigh energy cosmic rays? Do the ANITA detected events point to new physics? Shedding light on these questions requires instruments that can detect neutrinos above $10^7$ GeV with sufficient sensitivity - a daunting task. While most ultrahigh energy neutrino experiments are based on the detection of a radio signature from shower particles following a neutrino interaction, we believe that the detection of Cherenkov and fluorescence light from shower particles is an attractive alternative. Imaging air showers with Cherenkov and fluorescence light is a technique that is successfully used in several ultrahigh energy cosmic ray and very-high energy gamma-ray experiments. We performed a case study of an air-shower imaging system for the detection of earth-skimming tau neutrinos. The detector configuration we consider consists of an imaging system that is located on top of a mountain and is pointed at the horizon. From the results of this study we conclude that a sensitivity of $3\cdot10^{-9}$ GeV cm$^{-2}$s$^{-1}$sr$^{-1}$ can be achieved at $2\cdot10^8$ GeV with a relatively small and modular system after three years of observation. In this presentation we discuss key findings of our study and how they translate into design requirements for an imaging system we dub Trinity.

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arXiv2019

The glow of annihilating dark matter in Omega Centauri

Dark matter (DM) is the most abundant material in the Universe, but has so far been detected only via its gravitational effects. Several theories suggest that pairs of DM particles can annihilate into a flash of light at gamma-ray wavelengths. While gamma-ray emission has been observed from environments where DM is expected to accumulate, such as the centre of our Galaxy, other high energy sources can create a contaminating astrophysical gamma-ray background, thus making DM detection difficult. In principle, dwarf galaxies around the Milky Way are a better place to look -- they contain a greater fraction of DM with no astrophysical gamma-ray background -- but they are too distant for gamma-rays to have been seen. A range of observational evidence suggests that Omega Centauri (omega Cen or NGC 5139), usually classified as the Milky Way's largest globular cluster, is really the core of a captured and stripped dwarf galaxy. Importantly, Omega Cen is ten times closer to us than known dwarfs. Here we show that not only does Omega Cen contain DM with density as high as compact dwarf galaxies, but also that it emits gamma-rays with an energy spectrum matching that expected from the annihilation of DM particles with mass 31$\pm$4 GeV (68\% confidence limit). No astrophysical sources have been found that would otherwise explain Omega Cen's gamma-ray emission, despite deep multi-wavelength searches. We anticipate our results to be the starting point for even deeper radio observations of Omega Cen. If multi-wavelength searches continue to find no astrophysical explanations, this pristine, nearby clump of DM will become the best place to study DM interactions through forces other than gravity.

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arXiv2019

Trinity: An Air-Shower Imaging Instrument to detect Ultrahigh Energy Neutrinos

Trinity is a proposed air-shower imaging system optimized for the detection of earth-skimming ultrahigh energy tau neutrinos with energies between $10^7$ GeV and $10^{10}$ GeV. Trinity will pursue three major scientific objectives. 1) It will narrow in on possible source classes responsible for the astrophysical neutrino flux measured by IceCube. 2) It will help find the sources of ultrahigh-energy cosmic rays (UHECR) and understand the composition of UHECR. 3) It will test fundamental neutrino physics at the highest energies. Trinity uses the imaging technique, which is well established and successfully used by the very high-energy gamma-ray community (CTA, H.E.S.S., MAGIC, and VERITAS) and the UHECR community (Telescope Array, Pierre Auger)

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arXiv2019

An open source toolkit for the tracking, termination and recovery of high altitude balloon flights and payloads

We present an open source toolkit of flight-proven electronic devices which can be used to track, terminate and recover high altitude balloon flights and payloads. Comprising a beacon, pyrotechnic and non-pyrotechnic cut-down devices plus associated software, the toolkit can be used to: (i) track the location of a flight via Iridium satellite communication; (ii) release lift and/or float balloons manually or at pre-defined altitudes; (iii) locate the payload after descent. The size and mass of the toolkit make it suitable for use on weather or sounding balloon flights. We describe the technology readiness level of the toolkit, based on over 20 successful flights to altitudes of typically 32,000 m.

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arXiv2019

Gamma-rays from SS433: evidence for periodicity

In this paper we present our study of the gamma-ray emission from the microquasar SS433. Integrating over 9 years of Fermi-LAT \textsc{pass8} data, we detect SS433 with a significance of ~13$σ$ in the 200 to 500 MeV photon energy range, with evidence for an extension in the direction of the w1 X-ray `hotspot`. A temporal analysis reveals evidence for modulation of SS433's gamma-ray emission with the precession period of its relativistic jet. This suggests that at least some of SS433's gamma-ray emission originates close to the object rather than from the jet termination regions.

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arXiv2018

Gamma-ray emission from high Galactic latitude globular clusters

We analyse 8 years of PASS 8 Fermi-LAT data, in the 60 MeV - 300 GeV energy range, from 30 high Galactic latitude globular clusters. Six of these globular clusters are detected with a TS > 25, with NGC 6254 being detected as gamma-ray bright for the first time. The most significant detection is of the well-known globular cluster 47 Tuc, and we produce a refined spectral fit for this object with a log parabola model. NGC 6093, NGC 6752 and NGC 6254 are fitted with hard, flat power law models, NGC 7078 is best fitted with a soft power law and NGC 6218 is best fitted with a hard, broken power law. This variety of spectral models suggests that there is a variety of gamma-ray source types within globular clusters, in addition to the traditional millisecond pulsar interpretation. We identify a correspondence between diffuse X-ray emission in globular cluster cores and gamma-ray emission. This connection suggests that gamma-ray emission in globular clusters could also arise from unresolved X-ray sources or a relativistic electron population, perhaps generated by the millisecond pulsars. X-ray observations of further gamma-ray bright globular clusters would allow a functional relationship to be determined between diffuse X-ray and gamma-ray emission.

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arXiv2018

Overview, design, and flight results from SuperBIT: a high-resolution, wide-field, visible-to-near-UV balloon-borne astronomical telescope

Balloon-borne astronomy is a unique tool that allows for a level of image stability and significantly reduced atmospheric interference without the often prohibitive cost and long development time-scale that are characteristic of space-borne facility-class instruments. The Super-pressure Balloon-borne Imaging Telescope (SuperBIT) is a wide-field imager designed to provide 0.02" image stability over a 0.5 degree field-of-view for deep exposures within the visible-to-near-UV (300-900 um). As such, SuperBIT is a suitable platform for a wide range of balloon-borne observations, including solar and extrasolar planetary spectroscopy as well as resolved stellar populations and distant galaxies. We report on the overall payload design and instrumentation methodologies for SuperBIT as well as telescope and image stability results from two test flights. Prospects for the SuperBIT project are outlined with an emphasis on the development of a fully operational, three-month science flight from New Zealand in 2020.

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arXiv2018

Understanding the $γ$-ray emission from the globular cluster 47 Tuc: evidence for dark matter?

47 Tuc was the first globular cluster observed to be $γ$-ray bright, with the $γ$-rays being attributed to a population of unresolved millisecond pulsars (MSPs). Recent kinematic data, combined with detailed simulations, appears to be consistent with the presence of an intermediate mass black hole (IMBH) at the centre of 47 Tuc. Building upon this, we analyse 9 years of \textit{Fermi}-LAT observations to study the spectral properties of 47 Tuc with unprecedented accuracy and sensitivity. This 9-year $γ$-ray spectrum shows that 47 Tuc's $γ$-ray flux cannot be explained by MSPs alone, due to a systematic discrepancy between the predicted and observed flux. Rather, we find a significant preference (TS $=40$) for describing 47 Tuc's spectrum with a two source population model, consisting of an ensemble of MSPs and annihilating dark matter (DM) with an enhanced density around the IMBH, when compared to an MSP-only explanation. The best-fit DM mass of 34 GeV is essentially the same as the best-fit DM explanation for the Galactic centre "excess" when assuming DM annihilation into $b\bar{b}$ quarks. Our work constitutes the first possible evidence of dark matter within a globular cluster.

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arXiv2017

Discovery of a new extragalactic source population of energetic particles

We report the discovery of a statistically significant hardening in the Fermi-LAT $γ$-ray spectrum of Centaurus A's core, with the spectral index hardening from $Γ_{1}=2.73 \pm 0.02$ to $Γ_{1}=2.29 \pm 0.07$ at a break energy of ($2.6 \pm 0.3$) GeV. Using a likelihood analysis, we find no evidence for flux variability in Cen A's core lightcurve above or below the spectral break when considering the entire 8 year period. Interestingly, however, the first $\sim3.5$ years of the low energy lightcurve shows evidence of flux variability at the $\sim3.5 σ$ confidence level. To understand the origin of this spectral break, we assume that the low energy component below the break feature originates from leptons in Centaurus A's radio jet and we investigate the possibility that the high energy component above the spectral break is due to an additional source of very high energy particles near the core of Cen A. We show for the first time that the observed $γ$-ray spectrum of an Active Galactic Nucleus is compatible with either a very large localized enhancement (referred to as a spike) in the dark matter halo profile or a population of millisecond pulsars. Our work constitutes the first robust indication that new $γ$-ray production mechanisms can explain the emission from active galaxies and could provide tantalizing first evidence for the clustering of heavy dark matter particles around black holes.

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arXiv2017

On the prospects of cross-calibrating the Cherenkov Telescope Array with an airborne calibration platform

Recent advances in unmanned aerial vehicle (UAV) technology have made UAVs an attractive possibility as an airborne calibration platform for astronomical facilities. This is especially true for arrays of telescopes spread over a large area such as the Cherenkov Telescope Array (CTA). In this paper, the feasibility of using UAVs to calibrate CTA is investigated. Assuming a UAV at 1 km altitude above CTA, operating on astronomically clear nights with stratified, low atmospheric dust content, appropriate thermal protection for the calibration light source and an onboard photodiode to monitor its absolute light intensity, inter-calibration of CTA's telescopes of the same size class is found to be achievable with a 6-8 % uncertainty. For cross-calibration of different telescope size classes, a systematic uncertainty of 8-10 % is found to be achievable. Importantly, equipping the UAV with a multi-wavelength calibration light source affords us the ability to monitor the wavelength-dependent degradation of CTA telescopes' optical system, allowing us to not only maintain this 6-10 % uncertainty after the first few years of telescope deployment, but also to accurately account for the effect of multi-wavelength degradation on the cross-calibration of CTA by other techniques, namely with images of air showers and local muons. A UAV-based system thus provides CTA with several independent and complementary methods of cross-calibrating the optical throughput of individual telescopes. Furthermore, housing environmental sensors on the UAV system allows us to not only minimise the systematic uncertainty associated with the atmospheric transmission of the calibration signal, it also allows us to map the dust content above CTA as well as monitor the temperature, humidity and pressure profiles of the first kilometre of atmosphere above CTA with each UAV flight.

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arXiv2017

Fermi-LAT high $z$ AGN and the Extragalactic Background Light

Observations of distant gamma-ray sources are hindered by the presence of the extragalactic background light (EBL). In order to understand the physical processes that result in the observed spectrum of sources, it is imperative that a good understanding of the EBL is included. In this work, an investigation into the imprint of the EBL on the observed spectra of high redshift Fermi-LAT AGN is presented. By fitting the spectrum below $\sim$10 GeV, an estimation of the un-absorbed intrinsic source spectrum is obtained; by applying this spectrum to data up to 300 GeV, it is then possible to derive a scaling factor for different EBL models. A second approach uses 5 sources (PKS 0426-380, 4C +55.17, Ton 116, PG 1246+586 and RBS 1432) which were found to exhibit very high energy emission ($E_γ>100$ GeV). Through Monte Carlo simulations it is shown that the observation of VHE photons, despite the large distances of these objects, is consistent with current EBL models. Many of these sources would be observable with the upcoming ground based observatory the Cherenkov Telescope Array (CTA), leading to a better understanding of the EBL.

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arXiv2016

Gamma-ray Novae: Rare or Nearby?

Classical Novae were revealed as a surprise source of gamma-rays in Fermi LAT observations. During the first 8 years since the LAT was launched, 6 novae in total have been detected to > 5 sigma in gamma-rays, in contrast to the 69 discovered optically in the same period. We attempt to resolve this discrepancy by assuming all novae are gamma-ray emitters, and assigning peak one-day fluxes based on a flat distribution of the known emitters to a simulated population. To determine optical parameters, the spatial distribution and magnitudes of bulge and disc novae in M31 are scaled to the Milky Way, which we approximate as a disc with a 20 kpc radius and elliptical bulge with semi major axis 3 kpc and axis ratios 2:1 in the xy plane. We approximate Galactic reddening using a double exponential disc with vertical and radial scale heights of r_d = 5 kpc and z_d = 0.2 kpc, and demonstrate that even such a rudimentary model can easily reproduce the observed fraction of gamma-ray novae, implying that these apparently rare sources are in fact nearby and not intrinsically rare. We conclude that classical novae with m_R < 12 and within ~8 kpc are likely to be discovered in gamma-rays using the Fermi LAT.

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arXiv2016

The GCT camera for the Cherenkov Telescope Array

The Gamma-ray Cherenkov Telescope (GCT) is proposed for the Small-Sized Telescope component of the Cherenkov Telescope Array (CTA). GCT's dual-mirror Schwarzschild-Couder (SC) optical system allows the use of a compact camera with small form-factor photosensors. The GCT camera is ~0.4 m in diameter and has 2048 pixels; each pixel has a ~0.2 degree angular size, resulting in a wide field-of-view. The design of the GCT camera is high performance at low cost, with the camera housing 32 front-end electronics modules providing full waveform information for all of the camera's 2048 pixels. The first GCT camera prototype, CHEC-M, was commissioned during 2015, culminating in the first Cherenkov images recorded by a SC telescope and the first light of a CTA prototype. In this contribution we give a detailed description of the GCT camera and present preliminary results from CHEC-M's commissioning.

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arXiv2016

Localising the gamma-ray emission region during the June 2014 outburst of 3C 454.3

In May - July 2014, the flat spectrum radio quasar 3C 454.3 exhibited strong flaring behaviour. Observations with the Large Area Telescope detector on-board the Fermi Gamma-ray Space Telescope captured the $γ$-ray flux at energies 0.1 $\leq E_γ\leq$ 300 GeV increasing fivefold during this period, with two distinct peaks in emission. The $γ$-ray emission is analysed in detail, in order to study the emission characteristics and put constraints on the location of the emission region. We explore variability in the spectral shape of 3C 454.3, search for evidence of a spectral cutoff, quantify the significance of VHE emission and investigate whether or not an energy-dependence of the emitting electron cooling exists. $γ$-ray intrinsic doubling timescales as small as $τ_{int} = 0.68$ $\pm$ 0.01 h at a significance of > 5$σ$ are found, providing evidence of a compact emission region. Significant $E_{γ, emitted}\geq$ 35 GeV and $E_{γ, emitted}\geq$ 50 GeV emission is also observed. The location of the emission region can be constrained to $r\geq1.3$ $\times$ $R_{BLR}^{out}$, a location outside the broad-line region. The spectral variation of 3C 454.3 also suggests that these flares may be originating further downstream of the supermassive black hole than the emission before and after the flares.

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arXiv2015

Flasher and muon-based calibration of the GCT telescopes proposed for the Cherenkov Telescope Array

The GCT is a dual-mirror Small-Sized-Telescope prototype proposed for the Cherenkov Telescope Array. Calibration of the GCT's camera is primarily achieved with LED-based flasher units capable of producing $\sim4$ ns FWHM pulses of 400 nm light across a large dynamic range, from 0.1 up to 1000 photoelectrons. The flasher units are housed in the four corners of the camera's focal plane and illuminate it via reflection from the secondary mirror. These flasher units are adaptable to allow several calibration scenarios to be accomplished: camera flat-fielding, linearity measurements (up to and past saturation), and gain estimates from both single pe measurements and from the photon statistics at various high illumination levels. In these proceedings, the performance of the GCT flashers is described, together with ongoing simulation work to quantify the efficiency of using muon rings as an end-to-end calibration for the optical throughput of the GCT.

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arXiv2015

The Detection of Fermi AGN above 100 GeV using Clustering Analysis

The density-based clustering algorithm DBSCAN has been applied to the Fermi Large Area Telescope (LAT) dataset of $ E_γ \geqslant 100$ GeV events with $\lvert b\rvert>10^{\circ}$, in order to search for new very high energy (VHE) $γ$-ray sources. The clustering analysis returned 49 clusters, of which 21 correspond to already known VHE-emitting active galactic nuclei (AGN) within the TeVCat catalogue and a further 11 were found to be significant in a full Fermi analysis. Of these, 2 are previously detected Fermi VHE AGN, and 9 represent new VHE sources consisting of 6 BL Lac objects, one blazar of unknown type and 2 unassociated sources. Comparing these, along with the VHE AGN RBS 0679 and RBS 0970 previously detected with Fermi-LAT, to the current populations of AGN detected with ground-based instruments and Fermi suggests that the VHE-emitting AGN discovered in this study are very similar to the TeVCat AGN and therefore further observations with ground-based imaging atmospheric Cherenkov telescopes are recommended.

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arXiv2015

$γ$-ray observations of extraterrestrial neutrino track event positions

In this paper we report the results of a $γ$-ray study of IceCube's extraterrestrial neutrino candidates detected as track-like events. Using 70 months of Fermi-LAT observations, a likelihood analysis of all $1-300$ GeV photons within 5 degrees of the track-like neutrino candidates' origin was undertaken, to search for spatially coincident $γ$-ray emission. One of IceCube's HESE track events was found to be spatially coincident with a $γ$-ray bright active galactic nucleus (AGN), PKS 0723-008. We find however, that the chance probability for Fermi-LAT detected AGN to be spatially coincident with a single HESE track-like event is high ($\sim37$\%). We therefore find no evidence of $γ$-ray emission associated with the detection of IceCube's HESE track-like neutrino candidates. Upper limits were calculated in the energy range of $1-300$ GeV, assuming a point source origin for the neutrino events considered. The implications for the non-detection of $γ$-ray emission from the source of the HESE track-like events are briefly discussed. The large time period analysed in our study did however, reveal two new $γ$-ray point sources. With a flux of ($1.28 \pm 0.08$) $\times 10^{-9}$ photons cm$^{-2}$ s$^{-1}$, in the $1-300$ GeV energy range, and an associated TS value of 220.6, one of these new point sources is positionally coincident with the AGN PKS 1346-112. The other point source has a $1-300$ GeV flux of ($7.95 \pm 1.23$) $\times 10^{-10}$ photons cm$^{-2}$ s$^{-1}$ and an associated TS value of 92.4. This new point source is spatially coincident with the radio source NVSS J072534+021645 suggesting that it too is an AGN.

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arXiv2014

Very high energy $γ$-ray emission from RBS 0679

In this paper we report the \textit{Fermi} Large Area Telescope (LAT) detection of Very High Energy (VHE; $E_γ>100$ GeV) $γ$-ray emission from the BL Lac object RBS 0679. $5.3$ years of LAT observations revealed the presence of three VHE photon events within 0.1 degrees of RBS 0679, with a subsequent unbinned likelihood analysis finding RBS 0679 to be a source of VHE photons at $6.9$ standard deviations ($σ$). An unbinned likelihood analysis of the $0.1-100$ GeV data, binned in 28-day periods, finds both the flux and spectral index to be variable, with a `softer-when-brighter' trend in the global $γ$-ray characteristics. On the other hand, the 28-day periods in which the VHE photons were detected have spectral indices that are consistent with the $5.3$ year average suggesting that the observed VHE emission is not associated with a spectral hardening event. The discovery of RBS 0679 in the $100-300$ GeV energy range, combined with the non-detection above 390 GeV with the H.E.S.S. telescope array, suggest RBS 0679 to be an intriguing source that requires further follow-up observations with ground-based $γ$-ray observatories.

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