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G. Hughes

Publications and source records attributed to G. Hughes.

At least 37 records · Page 2Linked to original sources
arXiv2019

A Search for Pulsed Very High-Energy Gamma Rays from Thirteen Young Pulsars in Archival VERITAS Data

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.

astro-ph.HE↗
arXiv2019

Direct measurement of stellar angular diameters by the VERITAS Cherenkov Telescopes

The angular size of a star is a critical factor in determining its basic properties. Direct measurement of stellar angular diameters is difficult: at interstellar distances stars are generally too small to resolve by any individual imaging telescope. This fundamental limitation can be overcome by studying the diffraction pattern in the shadow cast when an asteroid occults a star, but only when the photometric uncertainty is smaller than the noise added by atmospheric scintillation. Atmospheric Cherenkov telescopes used for particle astrophysics observations have not generally been exploited for optical astronomy due to the modest optical quality of the mirror surface. However, their large mirror area makes them well suited for such high-time-resolution precision photometry measurements. Here we report two occultations of stars observed by the VERITAS Cherenkov telescopes with millisecond sampling, from which we are able to provide a direct measurement of the occulted stars' angular diameter at the $\leq0.1$ milliarcsecond scale. This is a resolution never achieved before with optical measurements and represents an order of magnitude improvement over the equivalent lunar occultation method. We compare the resulting stellar radius with empirically derived estimates from temperature and brightness measurements, confirming the latter can be biased for stars with ambiguous stellar classifications.

astro-ph.SR↗
arXiv2018

Teraelectronvolt pulsed emission from the Crab pulsar detected by MAGIC

Aims: To investigate the extension of the very-high-energy spectral tail of the Crab pulsar at energies above 400 GeV. Methods: We analyzed $\sim$320 hours of good quality data of Crab with the MAGIC telescope, obtained from February 2007 until April 2014. Results: We report the most energetic pulsed emission ever detected from the Crab pulsar reaching up to 1.5 TeV. The pulse profile shows two narrow peaks synchronized with the ones measured in the GeV energy range. The spectra of the two peaks follow two different power-law functions from 70 GeV up to 1.5 TeV and connect smoothly with the spectra measured above 10 GeV by the Large Area Telescope (LAT) on board of the Fermi satellite. When making a joint fit of the LAT and MAGIC data, above 10 GeV, the photon indices of the spectra differ by 0.5$\pm$0.1. Conclusions: We measured with the MAGIC telescopes the most energetic pulsed photons from a pulsar to date. Such TeV pulsed photons require a parent population of electrons with a Lorentz factor of at least $5\times 10^6$. These results strongly suggest IC scattering off low energy photons as the emission mechanism and a gamma-ray production region in the vicinity of the light cylinder.

astro-ph.HE↗
arXiv2018

Periastron Observations of TeV Gamma-Ray Emission from a Binary System with a 50-year Period

We report on observations of the pulsar / Be star binary system PSR J2032+4127 / MT91 213 in the energy range between 100 GeV and 20 TeV with the VERITAS and MAGIC imaging atmospheric Cherenkov telescope arrays. The binary orbit has a period of approximately 50 years, with the most recent periastron occurring on 2017 November 13. Our observations span from 18 months prior to periastron to one month after. A new, point-like, gamma-ray source is detected, coincident with the location of PSR J2032+4127 / MT91 213. The gamma-ray light curve and spectrum are well-characterized over the periastron passage. The flux is variable over at least an order of magnitude, peaking at periastron, thus providing a firm association of the TeV source with the pulsar / Be star system. Observations prior to periastron show a cutoff in the spectrum at an energy around 0.5 TeV. This result adds a new member to the small population of known TeV binaries, and it identifies only the second source of this class in which the nature and properties of the compact object are firmly established. We compare the gamma-ray results with the light curve measured with the X-ray Telescope (XRT) on board the Neil Gehrels \textit{Swift} Observatory and with the predictions of recent theoretical models of the system. We conclude that significant revision of the models is required to explain the details of the emission we have observed, and we discuss the relationship between the binary system and the overlapping steady extended source, TeV J2032+4130.

astro-ph.HE↗
arXiv2018

VERITAS and Fermi-LAT observations of new HAWC sources

The HAWC (High Altitude Water Cherenkov) collaboration recently published their 2HWC catalog, listing 39 very high energy (VHE; >100~GeV) gamma-ray sources based on 507 days of observation. Among these, there are nineteen sources that are not associated with previously known TeV sources. We have studied fourteen of these sources without known counterparts with VERITAS and Fermi-LAT. VERITAS detected weak gamma-ray emission in the 1~TeV-30~TeV band in the region of DA 495, a pulsar wind nebula coinciding with 2HWC J1953+294, confirming the discovery of the source by HAWC. We did not find any counterpart for the selected fourteen new HAWC sources from our analysis of Fermi-LAT data for energies higher than 10 GeV. During the search, we detected GeV gamma-ray emission coincident with a known TeV pulsar wind nebula, SNR G54.1+0.3 (VER J1930+188), and a 2HWC source, 2HWC J1930+188. The fluxes for isolated, steady sources in the 2HWC catalog are generally in good agreement with those measured by imaging atmospheric Cherenkov telescopes. However, the VERITAS fluxes for SNR G54.1+0.3, DA 495, and TeV J2032+4130 are lower than those measured by HAWC and several new HAWC sources are not detected by VERITAS. This is likely due to a change in spectral shape, source extension, or the influence of diffuse emission in the source region.

astro-ph.HE↗
arXiv2018

Measurement of Cosmic-ray Electrons at TeV Energies by VERITAS

Cosmic-ray electrons and positrons (CREs) at GeV-TeV energies are a unique probe of our local Galactic neighborhood. CREs lose energy rapidly via synchrotron radiation and inverse-Compton scattering processes while propagating within the Galaxy and these losses limit their propagation distance. For electrons with TeV energies, the limit is on the order of a kiloparsec. Within that distance there are only a few known astrophysical objects capable of accelerating electrons to such high energies. It is also possible that the CREs are the products of the annihilation or decay of heavy dark matter (DM) particles. VERITAS, an array of imaging air Cherenkov telescopes in southern Arizona, USA, is primarily utilized for gamma-ray astronomy, but also simultaneously collects CREs during all observations. We describe our methods of identifying CREs in VERITAS data and present an energy spectrum, extending from 300 GeV to 5 TeV, obtained from approximately 300 hours of observations. A single power-law fit is ruled out in VERITAS data. We find that the spectrum of CREs is consistent with a broken power law, with a break energy at 710 $\pm$ 40$_{stat}$ $\pm$ 140$_{syst}$ GeV.

astro-ph.HE↗
arXiv2018

Measurement of the Iron Spectrum in Cosmic Rays by VERITAS

We present a new measurement of the energy spectrum of iron nuclei in cosmic rays from 20 to 500 TeV. The measurement makes use of a template-based analysis method, which, for the first time, is applied to the energy reconstruction of iron-induced air showers recorded by the VERITAS array of imaging atmospheric Cherenkov telescopes. The event selection makes use of the direct Cherenkov light which is emitted by charged particles before the first interaction, as well as other parameters related to the shape of the recorded air shower images. The measured spectrum is well described by a power law $\frac{\mathrm{d} F}{\mathrm{d} E}=f_0\cdot \left(\frac{E}{E_0}\right)^{-γ}$ over the full energy range, with $γ= 2.82 \pm 0.30 \mathrm{(stat.)} ^{+0.24}_{-0.27} \mathrm{(syst.)}$ and $f_0 = \left( 4.82 \pm 0.98 \mathrm{(stat.)}^{+2.12}_{-2.70} \mathrm{(syst.)} \right)\cdot 10^{-7}$m$^{-2}$s$^{-1}$sr$^{-1}$TeV$^{-1}$ at $E_0=50$TeV, with no indication of a cutoff or spectral break. The measured differential flux is compatible with previous results, with improved statistical uncertainty at the highest energies.

astro-ph.HE↗
arXiv2018

VERITAS observations of the BL Lac object TXS 0506+056

On 2017 September 22, the IceCube Neutrino Observatory reported the detection of the high-energy neutrino event \icnu, of potential astrophysical origin. It was soon determined that the neutrino direction was consistent with the location of the gamma-ray blazar \txs~(3FGL J0509.4+0541), which was in an elevated gamma-ray emission state as measured by the \emph{Fermi} satellite. VERITAS observations of the neutrino/blazar region started on 2017 September 23 in response to the neutrino alert and continued through 2018 February 6. While no significant very-high-energy (VHE; E $>$ 100 GeV) emission was observed from the blazar by VERITAS in the two-week period immediately following the IceCube alert, TXS 0506+056 was detected by VERITAS with a significance of 5.8 standard deviations ($σ$) in the full 35-hour data set. The average photon flux of the source during this period was $(8.9 \pm 1.6) \times 10^{-12} \; \mathrm{cm}^{-2} \, \mathrm{s}^{-1}$, or 1.6\% of the Crab Nebula flux, above an energy threshold of 110 GeV, with a soft spectral index of $4.8 \pm 1.3$.

astro-ph.HE↗
arXiv2018

HESS J1943+213: An Extreme Blazar Shining Through The Galactic Plane

HESS J1943+213 is a very-high-energy (VHE; $>$100 GeV) $γ$-ray source in the direction of the Galactic Plane. Studies exploring the classification of the source are converging towards its identification as an extreme synchrotron BL Lac object. Here we present 38 hours of VERITAS observations of HESS J1943+213 taken over two years. The source is detected with $\sim$20 standard deviations significance, showing a remarkably stable flux and spectrum in VHE $γ$-rays. Multi-frequency very-long-baseline array (VLBA) observations of the source confirm the extended, jet-like structure previously found in the 1.6 GHz band with European VLBI Network and detect this component in the 4.6 GHz and the 7.3 GHz bands. The radio spectral indices of the core and the jet and the level of polarization derived from the VLBA observations are in a range typical for blazars. Data from VERITAS, $Fermi$-LAT, $Swift$-XRT, FLWO 48$''$ telescope, and archival infrared and hard X-ray observations are used to construct and model the spectral energy distribution (SED) of the source with a synchrotron-self-Compton model. The well-measured $γ$-ray peak of the SED with VERITAS and $Fermi$-LAT provides constraining upper limits on the source redshift. Possible contribution of secondary $γ$-rays from ultra-high-energy cosmic ray-initiated electromagnetic cascades to the $γ$-ray emission is explored, finding that only a segment of the VHE spectrum can be accommodated with this process. A variability search is performed across X-ray and $γ$-ray bands. No statistically significant flux or spectral variability is detected.

astro-ph.HE↗
arXiv2018

A Very High Energy $γ$-Ray Survey towards the Cygnus Region of the Galaxy

We present results from deep observations towards the Cygnus region using 300 hours of very-high-energy (VHE) $γ$-ray data taken with the VERITAS Cherenkov telescope array and over seven years of high-energy $γ$-ray data taken with the Fermi satellite at an energy above 1 GeV. As the brightest region of diffuse $γ$-ray emission in the northern sky, the Cygnus region provides a promising area to probe the origins of cosmic rays. We report the identification of a potential Fermi-LAT counterpart to VER J2031+415 (TeV J2032+4130), and resolve the extended VHE source VER J2019+368 into two source candidates (VER J2018+367* and VER J2020+368*) and characterize their energy spectra. The Fermi-LAT morphology of 3FGL 2021.0+4031e (the Gamma-Cygni supernova remnant) was examined and a region of enhanced emission coincident with VER J2019+407 was identified and jointly fit with the VERITAS data. By modeling 3FGL J2015.6+3709 as two sources, one located at the location of the pulsar wind nebula CTB 87 and one at the quasar QSO J2015+371, a continuous spectrum from 1 GeV to 10 TeV was extracted for VER J2016+371 (CTB 87). An additional 71 locations coincident with Fermi-LAT sources and other potential objects of interest were tested for VHE $γ$-ray emission, with no emission detected and upper limits on the differential flux placed at an average of 2.3% of the Crab Nebula ux. We interpret these observations in a multiwavelength context and present the most detailed $γ$-ray view of the region to date.

astro-ph.HE↗
arXiv2018

A strong limit on the very-high-energy emission from GRB 150323A

On 2015 March 23, VERITAS responded to a $Swift$-BAT detection of a gamma-ray burst, with observations beginning 270 seconds after the onset of BAT emission, and only 135 seconds after the main BAT emission peak. No statistically significant signal is detected above 140 GeV. The VERITAS upper limit on the fluence in a 40 minute integration corresponds to about 1% of the prompt fluence. Our limit is particularly significant since the very-high-energy (VHE) observation started only $\sim$2 minutes after the prompt emission peaked, and $Fermi$-LAT observations of numerous other bursts have revealed that the high-energy emission is typically delayed relative to the prompt radiation and lasts significantly longer. Also, the proximity of GRB~150323A ($z=0.593$) limits the attenuation by the extragalactic background light to $\sim 50$ % at 100-200 GeV. We conclude that GRB 150323A had an intrinsically very weak high-energy afterglow, or that the GeV spectrum had a turnover below $\sim100$ GeV. If the GRB exploded into the stellar wind of a massive progenitor, the VHE non-detection constrains the wind density parameter to be $A\gtrsim 3\times 10^{11}$ g cm$^{-1}$, consistent with a standard Wolf-Rayet progenitor. Alternatively, the VHE emission from the blast wave would be weak in a very tenuous medium such as the ISM, which therefore cannot be ruled out as the environment of GRB 150323A.

astro-ph.HE↗
arXiv2018

Multiwavelength observations of the blazar BL Lacertae: a new fast TeV gamma-ray flare

Combined with very-long-baseline interferometry measurements, the observations of fast TeV gamma-ray flares probe the structure and emission mechanism of blazar jets. However, only a handful of such flares have been detected to date, and only within the last few years have these flares been observed from lower-frequency-peaked BL~Lac objects and flat-spectrum radio quasars. We report on a fast TeV gamma-ray flare from the blazar BL~Lacertae observed by VERITAS, with a rise time of $\sim$2.3~hr and a decay time of $\sim$36~min. The peak flux above 200 GeV is $(4.2 \pm 0.6) \times 10^{-6} \;\text{photon} \;\text{m}^{-2}\; \text{s}^{-1}$ measured with a 4-minute-binned light curve, corresponding to $\sim$180\% of the flux which is observed from the Crab Nebula above the same energy threshold. Variability contemporaneous with the TeV gamma-ray flare was observed in GeV gamma-ray, X-ray, and optical flux, as well as in optical and radio polarization. Additionally, a possible moving emission feature with superluminal apparent velocity was identified in VLBA observations at 43 GHz, potentially passing the radio core of the jet around the time of the gamma-ray flare. We discuss the constraints on the size, Lorentz factor, and location of the emitting region of the flare, and the interpretations with several theoretical models which invoke relativistic plasma passing stationary shocks.

astro-ph.HE↗
arXiv2018

Very-high-energy gamma-rays from the Universe's middle age: detection of the z=0.940 blazar PKS 1441+25 with MAGIC

The flat-spectrum radio quasar PKS 1441+25 at a redshift of z = 0.940 is detected between 40 and 250 GeV with a significance of 25.5 σ using the MAGIC telescopes. Together with the gravitationally lensed blazar QSO B0218+357 (z = 0.944), PKS 1441+25 is the most distant very high energy (VHE) blazar detected to date. The observations were triggered by an outburst in 2015 April seen at GeV energies with the Large Area Telescope on board Fermi. Multi-wavelength observations suggest a subdivision of the high state into two distinct flux states. In the band covered by MAGIC, the variability time scale is estimated to be 6.4 +/- 1.9 days. Modeling the broadband spectral energy distribution with an external Compton model, the location of the emitting region is understood as originating in the jet outside the broad line region (BLR) during the period of high activity, while being partially within the BLR during the period of low (typical) activity. The observed VHE spectrum during the highest activity is used to probe the extragalactic background light at an unprecedented distance scale for ground-based gamma-ray astronomy.

astro-ph.GA↗
arXiv2017

Very-High-Energy $γ$-Ray Observations of the Blazar 1ES 2344+514 with VERITAS

We present very-high-energy $γ$-ray observations of the BL Lac object 1ES 2344+514 taken by the Very Energetic Radiation Imaging Telescope Array System (VERITAS) between 2007 and 2015. 1ES 2344+514 is detected with a statistical significance above background of $20.8σ$ in $47.2$ hours (livetime) of observations, making this the most comprehensive very-high-energy study of 1ES 2344+514 to date. Using these observations the temporal properties of 1ES 2344+514 are studied on short and long times scales. We fit a constant flux model to nightly- and seasonally-binned light curves and apply a fractional variability test, to determine the stability of the source on different timescales. We reject the constant-flux model for the 2007-2008 and 2014-2015 nightly-binned light curves and for the long-term seasonally-binned light curve at the $> 3σ$ level. The spectra of the time-averaged emission before and after correction for attenuation by the extragalactic background light are obtained. The observed time-averaged spectrum above 200 GeV is satisfactorily fitted (${χ^2/NDF = 7.89/6}$) by a power-law function with index $Γ= 2.46 \pm 0.06_{stat} \pm 0.20_{sys} $ and extends to at least 8 TeV. The extragalactic-background-light-deabsorbed spectrum is adequately fit (${χ^2/NDF = 6.73/6}$) by a power-law function with index $Γ= 2.15 \pm 0.06_{stat} \pm 0.20_{sys} $ while an F-test indicates that the power-law with exponential cutoff function provides a marginally-better fit ($χ^2/NDF $ = $2.56 / 5 $) at the 2.1$σ$ level. The source location is found to be consistent with the published radio location and its spatial extent is consistent with a point source.

astro-ph.HE↗
arXiv2017

Performance of the MAGIC telescopes under moonlight

MAGIC, a system of two imaging atmospheric Cherenkov telescopes, achieves its best performance under dark conditions, i.e. in absence of moonlight or twilight. Since operating the telescopes only during dark time would severely limit the duty cycle, observations are also performed when the Moon is present in the sky. Here we develop a dedicated Moon-adapted analysis to characterize the performance of MAGIC under moonlight. We evaluate energy threshold, angular resolution and sensitivity of MAGIC under different background light levels, based on Crab Nebula observations and tuned Monte Carlo simulations. This study includes observations taken under non-standard hardware configurations, such as reducing the camera photomultiplier tubes gain by a factor ~1.7 (Reduced HV settings) with respect to standard settings (Nominal HV) or using UV-pass filters to strongly reduce the amount of moonlight reaching the cameras of the telescopes. The Crab Nebula spectrum is correctly reconstructed in all the studied illumination levels, that reach up to 30 times brighter than under dark conditions. The main effect of moonlight is an increase in the analysis energy threshold and in the systematic uncertainties on the flux normalization. The sensitivity degradation is constrained to be below 10%, within 15-30% and between 60 and 80% for Nominal HV, Reduced HV and UV-pass filter observations, respectively. No worsening of the angular resolution was found. Thanks to observations during moonlight, the maximal duty cycle of MAGIC can be increased from ~18%, under dark nights only, to up to ~40% in total with only moderate performance degradation.

astro-ph.IM↗
arXiv2017

Constraints on particle acceleration in SS433/W50 from MAGIC and H.E.S.S. observations

The large jet kinetic power and non-thermal processes occurring in the microquasar SS 433 make this source a good candidate for a very high-energy (VHE) gamma-ray emitter. Gamma-ray fluxes have been predicted for both the central binary and the interaction regions between jets and surrounding nebula. Also, non-thermal emission at lower energies has been previously reported. We explore the capability of SS 433 to emit VHE gamma rays during periods in which the expected flux attenuation due to periodic eclipses and precession of the circumstellar disk periodically covering the central binary system is expected to be at its minimum. The eastern and western SS433/W50 interaction regions are also examined. We aim to constrain some theoretical models previously developed for this system. We made use of dedicated observations from MAGIC and H.E.S.S. from 2006 to 2011 which were combined for the first time and accounted for a total effective observation time of 16.5 h. Gamma-ray attenuation does not affect the jet/medium interaction regions. The analysis of a larger data set amounting to 40-80 h, depending on the region, was employed. No evidence of VHE gamma-ray emission was found. Upper limits were computed for the combined data set. We place constraints on the particle acceleration fraction at the inner jet regions and on the physics of the jet/medium interactions. Our findings suggest that the fraction of the jet kinetic power transferred to relativistic protons must be relatively small to explain the lack of TeV and neutrino emission from the central system. At the SS433/W50 interface, the presence of magnetic fields greater 10$μ$G is derived assuming a synchrotron origin for the observed X-ray emission. This also implies the presence of high-energy electrons with energies up to 50 TeV, preventing an efficient production of gamma-ray fluxes in these interaction regions.

astro-ph.HE↗
arXiv2017

MAGIC observations of the microquasar V404 Cygni during the 2015 outburst

The microquasar V404 Cygni underwent a series of outbursts in 2015, June 15-31, during which its flux in hard X-rays (20-40 keV) reached about 40 times the Crab Nebula flux. Because of the exceptional interest of the flaring activity from this source, observations at several wavelengths were conducted. The MAGIC telescopes, triggered by the INTEGRAL alerts, followed-up the flaring source for several nights during the period June 18-27, for more than 10 hours. One hour of observation was conducted simultaneously to a giant 22 GHz radio flare and a hint of signal at GeV energies seen by Fermi-LAT. The MAGIC observations did not show significant emission in any of the analysed time intervals. The derived flux upper limit, in the energy range 200--1250 GeV, is 4.8$\times 10^{-12}$ ph cm$^{-2}$ s$^{-1}$. We estimate the gamma-ray opacity during the flaring period, which along with our non-detection, points to an inefficient acceleration in the V404\,Cyg jets if VHE emitter is located further than $1\times 10^{10}$ cm from the compact object.

astro-ph.HE↗