We present results from deep observations toward the Cygnus region using 300 hr of very high energy (VHE) γ-ray data taken with the VERITAS Cerenkov telescope array and over 7 yr 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 J2021.0+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 flux. We interpret these observations in a multiwavelength context and present the most detailed γ-ray view of the region to date.
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, 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±40stat±140syst GeV.
ABSTRACT
We present a new Australian Square Kilometre Array Pathfinder (ASKAP) sample of 14 radio supernova remnants (SNR) candidates in the Large Magellanic Cloud (LMC). This new sample is a ...significant increase to the known number of older, larger, and low surface brightness LMC SNRs. We employ a multifrequency search for each object and found possible traces of optical and occasionally X-ray emission in several of these 14 SNR candidates. One of these 14 SNR candidates (MCSNR J0522 – 6543) has multifrequency properties that strongly indicate a bona fide SNR. We also investigate a sample of 20 previously suggested LMC SNR candidates and confirm the SNR nature of MCSNR J0506 – 6815. We detect lower surface brightness SNR candidates which were likely formed by a combination of shock waves and strong stellar winds from massive progenitors (and possibly surrounding OB stars). Some of our new SNR candidates are also found in lower density environments in which SNe type Ia explode inside a previously excavated interstellar medium.
ABSTRACT
We present new and archival Australia Telescope Compact Array and Atacama Large Millimeter/submillimeter Array data of the Small Magellanic Cloud supernova remnant 1E 0102.2–7219 at 2100, ...5500, 9000, and 108 000 MHz; as well as H i data provided by the Australian Square Kilometre Array Pathfinder. The remnant shows a ring-like morphology with a mean radius of 6.2 pc. The 5500 MHz image reveals a bridge-like structure, seen for the first time in a radio image. This structure is also visible in both optical and X-ray images. In the 9000 MHz image, we detect a central feature that has a flux density of 4.3 mJy but rule out a pulsar wind nebula origin, due to the lack of significant polarization towards the central feature with an upper limit of 4 per cent. The mean fractional polarization for 1E 0102.2–7219 is 7 ± 1 and 12 ± 2 per cent for 5500 and 9000 MHz, respectively. The spectral index for the entire remnant is −0.61 ± 0.01. We estimate the line-of-sight magnetic field strength in the direction of 1E 0102.2–7219 of ∼44 µG with an equipartition field of $65\pm 5\, \mu$G. This latter model uses the minimum energy of the sum of the magnetic field and cosmic ray electrons only. We detect an H i cloud towards this remnant at the velocity range of ∼160–180 km s−1 and a cavity-like structure at the velocity of 163.7–167.6 km s−1. We do not detect CO emission towards 1E 0102.2–7219.
We conduct a search for periodic emission in the very high-energy (VHE) gamma-ray band (E > 100 GeV) from a total of 13 pulsars in an archival VERITAS data set with a total exposure of over 450 hr. ...The set of pulsars includes many of the brightest young gamma-ray pulsars visible in the Northern Hemisphere. The data analysis resulted in nondetections 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.
HESS J0632+057 is a gamma-ray binary composed of a compact object orbiting a Be star with a period of about 315 days. Extensive X-ray and TeV gamma-ray observations have revealed a peculiar light ...curve containing two peaks, separated by a dip. We present the results of simultaneous observations in hard X-rays with NuSTAR and in TeV gamma-rays with VERITAS, performed in 2017 November and December. These observations correspond to the orbital phases φ 0.22 and 0.3, where the fluxes are rising toward the first light-curve peak. A significant variation of the spectral index from 1.77 0.05 to 1.56 0.05 is observed in the X-ray data. The multiwavelength spectral energy distributions (SED) derived from the observations are interpreted in terms of a leptonic model, in which the compact object is assumed to be a pulsar and nonthermal radiation is emitted by high-energy electrons accelerated at the shock formed by the collision between the stellar and pulsar wind. The results of the SED fitting show that our data can be consistently described within this scenario, and allow us to estimate the magnetization of the pulsar wind at the location of the shock formation. The constraints on the pulsar wind magnetization provided by our results are shown to be consistent with those obtained from other systems.
We present a new measurement of the energy spectrum of iron nuclei in cosmic rays from 20 TeV 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 dFdE=f0·(EE0)−γ over the full energy range, with γ=2.82±0.30(stat)−0.27+0.24(syst) and f0=(4.82±0.98(stat)−2.70+2.12(syst))×10−7 m−2 s−1 TeV−1 sr−1 at E0=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.
Recurrent novae are repeating thermonuclear explosions in the outer layers of white dwarfs, due to the accretion of fresh material from a binary companion. The shock generated when ejected material ...slams into the companion star's wind can accelerate particles. We report very-high-energy (VHE; Formula: see text) gamma rays from the recurrent nova RS Ophiuchi, up to 1 month after its 2021 outburst, observed using the High Energy Stereoscopic System (H.E.S.S.). The temporal profile of VHE emission is similar to that of lower-energy giga-electron volt emission, indicating a common origin, with a 2-day delay in peak flux. These observations constrain models of time-dependent particle energization, favoring a hadronic emission scenario over the leptonic alternative. Shocks in dense winds provide favorable environments for efficient acceleration of cosmic rays to very high energies.
Context.
In recent years, winds from massive stars have been considered promising sites for investigating relativistic particle acceleration. In particular, the resulting bow-shaped shocks from the ...interaction of the supersonic winds of runaway stars with interstellar matter have been intensively observed at many different wavelengths, from radio to
γ
-rays.
Aims.
In this study we investigate the O4If star, BD+43° 3654, the bow shock of which is, so far, the only one proven to radiate both thermal and non-thermal emission at radio frequencies. In addition, we consider NGC 7635, the Bubble Nebula, as a bow shock candidate and examine its apex for indications of thermal and non-thermal radio emission.
Methods.
We observed both bow shocks in radio frequencies with the Very Large Array (VLA) in the C and X bands (4–8 GHz and 8–12 GHz) and with the Effelsberg telescope at 4–8 GHz. We analysed single-dish and interferometric results individually, in addition to their combined emission, obtained spectral index maps for each source, and calculated their spectral energy distributions.
Results.
We find that both sources emit non-thermal emission in the radio regime, with the clearest evidence for NGC 7635, whose radio emission has a strongly negative spectral index along the northern rim of the bubble. We present the first high-resolution maps of radio emission from NGC 7635, finding that the morphology closely follows the optical nebular emission. Our results are less conclusive for the bow shock of BD+43° 3654, as its emission becomes weaker and faint at higher frequencies in VLA data. Effelsberg data show a much larger emitting region (albeit a region of thermal emission) than is detected with the VLA for this source.
Conclusions.
Our results extend the previous radio results from the BD+43° 3654 bow shock to higher frequencies, and with our NGC 7635 results we double the number of bow shocks around O stars with detected non-thermal emission, from one to two. Modelling of the multi-wavelength data for both sources shows that accelerated electrons at the wind termination shock are a plausible source for the non-thermal radio emission, but energetics arguments suggest that any non-thermal X-ray and
γ
-ray emission could be significantly below existing upper limits. Enhanced synchrotron emission from compressed galactic cosmic rays in the radiative bow shock could also explain the radio emission from the BD+43° 3654 bow shock, but not from NGC 7635. The non-detection of point-like radio emission from BD+43° 3654 puts an upper limit on the mass-loss rate of the star that is lower than values quoted in the literature.
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