arXiv subjects

J. Carr

Publications and source records attributed to J. Carr.

At least 91 records · Page 5Linked to original sources

arXiv2016

The supernova remnant W49B as seen with H.E.S.S. and Fermi-LAT

The supernova remnant (SNR) W49B originated from a core-collapse supernova that occurred between one and four thousand years ago, and subsequently evolved into a mixed-morphology remnant, which is interacting with molecular clouds (MC). $γ$-ray observations of SNR/MC associations are a powerful tool to constrain the origin of Galactic cosmic-rays, as they can probe the acceleration of hadrons through their interaction with the surrounding medium and subsequent emission of non-thermal photons. The detection of a $γ$-ray source coincident with W49B at very high energies (VHE; E > 100 GeV) with the H.E.S.S. Cherenkov telescopes is reported together with a study of the source with 5 years of Fermi-LAT high energy $γ$-ray (0.06 - 300 GeV) data. The smoothly-connected combined source spectrum, measured from 60 MeV to multi-TeV energies, shows two significant spectral breaks at $304\pm20$ MeV and $8.4_{-2.5}^{+2.2}$ GeV, the latter being constrained by the joint fit from the two instruments. The detected spectral features are similar to those observed in several other SNR/MC associations and are found to be indicative of $γ$-ray emission produced through neutral-pion decay.

astro-ph.HE↗

arXiv2016

Search for dark matter annihilations towards the inner Galactic halo from 10 years of observations with H.E.S.S

The inner region of the Milky Way halo harbors a large amount of dark matter (DM). Given its proximity, it is one of the most promising targets to look for DM. We report on a search for the annihilations of DM particles using $γ$-ray observations towards the inner 300 parsecs of the Milky Way, with the H.E.S.S. array of ground-based Cherenkov telescopes. The analysis is based on a 2D maximum likelihood method using Galactic center (GC) data accumulated by H.E.S.S. over the last 10 years (2004-2014), and does not show any significant $γ$-ray signal above background. Assuming Einasto and Navarro-Frenk-White DM density profiles at the GC, we derive upper limits on the annihilation cross section $\langle σv\rangle$. These constraints are the strongest obtained so far in the TeV DM mass range and improve upon previous limits by a factor 5. For the Einasto profile, the constraints reach $\langle σv\rangle$ values of $\rm 6\times10^{-26} cm^3s^{-1}$ in the $W^+W^-$ channel for a DM particle mass of 1.5 TeV, and $\rm 2\times10^{-26} cm^3s^{-1}$ in the $τ^+τ^-$ channel for 1 TeV mass. For the first time, ground-based $γ$-ray observations have reached sufficient sensitivity to probe $\langle σv\rangle$ values expected from the thermal relic density for TeV DM particles.

astro-ph.HE↗

arXiv2016

A search for very high-energy flares from the microquasars GRS 1915+105, Circinus X-1, and V4641 Sgr using contemporaneous H.E.S.S. and RXTE observations

Microquasars are potential $γ$-ray emitters. Indications of transient episodes of $γ$-ray emission were recently reported in at least two systems: Cyg X-1 and Cyg X-3. The identification of additional $γ$-ray-emitting microquasars is required to better understand how $γ$-ray emission can be produced in these systems. Theoretical models have predicted very high-energy (VHE) $γ$-ray emission from microquasars during periods of transient outburst. Observations reported herein were undertaken with the objective of observing a broadband flaring event in the $γ$-ray and X-ray bands. Contemporaneous observations of three microquasars, GRS 1915+105, Circinus X-1, and V4641 Sgr, were obtained using the High Energy Spectroscopic System (H.E.S.S.) telescope array and the Rossi X-ray Timing Explorer (RXTE) satellite. X-ray analyses for each microquasar were performed and VHE $γ$-ray upper limits from contemporaneous H.E.S.S. observations were derived. No significant $γ$-ray signal has been detected in any of the three systems. The integral $γ$-ray photon flux at the observational epochs is constrained to be $I(> 560 {\rm\ GeV}) < 7.3\times10^{-13}$ cm$^{-2}$ s$^{-1}$, $I(> 560 {\rm\ GeV}) < 1.2\times10^{-12}$ cm$^{-2}$ s$^{-1}$, and $I(> 240 {\rm\ GeV}) < 4.5\times10^{-12}$ cm$^{-2}$ s$^{-1}$ for GRS 1915+105, Circinus X-1, and V4641 Sgr, respectively. The $γ$-ray upper limits obtained using H.E.S.S. are examined in the context of previous Cherenkov telescope observations of microquasars. The effect of intrinsic absorption is modelled for each target and found to have negligible impact on the flux of escaping $γ$-rays. When combined with the X-ray behaviour observed using RXTE, the derived results indicate that if detectable VHE $γ$-ray emission from microquasars is commonplace, then it is likely to be highly transient.

astro-ph.HE↗

arXiv2016

Extended VHE gamma-ray emission towards SGR1806-20, LBV1806-20, and stellar cluster Cl*1806-20

Using the High Energy Spectroscopic System (H.E.S.S.) telescopes we have discovered a steady and extended very high-energy (VHE) gamma-ray source towards the luminous blue variable candidate LBV1806-20, massive stellar cluster Cl*1806-20, and magnetar SGR1806-20. The new VHE source, HESSJ1808-204, was detected at a statistical significance of >6sigma (post-trial) with a photon flux normalisation $(2.9 \pm 0.4_{\rm stat} \pm 0.5_{\rm sys})\times 10^{-13}$\,ph\,cm$^{-2}$\,s$^{-1}$\,TeV$^{-1}$ at 1TeV and a power-law photon index of $2.3\pm0.2_{\rm stat}\pm 0.3_{\rm sys}$. The luminosity of this source (0.2 to 10 TeV; scaled to distance d=8.7kpc) is $L_{\rm VHE}\sim1.6 \times 10^{34}(d/{8.7kpc})^2$\,erg\,s$^{-1}$. The VHE gamma-ray emission is extended and is well fit by a single Gaussian with statistical standard deviation of $0.095^\circ \pm 0.015^\circ$. This extension is similar to that of the synchrotron radio nebula G10.0-0.3, which is thought to be powered by LBV1806-20. The VHE gamma-ray luminosity could be provided by the stellar wind luminosity of LBV1806-20 by itself and/or the massive star members of Cl*1806-20. Alternatively, magnetic dissipation (e.g. via reconnection) from SGR1806-20 can potentially account for the VHE luminosity. The origin and hadronic and/or leptonic nature of the accelerated particles responsible for HESSJ1808-204 is not yet clear. If associated with SGR1806-20, the potentially young age of the magnetar (650yr) can be used to infer the transport limits of these particles to match the VHE source size. This discovery provides new interest in the potential for high-energy particle acceleration from magnetars, massive stars, and/or stellar clusters.

astro-ph.HE↗

arXiv2016

Limits on Dark Matter Annihilation in the Sun using the ANTARES Neutrino Telescope

A search for muon neutrinos originating from dark matter annihilations in the Sun is performed using the data recorded by the ANTARES neutrino telescope from 2007 to 2012. In order to obtain the best possible sensitivities to dark matter signals, an optimisation of the event selection criteria is performed taking into account the background of atmospheric muons, atmospheric neutrinos and the energy spectra of the expected neutrino signals. No significant excess over the background is observed and $90\%$ C.L. upper limits on the neutrino flux, the spin--dependent and spin--independent WIMP-nucleon cross--sections are derived for WIMP masses ranging from $ \rm 50$ GeV to $\rm 5$ TeV for the annihilation channels $\rm WIMP + WIMP \to b \bar b, W^+ W^-$ and $\rm τ^+ τ^-$.

astro-ph.HE↗

arXiv2016

A search for Secluded Dark Matter in the Sun with the ANTARES neutrino telescope

A search for Secluded Dark Matter annihilation in the Sun using 2007-2012 data of the ANTARES neutrino telescope is presented. Three different cases are considered: a) detection of dimuons that result from the decay of the mediator, or neutrino detection from: b) mediator that decays into a dimuon and, in turn, into neutrinos, and c) mediator that decays directly into neutrinos. As no significant excess over background is observed, constraints are derived on the dark matter mass and the lifetime of the mediator.

hep-ex↗

arXiv2016

High-energy Neutrino follow-up search of Gravitational Wave Event GW150914 with ANTARES and IceCube

We present the high-energy-neutrino follow-up observations of the first gravitational wave transient GW150914 observed by the Advanced LIGO detectors on Sept. 14th, 2015. We search for coincident neutrino candidates within the data recorded by the IceCube and ANTARES neutrino detectors. A possible joint detection could be used in targeted electromagnetic follow-up observations, given the significantly better angular resolution of neutrino events compared to gravitational waves. We find no neutrino candidates in both temporal and spatial coincidence with the gravitational wave event. Within 500 s of the gravitational wave event, the number of neutrino candidates detected by IceCube and ANTARES were three and zero, respectively. This is consistent with the expected atmospheric background, and none of the neutrino candidates were directionally coincident with GW150914. We use this non-detection to constrain neutrino emission from the gravitational-wave event.

astro-ph.HE↗

arXiv2016

Acceleration of petaelectronvolt protons in the Galactic Centre

Galactic cosmic rays reach energies of at least a few Peta-electronvolts (1 PeV =$10^\mathbf{15}$ electron volts). This implies our Galaxy contains PeV accelerators (PeVatrons), but all proposed models of Galactic cosmic-ray accelerators encounter non-trivial difficulties at exactly these energies. Tens of Galactic accelerators capable of accelerating particle to tens of TeV (1 TeV =$10^\mathbf{12}$ electron volts) energies were inferred from recent gamma-ray observations. None of the currently known accelerators, however, not even the handful of shell-type supernova remnants commonly believed to supply most Galactic cosmic rays, have shown the characteristic tracers of PeV particles: power-law spectra of gamma rays extending without a cutoff or a spectral break to tens of TeV. Here we report deep gamma-ray observations with arcminute angular resolution of the Galactic Centre regions, which show the expected tracer of the presence of PeV particles within the central 10~parsec of the Galaxy. We argue that the supermassive black hole Sagittarius A* is linked to this PeVatron. Sagittarius A* went through active phases in the past, as demonstrated by X-ray outbursts and an outflow from the Galactic Centre. Although its current rate of particle acceleration is not sufficient to provide a substantial contribution to Galactic cosmic rays, Sagittarius A* could have plausibly been more active over the last $\gtrsim 10^{6-7}$ years, and therefore should be considered as a viable alternative to supernova remnants as a source of PeV Galactic cosmic rays.

astro-ph.HE↗

arXiv2016

Murchison Widefield Array Limits on Radio Emission from ANTARES Neutrino Events

We present a search, using the Murchison Widefield Array (MWA), for electromagnetic counterparts to two candidate high energy neutrino events detected by the ANTARES neutrino telescope in 2013 November and 2014 March. These events were selected by ANTARES because they are consistent, within 0.4 degrees, with the locations of galaxies within 20 Mpc of Earth. Using MWA archival data at frequencies between 118 and 182 MHz, taken ~20 days prior to, at the same time as, and up to a year after the neutrino triggers, we look for transient or strongly variable radio sources consistent with the neutrino positions. No such counterparts are detected, and we set a 5 sigma upper limit for low-frequency radio emission of ~1E37 erg/s for progenitors at 20 Mpc. If the neutrino sources are instead not in nearby galaxies, but originate in binary neutron star coalescences, our limits place the progenitors at z > 0.2. While it is possible, due to the high background from atmospheric neutrinos, that neither event is astrophysical, the MWA observations are nevertheless among the first to follow up neutrino candidates in the radio, and illustrate the promise of wide-field instruments like MWA to detect electromagnetic counterparts to such events.

astro-ph.HE↗

arXiv2016

Constraints on the neutrino emission from the Galactic Ridge with the ANTARES telescope

Compelling evidence for the existence of astrophysical neutrinos has been reported by the IceCube collaboration. Some features of the energy and declination distributions of IceCube events hint at a North/South asymmetry of the neutrino flux. This could be due to the presence of the bulk of our Galaxy in the Southern hemisphere. The ANTARES neutrino telescope, located in the Mediterranean Sea, has been taking data since 2007. It offers the best sensitivity to muon neutrinos produced by galactic cosmic ray interactions in this region of the sky. In this letter a search for an extended neutrino flux from the Galactic Ridge region is presented. Different models of neutrino production by cosmic ray propagation are tested. No excess of events is observed and upper limits for different neutrino flux spectral indices are set. This constrains the number of IceCube events possibly originating from the Galactic Ridge. A simple power-law extrapolation of the Fermi-LAT flux to associated IceCube High Energy Starting Events is excluded at 90% confidence level.

astro-ph.HE↗

arXiv2016

Detailed spectral and morphological analysis of the shell type SNR RCW 86

Aims: We aim for an understanding of the morphological and spectral properties of the supernova remnant RCW~86 and for insights into the production mechanism leading to the RCW~86 very high-energy gamma-ray emission. Methods: We analyzed High Energy Spectroscopic System data that had increased sensitivity compared to the observations presented in the RCW~86 H.E.S.S. discovery publication. Studies of the morphological correlation between the 0.5-1~keV X-ray band, the 2-5~keV X-ray band, radio, and gamma-ray emissions have been performed as well as broadband modeling of the spectral energy distribution with two different emission models. Results:We present the first conclusive evidence that the TeV gamma-ray emission region is shell-like based on our morphological studies. The comparison with 2-5~keV X-ray data reveals a correlation with the 0.4-50~TeV gamma-ray emission.The spectrum of RCW~86 is best described by a power law with an exponential cutoff at $E_{cut}=(3.5\pm 1.2_{stat})$ TeV and a spectral index of $Γ$~$1.6\pm 0.2$. A static leptonic one-zone model adequately describes the measured spectral energy distribution of RCW~86, with the resultant total kinetic energy of the electrons above 1 GeV being equivalent to $\sim$0.1\% of the initial kinetic energy of a Type I a supernova explosion. When using a hadronic model, a magnetic field of $B$~100$μ$G is needed to represent the measured data. Although this is comparable to formerly published estimates, a standard E$^{-2}$ spectrum for the proton distribution cannot describe the gamma-ray data. Instead, a spectral index of $Γ_p$~1.7 would be required, which implies that ~$7\times 10^{49}/n_{cm^{-3}}$erg has been transferred into high-energy protons with the effective density $n_{cm^{-3}}=n/ 1$ cm^-3. This is about 10\% of the kinetic energy of a typical Type Ia supernova under the assumption of a density of 1~cm^-3.

astro-ph.HE↗

arXiv2015

First combined search for neutrino point-sources in the Southern Hemisphere with the ANTARES and IceCube neutrino telescopes

We present the results of searches for point-like sources of neutrinos based on the first combined analysis of data from both the ANTARES and IceCube neutrino telescopes. The combination of both detectors which differ in size and location forms a window in the Southern sky where the sensitivity to point sources improves by up to a factor of two compared to individual analyses. Using data recorded by ANTARES from 2007 to 2012, and by IceCube from 2008 to 2011, we search for sources of neutrino emission both across the Southern sky and from a pre-selected list of candidate objects. No significant excess over background has been found in these searches, and flux upper limits for the candidate sources are presented for $E^{-2.5}$ and $E^{-2}$ power-law spectra with different energy cut-offs.

hep-ex↗

arXiv2015

Prospects for Indirect Dark Matter Searches with the Cherenkov Telescope Array (CTA)

The Cherenkov Telescope Array (CTA) will have a unique chance of discovery for a large range of masses in Weakly Interacting Massive Particles models of dark matter. The principal target for dark matter searches with CTA is the centre of the Galactic Halo. The best strategy is to perform CTA observations within a few degrees of the Galactic Centre, with the Galactic Centre itself and the most intense diffuse emission regions removed from the analysis. Assuming a cuspy dark matter density profile for the Milky Way, 500 hours of observations in this region provide sensitivities to and below the thermal cross-section of dark matter annihilations, for masses between a few hundred GeV and a few tens of TeV; therefore CTA will have a significant chance of discovery in some models. Since the dark matter density in the Milky Way is far from certain in the inner kpc region, other targets are also proposed for observation, like ultra-faint dwarf galaxies such as Segue 1 with 100 hours per year proposed. Beyond these two observational targets, further alternatives, such as Galactic dark clumps, will be considered closer to the actual date of CTA operations. Sensitivity predictions for dark matter searches are given on the various targets taking into account the latest instrument response functions expected for CTA together with a discussion on the systematic uncertainties from the backgrounds.

astro-ph.HE↗

arXiv2015

Optical and X-ray early follow-up of ANTARES neutrino alerts

High-energy neutrinos could be produced in the interaction of charged cosmic rays with matter or radiation surrounding astrophysical sources. Even with the recent detection of extraterrestrial high-energy neutrinos by the IceCube experiment, no astrophysical neutrino source has yet been discovered. Transient sources, such as gamma-ray bursts, core-collapse supernovae, or active galactic nuclei are promising candidates. Multi-messenger programs offer a unique opportunity to detect these transient sources. By combining the information provided by the ANTARES neutrino telescope with information coming from other observatories, the probability of detecting a source is enhanced, allowing the possibility of identifying a neutrino progenitor from a single detected event. A method based on optical and X-ray follow-ups of high-energy neutrino alerts has been developed within the ANTARES collaboration. This program, denoted as TAToO, triggers a network of robotic optical telescopes (TAROT and ROTSE) and the Swift-XRT with a delay of only a few seconds after a neutrino detection, and is therefore well-suited to search for fast transient sources. To identify an optical or X-ray counterpart to a neutrino signal, the images provided by the follow-up observations are analysed with dedicated pipelines. A total of 42 alerts with optical and 7 alerts with X-ray images taken with a maximum delay of 24 hours after the neutrino trigger have been analysed. No optical or X-ray counterparts associated to the neutrino triggers have been found, and upper limits on transient source magnitudes have been derived. The probability to reject the gamma-ray burst origin hypothesis has been computed for each alert.

astro-ph.HE↗

arXiv2015

Time calibration with atmospheric muon tracks in the ANTARES neutrino telescope

The ANTARES experiment consists of an array of photomultipliers distributed along 12 lines and located deep underwater in the Mediterranean Sea. It searches for astrophysical neutrinos collecting the Cherenkov light induced by the charged particles, mainly muons, produced in neutrino interactions around the detector. Since at energies of $\sim$10 TeV the muon and the incident neutrino are almost collinear, it is possible to use the ANTARES detector as a neutrino telescope and identify a source of neutrinos in the sky starting from a precise reconstruction of the muon trajectory. To get this result, the arrival times of the Cherenkov photons must be accurately measured. A to perform time calibrations with the precision required to have optimal performances of the instrument is described. The reconstructed tracks of the atmospheric muons in the ANTARES detector are used to determine the relative time offsets between photomultipliers. Currently, this method is used to obtain the time calibration constants for photomultipliers on different lines at a precision level of 0.5 ns. It has also been validated for calibrating photomultipliers on the same line, using a system of LEDs and laser light devices.

physics.ins-det↗

arXiv2015

Search for muon-neutrino emission from GeV and TeV gamma-ray flaring blazars using five years of data of the ANTARES telescope

The ANTARES telescope is well-suited for detecting astrophysical transient neutrino sources as it can observe a full hemisphere of the sky at all times with a high duty cycle. The background due to atmospheric particles can be drastically reduced, and the point-source sensitivity improved, by selecting a narrow time window around possible neutrino production periods. Blazars, being radio-loud active galactic nuclei with their jets pointing almost directly towards the observer, are particularly attractive potential neutrino point sources, since they are among the most likely sources of the very high-energy cosmic rays. Neutrinos and gamma rays may be produced in hadronic interactions with the surrounding medium. Moreover, blazars generally show high time variability in their light curves at different wavelengths and on various time scales. This paper presents a time-dependent analysis applied to a selection of flaring gamma-ray blazars observed by the FERMI/LAT experiment and by TeV Cherenkov telescopes using five years of ANTARES data taken from 2008 to 2012. The results are compatible with fluctuations of the background. Upper limits on the neutrino fluence have been produced and compared to the measured gamma-ray spectral energy distribution.

astro-ph.HE↗

arXiv2015

Search of Dark Matter Annihilation in the Galactic Centre using the ANTARES Neutrino Telescope

A search for high-energy neutrinos coming from the direction of the Galactic Centre is performed using the data recorded by the ANTARES neutrino telescope from 2007 to 2012. The event selection criteria are chosen to maximise the sensitivity to possible signals produced by the self-annihilation of weakly interacting massive particles accumulated around the centre of the Milky Way with respect to the atmospheric background. After data unblinding, the number of neutrinos observed in the line of sight of the Galactic Centre is found to be compatible with background expectations. The 90% C.L. upper limits in terms of the neutrino+anti-neutrino flux, $\rm Φ_{ν_μ+\barν_μ}$, and the velocity averaged annihilation cross-section, $\rm <σ_{A}v>$, are derived for the WIMP self-annihilation channels into $\rm b\bar{b},W^{+}W^{-},τ^{+}τ^{-},μ^{+}μ^{-},ν\barν$. The ANTARES limits for $\rm <σ_{A}v>$ are shown to be the most stringent for a neutrino telescope over the WIMP masses $\rm 25\,GeV < M_{WIMP} < 10\,TeV$.

astro-ph.HE↗

arXiv2015

ANTARES Constrains a Blazar Origin of Two IceCube PeV Neutrino Events

The source(s) of the neutrino excess reported by the IceCube Collaboration is unknown. The TANAMI Collaboration recently reported on the multiwavelength emission of six bright, variable blazars which are positionally coincident with two of the most energetic IceCube events. Objects like these are prime candidates to be the source of the highest-energy cosmic rays, and thus of associated neutrino emission. We present an analysis of neutrino emission from the six blazars using observations with the ANTARES neutrino telescope.The standard methods of the ANTARES candidate list search are applied to six years of data to search for an excess of muons --- and hence their neutrino progenitors --- from the directions of the six blazars described by the TANAMI Collaboration, and which are possibly associated with two IceCube events. Monte Carlo simulations of the detector response to both signal and background particle fluxes are used to estimate the sensitivity of this analysis for different possible source neutrino spectra. A maximum-likelihood approach, using the reconstructed energies and arrival directions of through-going muons, is used to identify events with properties consistent with a blazar origin.Both blazars predicted to be the most neutrino-bright in the TANAMI sample (1653$-$329 and 1714$-$336) have a signal flux fitted by the likelihood analysis corresponding to approximately one event. This observation is consistent with the blazar-origin hypothesis of the IceCube event IC14 for a broad range of blazar spectra, although an atmospheric origin cannot be excluded. No ANTARES events are observed from any of the other four blazars, including the three associated with IceCube event IC20. This excludes at a 90\% confidence level the possibility that this event was produced by these blazars unless the neutrino spectrum is flatter than $-2.4$.

astro-ph.HE↗