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Register a new userA Search for Cosmic Neutrino and Gamma-Ray Emitting Transients in 7.3 Years of ANTARES and Fermi LAT Data
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We analyze 7.3 years of ANTARES high-energy neutrino and Fermi LAT {gamma}-ray data in search of cosmic neutrino + {gamma}-ray ({ u}+{gamma}) transient sources or source populations. Our analysis has the potential to detect either individual { u}+{gamma} transient sources (durations {delta}t < 1000~s), if they exhibit sufficient {gamma}-ray or neutrino multiplicity, or a statistical excess of { u}+{gamma} transients of lower multiplicities. Treating ANTARES track and cascade event types separately, we establish detection thresholds by Monte Carlo scrambling of the neutrino data, and determine our analysis sensitivity by signal injection against scrambled datasets. We find our analysis is sensitive to { u}+{gamma} transient populations responsible for $>$5% of the observed gamma-coincident neutrinos in the track data at 90% confidence. Applying our analysis to the unscrambled data reveals no individual { u}+{gamma} events of high significance; two ANTARES track + Fermi {gamma}-ray events are identified that exceed a once per decade false alarm rate threshold ($p=17%$). No evidence for subthreshold { u}+{gamma} source populations is found among the track ($p=39%$) or cascade ($p=60%$) events. While TXS 0506+056, a blazar and variable (non-transient) Fermi {gamma}-ray source, has recently been identified as the first source of high-energy neutrinos, the challenges in reconciling observations of the Fermi {gamma}-ray sky, the IceCube high-energy cosmic neutrinos, and ultra-high energy cosmic rays using only blazars suggest a significant contribution by other source populations. Searches for transient sources of high-energy neutrinos remain interesting, with the potential for neutrino clustering or multimessenger coincidence searches to lead to discovery of the first { u}+{gamma} transients.
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We present results of an archival coincidence analysis between Fermi LAT gamma-ray data and public neutrino data from the IceCube neutrino observatorys 40-string (IC40) and 59-string (IC59) observing runs. Our analysis has the potential to detect either a statistical excess of neutrino + gamma-ray ($ u$+$gamma$) emitting transients or, alternatively, individual high gamma-multiplicity events, as might be produced by a neutrino observed by IceCube coinciding with a LAT-detected gamma-ray burst. Dividing the neutrino data into three datasets by hemisphere (IC40, IC59-North, and IC59-South), we construct uncorrelated null distributions by Monte Carlo scrambling of the neutrino datasets. We carry out signal-injection studies against these null distributions, demonstrating sensitivity to individual $ u$+$gamma$ events of sufficient gamma-ray multiplicity, and to $ u$+$gamma$ transient populations responsible for $>$14% (IC40), $>$9% (IC59-North), or $>$8% (IC59-South) of the gamma-coincident neutrinos observed in these datasets, respectively. Analyzing the unscrambled neutrino data, we identify no individual high-significance neutrino + high gamma-multiplicity events, and no significant deviations from the test statistic null distributions. However, we observe a similar and unexpected pattern in the IC59-North and IC59-South residual distributions that we conclude reflects a possible correlation ($p=7.0%$) between IC59 neutrino positions and persistently bright portions of the Fermi gamma-ray sky. This possible correlation should be readily testable using eight years of further data already collected by IceCube. We are currently working with Astrophysical Multimessenger Observatory Network (AMON) partner facilities to generate low-latency $ u$+$gamma$ alerts from Fermi LAT gamma-ray, IceCube and ANTARES neutrino data and distribute these in real time to AMON follow-up partners.
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.
The Large Area Telescope (LAT) on Fermi has detected ~150 gamma-ray pulsars, about a third of which were discovered in blind searches of the $gamma$-ray data. Because the angular resolution of the LAT is relatively poor and blind searches for pulsars (especially millisecond pulsars, MSPs) are very sensitive to an error in the position, one must typically scan large numbers of locations. Identifying plausible X-ray counterparts of a putative pulsar drastically reduces the number of trials, thus improving the sensitivity of pulsar blind searches with the LAT. I discuss our ongoing program of Swift, XMM-Newton, and Chandra observations of LAT unassociated sources in the context of our blind searches for gamma-ray pulsars.
We present a novel interpretation of the gamma-ray diffuse emission measured by H.E.S.S. in the Galactic Center (GC) region and the Galactic ridge. Our starting base is an updated analysis of PASS8 Fermi-LAT data, which allows to extend down to few GeV the spectra measured by H.E.S.S. and to infer the primary CR radial distribution above 100 GeV. We compare those results with a CR transport model assuming a harder scaling of the diffusion coefficient with rigidity in the inner Galaxy. Such a behavior reproduces the radial dependence of the CR spectral index recently inferred from Fermi-LAT measurements in the inner GP. We find that, in this scenario, the bulk of the Galactic ridge emission can be naturally explained by the interaction of the diffuse, steady-state Galactic CR sea interacting with the gas present in the Central molecular zone. The evidence of a GC PeVatron is significantly weaker than that inferred adopting a conventional (softer) CR sea.
Fast Radio Bursts (FRBs) are a mysterious flash phenomenon detected in radio wavelengths with a duration of only a few milliseconds, and they may also have prompt gamma-ray flashes. Here we carry out a blind search for msec-duration gamma-ray flashes using the 7-year Fermi Large Area Telescope (Fermi-LAT) all-sky gamma-ray data. About 100 flash candidates are detected, but after removing those associated with bright steady point sources, we find no flash events at high Galactic latitude region (|b|>20 deg). Events at lower latitude regions are consistent with statistical flukes originating from the diffuse gamma-ray background. From these results, we place an upper limit on the GeV gamma-ray to radio flux ratio of FRBs as xi equiv (nu L_nu)_gamma / (nu L_nu)_radio < 10^8, depending on the assumed FRB rate evolution. This limit is comparable with the largest value found for pulsars, though xi of pulsars is distributed in a wide range. We also compare this limit with the spectral energy distribution of the 2004 giant flare of the magnetar SGR 1806-20.
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