39 research outputs found
Multi-messenger observations of a binary neutron star merger
On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta
Erratum: Milagro observations of multi-TeV emission from galactic sources in the fermi bright source list (Astrophysical Journal (2009) 700 (L127))
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Constraints on the emission model of the "naked-eye burst" GRB080319B
On 2008 March 19, one of the brightest gamma-ray bursts (GRBs) ever recorded was detected by several ground- and space-based instruments spanning the electromagnetic spectrum from radio to gamma rays. With a peak visual magnitude of 5.3, GRB080319B was dubbed the "naked-eye" GRB, as an observer under dark skies could have seen the burst without the aid of an instrument. Presented here are results from observations of the prompt phase of GRB080319B taken with the Milagro TeV observatory. The burst was observed at an elevation angle of 47°. Analysis of the data is performed using both the standard air shower method and the scaler or single-particle technique, which results in a sensitive energy range that extends from ∼5GeV to >20TeV. These observations provide the only direct constraints on the properties of the high-energy gamma-ray emission from GRB080319B at these energies. No evidence for emission is found in the Milagro data, and upper limits on the gamma-ray flux above 10GeV are derived. The limits on emission between ∼25 and 200GeV are incompatible with the synchrotron self-Compton model of gamma-ray production and disfavor a corresponding range (2 eV-16 eV) of assumed synchrotron peak energies. This indicates that the optical photons and soft (∼650 keV) gamma rays may not be produced by the same electron population. © 2012. The American Astronomical Society. All rights reserved..link_to_subscribed_fulltex
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The Norman Transcript
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The study of TeV variability and the duty cycle of Mrk 421 from 3 Yr of observations with the Milagro observatory
TeV-flaring activity with timescales as short as tens of minutes and an orphan TeV flare have been observed from the blazar Markarian 421 (Mrk 421). The TeV emission from Mrk 421 is believed to be produced by leptonic synchrotron self-Compton (SSC) emission. In this scenario, correlations between the X-ray and the TeV fluxes are expected, TeV orphan flares are hardly explained, and the activity (measured as duty cycle) of the source at TeV energies is expected to be equal to or less than that observed in X-rays if only SSC is considered. To estimate the TeV duty cycle of Mrk 421 and to establish limits on its variability at different timescales, we continuously observed Mrk 421 with the Milagro observatory. Mrk 421 was detected by Milagro with a statistical significance of 7.1 standard deviations between 2005 September 21 and 2008 March 15. The observed spectrum is consistent with previous observations by VERITAS. We estimate the duty cycle of Mrk 421 for energies above 1 TeV for different hypotheses of the baseline flux and for different flare selections and we compared our results with the X-ray duty cycle estimated by Resconi et al. The robustness of the results is discussed. © 2014. The American Astronomical Society. All rights reserved.
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First results of a study of TeV emission from GRBs in Milagrito
Milagrito, a detector sensitive to gamma-rays at TeV energies, monitored the
northern sky during the period February 1997 through May 1998. With a large
field of view and high duty cycle, this instrument was used to perform a search
for TeV counterparts to gamma-ray bursts. Within the Milagrito field of view 54
gamma-ray bursts at keV energies were observed by the Burst And Transient
Satellite Experiment (BATSE) aboard the Compton Gamma-Ray Observatory. This
paper describes the results of a preliminary analysis to search for TeV
emission correlated with BATSE detected bursts. Milagrito detected an excess of
events coincident both spatially and temporally with GRB 970417a, with chance
probability within the BATSE error radius. No other
significant correlations were detected. Since 54 bursts were examined the
chance probability of observing an excess with this significance in any of
these bursts is . The statistical aspects and physical
implications of this result are discussed
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Instrumented Water Tanks can Improve Air Shower Detector Sensitivity
Previous works have shown that water Cherenkov detectors have superior
sensitivity to those of scintillation counters as applied to detecting
extensive air showers (EAS). This is in large part due to their much higher
sensitivity to EAS photons which are more than five times more numerous than
EAS electrons. Large area water Cherenkov detectors can be constructed
relatively cheaply and operated reliably. A sparse detector array has been
designed which uses these types of detectors to substantially increase the area
over which the Milagro Gamma Ray Observatory collects EAS information.
Improvements to the Milagro detector's performance characteristics and
sensitivity derived from this array and preliminary results from a prototype
array currently installed near the Milagro detector will be presented