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Meegan, C. A.

Publications and source records attributed to Meegan, C. A..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2019

A Fermi Gamma-Ray Burst Monitor Search for Electromagnetic Signals Coincident with Gravitational-Wave Candidates in Advanced LIGOs First Observing Run

We present a search for prompt gamma-ray counterparts to compact binary coalescence gravitational wave (GW)candidates from Advanced LIGO's first observing run (O1). As demonstrated by the multimessenger observations ofGW170817/GRB 170817A, electromagnetic and GW observations provide complementary information about theastrophysical source, and in the case of weaker candidates, may strengthen the case for an astrophysical origin. Here weinvestigate low-significance GWcandidates from the O1 compact binary coalescence searches using the Fermi Gamma-Ray Burst Monitor (GBM), leveraging its all sky and broad energy coverage. Candidates are ranked and compared tobackground to measure the significance. Those with false alarm rates (FARs) of less than 105 Hz (about one per day,yielding a total of 81 candidates) are used as the search sample for gamma-ray follow-up. No GW candidates werefound to be coincident with gamma-ray transients independently identified by blind searches of the GBM data. Inaddition, GW candidate event times were followed up by a separate targeted search of GBM data. Among the resultingGBM events, the two with the lowest FARs were the gamma-ray transient GW150914-GBM presented in Connaughtonet al. and a solar flare in chance coincidence with a GW candidate.

Burns, E.↗
NASA NTRS2017

Searching the Gamma-Ray Sky for Counterparts to Gravitational Wave Sources Fermi Gamma-Ray Burst Monitor and Large Area Telescope Observations of LVT151012 and GW151226

We present the Fermi Gamma-ray Burst Monitor (GBM) and Large Area Telescope (LAT) observations of the LIGO binary black hole merger event GW151226 and candidate LVT151012. At the time of the LIGO triggers on LVT151012 and GW151226, GBM was observing 68% and 83% of the localization regions, and LAT was observing 47% and 32%, respectively. No candidate electromagnetic counterparts were detected by either the GBM or LAT. We present a detailed analysis of the GBM and LAT data over a range of timescales from seconds to years, using automated pipelines and new techniques for characterizing the flux upper bounds across large areas of the sky. Due to the partial GBM and LAT coverage of the large LIGO localization regions at the trigger times for both events, differences in source distances and masses, as well as the uncertain degree to which emission from these sources could be beamed, these non-detections cannot be used to constrain the variety of theoretical models recently applied to explain the candidate GBM counterpart to GW150914.

gamma rays: general – gravitational waves – me↗
NASA NTRS2014

When a Standard Candle Flickers: Hard X-Ray Variations in the Crab Nebula

In the first two years of science operations of the Fermi Gamma-ray Burst Monitor (GBM), August 2008 to August 2010, an approximately 7% (70 mcrab) decline was discovered in the overall Crab nebula flux in the 15 - 50 keV band, measured with the Earth occultation technique. This decline was independently confirmed with four other instruments: the RXTE/PCA, Swift/BAT, INTEGRAL/IBIS, and INTEGRAL/SPI. The pulsed flux measured with RXTE/PCA from 1999-2010 was consistent with the pulsar spin-down, indicating that the observed changes were nebular. From 2001 to 2010, the Crab nebula flux measured with RXTE/PCA was particularly variable, changing by up to approximately 3.5% per year in the 15-50 keV band. These variations were confirmed with INTEGRAL/SPI starting in 2003, Swift/BAT starting in 2005, and Fermi GBM starting in 2008. Before 2001 and since 2010, the Crab nebula flux has appeared more stable, varying by less than 2% per year. At higher energies, above 50 keV, the Crab flux appears to be slowly recovering to its 2008 levels. I will present updated light curves in multiple energy bands for the Crab nebula, including recent data from Fermi GBM, Swift/BAT, INTEGRAL, MAXI, and NuSTAR and a 16-year long light curve from RXTE/PCA.

Wilson-Hodge, Colleen A.↗
NASA NTRS2013

Hard X-ray Variations in the Crab Nebula

In the first two years of science operations of the Fermi Gamma-ray Burst Monitor (GBM), August 2008 to August 2010, approximately 7% (70 mcrab) decline was discovered in the overall Crab Nebula flux in the 15 - 50 keV band, measured with the Earth occultation technique. This decline was independently confirmed with four other instruments: the RXTE/PCA, Swift/BAT, INTEGRAL/IBIS, and INTEGRAL/SPI. The pulsed flux measured with RXTE/PCA from 1999-2010 was consistent with the pulsar spin-down, indicating that the observed changes were nebular. From 2001 to 2010, the Crab nebula flux measured with RXTE/PCA was particularly variable, changing by up to approximately 3.5% per year in the 15-50 keV band. These variations were confirmed with INTEGRAL/SPI starting in 2003, Swift/BAT starting in 2005, and Fermi GBM starting in 2008. Before 2001 and since 2010, the Crab nebula flux has appeared more stable, varying by less than 2% per year. I will present updated light curves in multiple energy bands for the Crab nebula, including recent data from Fermi GBM, Swift/BAT, INTEGRAL and MAXI, and a 16-year long light curve from RXTE/PCA.

Wilson-Hodge, Colleen A.↗
NASA NTRS2013

Hard X-ray Variations in the Crab Nebula

In the first two years of science operations of the Fermi Gamma-ray Burst Monitor (GBM), August 2008 to August 2010, approximately 7% (70 mcrab) decline was discovered in the overall Crab Nebula flux in the 15 - 50 keV band, measured with the Earth occultation technique. This decline was independently confirmed with four other instruments: the RXTE/PCA, Swift/BAT, INTEGRAL/IBIS, and INTEGRAL/SPI. The pulsed flux measured with RXTE/PCA from 1999-2010 was consistent with the pulsar spin-down, indicating that the observed changes were nebular. From 2001 to 2010, the Crab nebula flux measured with RXTE/ PCA was particularly variable, changing by up to approximately 3.5% per year in the 15-50 keV band. These variations were confirmed with INTEGRAL/SPI starting in 2003, Swift/BAT starting in 2005, and Fermi GBM starting in 2008. Before 2001 and since 2010, the Crab nebula flux has appeared more stable, varying by less than 2% per year. I will present updated light curves in multiple energy bands for the Crab Nebula, including recent data from Fermi GBM, Swift/BAT, INTEGRAL and MAXI, and a 16-year long light curve from RXTE/PCA.

Wilson-Hodge, Colleen A.↗
NASA NTRS2012

Hard X-ray Variations in the Crab Nebula

In the first two years of science operations of the Fermi Gamma-ray Burst Monitor (GBM), August 2008 to August 2010, a approx.7% (70 mcrab) decline was discovered in the overall Crab Nebula flux in the 15 - 50 keV band, measured with the Earth occultation technique. This decline was independently confirmed with four other instruments: the RXTE/PCA, Swift/BAT, INTEGRAL/IBIS, and INTEGRAL/SPI. The pulsed flux measured with RXTE/PCA from 1999-2010 was consistent with the pulsar spin-down, indicating that the observed changes were nebular. From 2001 to 2010, the Crab nebula flux measured with RXTE/PCA was particularly variable, changing by up to approx.3.5% per year in the 15-50 keV band. These variations were confirmed with INTEGRAL/SPI starting in 2003, Swift/BAT starting in 2005, and Fermi GBM starting in 2008. Before 2001 and since 2010, the Crab nebula flux has appeared more stable, varying by less than 2% per year. I will present updated light curves in multiple energy bands for the Crab nebula, including recent data from Fermi GBM, Swift/BAT, and MAXI, and a 16-year long light curve from RXTE/PCA.

Wilson-Hodge, Colleen A.↗
NASA NTRS2012

SGR J1550-5418 Bursts Detected with the Fermi Gamma-Ray Burst Monitor during Its Most Prolific Activity

We have performed detailed temporal and time-integrated spectral analysis of 286 bursts from SGR J1550-5418 detected with the Fermi Gamma-ray Burst Monitor (GBM) in 2009 January, resulting in the largest uniform sample of temporal and spectral properties of SGR J1550-5418 bursts. We have used the combination of broadband and high time-resolution data provided with GBM to perform statistical studies for the source properties.We determine the durations, emission times, duty cycles, and rise times for all bursts, and find that they are typical of SGR bursts. We explore various models in our spectral analysis, and conclude that the spectra of SGR J15505418 bursts in the 8-200 keV band are equally well described by optically thin thermal bremsstrahlung (OTTB), a power law (PL) with an exponential cutoff (Comptonized model), and two blackbody (BB) functions (BB+BB). In the spectral fits with the Comptonized model, we find a mean PL index of -0.92, close to the OTTB index of -1. We show that there is an anti-correlation between the Comptonized E(sub peak) and the burst fluence and average flux. For the BB+BBfits, we find that the fluences and emission areas of the two BB functions are correlated. The low-temperature BB has an emission area comparable to the neutron star surface area, independent of the temperature, while the high temperature BB has a much smaller area and shows an anti-correlation between emission area and temperature.We compare the properties of these bursts with bursts observed from other SGR sources during extreme activations, and discuss the implications of our results in the context of magnetar burst models.

bursts↗
NASA NTRS2012

When a Standard Candle Flickers: Crab Nebula Variations in Hard X-rays

RXTE played a crucial role in our surprising discovery that the Crab Nebula is variable in hard X-rays. In the first two years of science operations of the Fermi Gamma-ray Burst Monitor (GBM), August 2008-2010, a approx.7% (70 mcrab) decline was discovered in the overall Crab Nebula flux in the 15 - 50 keV band, measured with the Earth occultation technique. This decline was independently confirmed in the ~15-50 keV band with four other instruments: the RXTE/PCA, Swift/BAT, INTEGRAL/IBIS, and INTEGRAL/SPI. The pulsed flux measured with RXTE/PCA from 1999-2010 was consistent with the pulsar spin-down, indicating that the observed changes were nebular. From 2001 to 2010, the Crab nebula flux measured with RXTE/PCA was particularly variable, changing by up to approx.3.5% per year in the 15-50 keV band. These variations were confirmed with INTEGRAL/SPI starting in 2003 and Swift/BAT starting in 2005. Before 2001 and since 2010, the Crab nebula 15-50 keV flux measured with RXTE/PCA appeared more stable, varying by less than 2% per year. In this talk I will present Crab light curves including RXTE data for the entire 16-year mission in multiple energy bands.

Wilson-Hodge, Collen A.↗
NASA NTRS2011

When a Standard Candle Flickers: Crab Nebula Variations in Hard X-rays

In the first two years of science operations of the Fermi Gamma-ray Burst Monitor (GBM) since August 2008, a ~7% (70 mcrab) decline was observed in the overall Crab Nebula flux in the 15 - 50 keV band, measured with the Earth occultation technique. This decline is independently confirmed in the ~15-50 keV band with four other instruments: Swift/BAT, the RXTE/PCA, INTEGRAL/IBIS, and INTEGRAL/SPI. A similar decline is also observed in the ~3-15 keV data from the RXTE/PCA and in the 50-100 keV band with GBM, Swift/BAT, INTEGRAL/IBIS, and INTEGRAL/SPI. The pulsed flux measured with RXTE/PCA since 1999 is consistent with the pulsar spin-down, indicating that the observed changes are nebular. Correlated variations in the Crab Nebula flux on a ~3 year timescale are also seen independently with the PCA, BAT, IBIS, and SPI from 2005 to 2008, with a flux minimum in April 2007. As of April 2011, the Crab nebula flux has stopped declining and may be beginning to increase. We will present updated results on our multi-instrument study of long-term Crab nebula variations.

Wilson-Hodge, Colleen A.↗
NASA NTRS2010

GBM Observations of Terrestrial Gamma-Ray Flashes

The TGF detection rate of Gamma-ray Burst Monitor (GBM) has been increased twice since launch. The most recent improvement is from a new operating mode in which data for individual photons are down-linked for selected portions of the orbit, enabling a more sensitive ground-based search for TGFs. The new search has increased the TGF detection rate and is finding TGFs more than five times fainter than the TGFs of the previous GBM sample. We summarize the properties of the original GBM TGF sample and compare to the less intense TGFs now being detected. In addition to gamma-ray TGFs, GBM is observing distant TGFs from the propagation of charged particles along geomagnetic field lines. Strong 511 keV annihilation lines have been observed, demonstrating that both electrons and positrons are present in the particle beams. Spectral fits to these electron/positron TGFs will be shown.

Briggs, M. S.↗
NASA NTRS2010

When a Standard Candle Flickers

The Crab is the only bright steady source in the X-ray sky. The Crab consists of a pulsar wind nebula, a synchrotron nebula, and a cloud of expanding ejecta. On small scales, the Crab is extremely complex and turbulent. X-ray astronomers have often used the Crab as a standard candle to calibrate instruments, assuming its spectrum and overall flux remains constant over time. Four instruments (Fermi/GBM, RXTE/PCA, Swift/BAT, INTEGRAL/ISGRI) show a approx.5% (50 m Crab) decline in the Crab from 2008-2010. This decline appears to be larger with increasing energy and is not present in the pulsed flux, implying changes in the shock acceleration, electron population or magnetic field in the nebula. The Crab is known to be dynamic on small scales, so it is not too surprising that its total flux varies as well. Caution should be taken when using the Crab for in-orbit calibrations.

Wilson-Hodge, Colleen A.↗
NASA NTRS2004

GLAST Burst Monitor Trigger Classification Algorithm

The Gamma Ray Large Area Space Telescope (GLAST), currently set for launch in the first quarter of 2007, will consist of two instruments, the GLAST Burst Monitor (GBM) and the Large Area Telescope (LAT). One of the goals of the GBM is to identify and locate gamma-ray bursts using on-board software. The GLAST observatory can then be re-oriented to allow observations by the LAT. A Bayesian analysis will be used to distinguish gamma-ray bursts from other triggering events, such as solar flares, magnetospheric particle precipitation, soft gamma repeaters (SGRs), and Cygnus X-1 flaring. The trigger parameters used in the analysis are the burst celestial coordinates, angle from the Earth's horizon, spectral hardness, and the spacecraft geomagnetic latitude. The algorithm will be described and the results of testing will be presented.

Perrin, D. J.↗
NASA NTRS2003

Analysis Methods and Results for Weak Gamma-Ray Bursts in the BATSE Data

We report initial results on the statistical properties of the dimmest gamma-ray bursts (GRBs) observed with the Burst and Transient Source Experiment (BATSE), using new ground-based methods to obtain a sample of GRBs from 502 days of BATSE data. Using the most sensitive ground-based detection of GRBs, the sample extends to GRBs much fainter than those detected by the on-board trigger, but because of the temporal resolution of the data, the sample is limited to GRBs of duration of at least 2(approx.)s. For each detected event, Bayesian probabilities are calculated for the event to belong to each of seven classes of differing physical origins. The sample of GRB candidates is defined by the requirement that the Bayesian probability for belonging to the GRB class is higher than 0.5. The intensity distribution of the GRB sample is corrected using a Monte Carlo simulation of the post-flight detection efficiency. The dimmest BATSE bursts of the sample continue the hardness-intensity trend seen in brighter GREs and are consistent with isotropy.

Mitrofanov, I. G.↗
NASA NTRS2001

The GLAST Burst Monitor

The GLAST Burst Monitor The Gamma Ray Large Area Space Telescope, scheduled for launch in 2006, comprises a Large Area Telescope (LAT) and a GLAST Burst Monitor (GBM). The LAT is a pair telescope with unprecedented sensitivity in the 20 Mev to 300 GeV energy range. The GLAST Burst Monitor (GBM) consists of an array of NaI and BGO scintillation detectors operating in the 10 kev to 30 MeV range and covering a wide field of view. The GBM will enhance LAT observations of GRBs by extending the spectral coverage into the range of current GRB databases, and will provide a trigger for re-orienting the spacecraft to observe delayed emission from bursts outside the LAT field of view.

Meegan, C. A.↗
NASA NTRS2001

Observations of Gamma-Ray Bursts

Observations of Gamma-ray bursts (GRBs) remain one of the most baffling phenomena in astrophysics. This talk will summarize the observations of GRBs with particular emphasis on those that present the greatest difficulty for theoretical interpretation. These include the short and highly variable temporal structure, the hard non-thermal spectra, and the enormous total energy output.

Meegan, C. A.↗
NASA NTRS2000

General Differences between Early and Late Stages of BATSE Gamma-Ray Bursts

The early and late stages of gamma-ray bursts are studied in a statistical analysis of the large sample of long BATSE events. The primary peak is used as the boundary between the early and late stages of emission. Significant differences are found between the stages: the early stage is shorter, it has harder emission, and it becomes a smaller fraction of the total burst duration for burst groups of decreasing intensity.

Mitrofanov, I. G.↗