Engineering topics

Bhat, P. N.

Publications and source records attributed to Bhat, P. N..

At least 37 records · Page 2

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 NTRS2010

Fermi GBM Observations of Terrestrial Gamma Flashes

In its first two years of operation, the Fermi Gamma Ray Burst Monitor (GBM) has observed more than 77 Terrestrial Gamma Flashes (TGFs). The thick Bismuth Germanate (BGO) detectors are excellent for TGF spectroscopy, having a high probability of recording the full energy of an incident photon, spanning a broad energy range from 150 keV to 40 MeV, and recording a large number of photons per TGF. Correlations between GBM TGF triggers and lightning sferics detected with the World-Wide Lightning Location Network indicate that TGFs and lightning are simultaneous to within tens of microseconds. The energy spectra of some TGFs have strong 511 keV positron annihilation lines, indicating that these TGFs contain a large fraction of positrons

Wilson-Hodge, Colleen A.↗

NASA NTRS2010

Fermi GBM Observations of Terrestrial Gamma Flashes

In its first two years of operation, the Fermi Gamma Ray Burst Monitor (GBM) has observed 79 Terrestrial Gamma Flashes (TGFs). The thick Bismuth Germanate (BGO) detectors are excellent for TGF spectroscopy, having a high probability of recording the full energy of an incident photon, spanning a broad energy range from 150 keV to 40 MeV, and recording a large number of photons per TGF. Correlations between GBM TGF triggers and lightning sferics detected with the World-Wide Lightning Location Network indicate that TGFs and lightning are simultaneous to within tens of microseconds.

Wilson-Hodge, Colleen A.↗

NASA NTRS2010

Fermi GBM Observations of Terrestrial Gamma-Ray Flashes

This slide presentation explores the relationship between Terrestrial Gamma-Ray Flashes (TGF) and lightning. Using data from the World-Wide Lightning Location Network (WWLLN), and the gamma ray observations from Fermi's Gamma-ray Burst Monitor (GBM), the study reviews any causal relationship between TGFs and lightning. The conclusion of the study is that the TGF and lightning are simultaneous with out a causal relationship.

Wilson-Hodge, Colleen A.↗

NASA NTRS2010

Terrestrial Gamma-ray Flashes (TGFs) Observed with the Fermi-Gamma-ray Burst Monitor: Temporal and Spectral Properties

The Gamma-ray Burst Monitor (GBM) on the Fermi Gamma-ray Space Telescope Observatory (Fermi) was detecting ~2.1 TGFs per week. This rate has increased by a factor of ~8 since new flight software was uploaded to the spacecraft in November 2009 in order to increase the sensitivity of GBM to TGFs. Further upgrades to Fermi-GBM to allow observations of weaker TGFs are in progress. The high time resolution (2 s) allows temporal features to be resolved so that some insight may be gained on the origin and transport of the gamma-ray photons through the atmosphere. The absolute time of the TGFs, known to several microseconds, also allows accurate correlations of TGFs with lightning networks and other lightning-related phenomena. The thick bismuth germanate (BGO) scintillation detectors of the GBM system have observed photon energies from TGFs at energies above 40 MeV. New results on the some temporal aspects of TGFs will be presented along with spectral characteristics and properties of several electron-positron TGF events that have been identified.

Fishman, G. J.↗

NASA NTRS2010

Terrestrial Gamma-Ray Flashes (TGFs) Observed with the Fermi-Gamma-Ray Burst Monitor: The First Hundred TGFs

The Gamma-ray Burst Monitor (GBM) on the Fermi Gamma-ray Space Telescope Observatory (Fermi) is now detecting ~2.1 TGFs per week. At this rate, nearly a hundred TGFs will have been detected by the time of this Meeting. This rate has increased by a factor of ~8 since new flight software was uploaded to the spacecraft in November 2009 in order to increase the sensitivity of GBM to TGFs. The high time resolution (2 microseconds) allows temporal features to be resolved so that some insight may be gained on the origin and transport of the gamma-ray photons through the atmosphere. The absolute time of the TGFs, known to several microseconds, also allows accurate correlations of TGFs with lightning networks and other lightning-related phenomena. The thick bismuth germanate (BGO) scintillation detectors of the GBM system have observed photon energies from TGFs at energies above 40 MeV. New results on the some temporal aspects of TGFs will be presented.

Fishman, G J.↗

NASA NTRS2010

All-Sky Earth Occultation Observations with the Fermi Gamma Ray Burst Monitor

Using the Gamma Ray Burst Monitor (GBM) on-board Fermi, we are monitoring the hard X-ray/soft gamma ray sky using the Earth occultation technique. Each time a source in our catalog is occulted by (or exits occultation by) the Earth, we measure its flux using the change in count rates due to the occultation. Currently we are using CTIME data with 8 energy channels spanning 8 keV to 1 MeV for the GBM NaI detectors and spanning 150 keV to 40 MeV for the GBM BGO detectors. Our preliminary catalog consists of galactic X-ray binaries, the Crab Nebula, and active galactic nuclei. New sources are added to our catalog as they become active or upon request. In addition to Earth occultations, we have observed numerous occultations with Fermi's solar panels. We will present early results. Regularly updated results will be found on our website http://gammaray.nsstc.nasa.gov/gbm/science/occultation.

Wilson-Hodge, C. A.↗

NASA NTRS2009

A Catalog of Terrestrial Gamma-Ray Flashes Observed with the Fermi- Gamma-Ray Burst Monitor: The First Sixteen Months of Operation

The Gamma-ray Burst Monitor (GBM) on the Fermi Gamma-ray Space Telescope Observatory (Fermi) has been detecting on the average about one terrestrial gamma-ray flash every four weeks. This catalog presents the basic characteristics of observed TGFs from the beginning of the Fermi-GBM operation in 2008 July until 2009 October. The thick bismuth germanate (BGO) scintillation detectors of the GBM system have observed photon energies from TGFs at energies above 30 MeV. It is found that the TGF pulses are typically shorter than previously reported, and in several cases less than 0.2ms. Extremely high counting rates are encountered ~200kcps or higher per detector during portions of some TGFs. These high rates require considerable corrections (with inherent assumptions) to the observed data in order to derive the true counting rates.

Fishman, Gerald J.↗

NASA NTRS2009

All-Sky Earth Occultation Observations with the Fermi Gamma-Ray Burst Monitor

Using the Gamma Ray Burst Monitor (GBM) on-board Fermi, we are monitoring the hard X-ray/ soft gamma ray sky using the Earth occultation technique. Each time a source in our catalog is occulted by (or exits occultation by) the Earth, we measure its flux using the change in count rates due to the occultation. Currently we are using CTIME data with 8 energy channels spanning 8 keV to 1 MeV for the GBM NaI detectors and spanning 150 keV to 40 MeV for the GBM BGO detectors. Our preliminary catalog consists of galactic X-ray binaries, the Crab Nebula, and active galactic nuclei. In addition, to Earth occultations, we have observed numerous occultations with Fermi's solar panels.

Wilson-Hodge, C. A.↗

NASA NTRS2009

All-Sky Earth Occultation Observations with the Fermi Gamma-Ray Burst Monitor

Using the Gamma Ray Burst Monitor (GBM) on-board Fermi, we are monitoring the hard X-ray/soft gamma ray sky using the Earth occultation technique. Each time a source in our catalog is occulted by (or exits occultation by) the Earth, we measure its flux by determining the change in count rate due to the occultation. Currently we are using CTIME data with 8 energy channels spanning 8 keV to 1 MeV for the GBM NaI detectors and spanning 150 keV to 40 MeV for the GBM BGO detectors. Our preliminary catalog consists of galactic X-ray binaries, the Crab Nebula, and active galactic nuclei. We will present early results. Regularly updated results can be found on our website http://gammaray.nsstc.nasa.gov/gbm/science/occultation.

Wilson-Hodge, C. A.↗

NASA NTRS1994

Discovery of Intense Gamma-Ray Flashes of Atmospheric Origin

Detectors aboard the Compton Gamma Ray Observatory have observed an unexplained terrestrial phenomenon: brief, intense flashes of gamma rays. These flashes must originate in the atmosphere at altitudes above at least 30 kilometers in order to escape atmospheric absorption and reach the orbiting detectors. At least a dozen such events have been detected over the past 2 years. The photon spectra from the events are very hard (peaking in the high-energy portion of the spectrum) and are consistent with bremsstrahlung emission from energetic (million-electron volt) electrons. The most likely origin of these high- energy electrons, although speculative at this time, is a rare type of high-altitude electrical discharge above thunderstorm regions.

Fishman, G. J.↗

NASA NTRS1994

Spectral evolution of a subclass of gamma-ray bursts observed by batse

Among the gamma-ray bursts (GRBs) observed by the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory we define a subclass of bursts based on similar morphology: a sharp rise followed by a longer decay time. About 7% of all the gamma-ray bursts observed by BATSE fall into this subclass. We study the spectral evolution of these bursts by fitting models to time-segmented burst spectra and find no clear distinction between the spectral evolutionary properties of this subclass and those of other bursts. Further, we study the high time resolution spectral evolution of this subclass of GRBs using their spectral hardness ratios. A majority of the bursts show hardness ratio leading the counting rate and also display a continuous hard to soft evolution. The time lag between the counting rate and the hardness ratio is found to be directly correlated with the rise time of the counting rate profile. We also find, for the first time, evidence for spectral variation in a timescale of 64 ms.

Bhat, P. N.↗

NASA NTRS1994

Discovery of intense gamma-ray flashes of atmospheric origin

Observations have been made of a new terrestrial phenomenon: brief (approx. millisecond), intense flashes of gamma rays, observed with space-borne detectors. These flashes must originate at altitudes in the atmosphere above at least 30 km in order to be observable by orbiting detectors aboard the Compton Gamma-Ray Observatory (CGRO). At least a dozen events have been detected over the past 2 years. The photon spectra from the events are very hard and are consistent with bremsstrahlung emission from energetic (MeV) electrons. The most likely origin of these high energy electrons, while speculative at this time, is a rare type of high altitude electrical discharge above thunderstorm regions.

Fishman, G. J.↗

NASA NTRS1992

Evidence of sub-millisecond structure in a gamma-ray burst

It is reported here that GRB910711, the gamma-ray burst (GRB) with apparently the shortest duration yet seen by the BATSE, has a time profile that shows significant submillisecond structure. The responses to this burst in the BATSE detectors show that the burst is both narrower and of higher energy than is indicated by a light curve summed over all detectors. A narrow spike of duration 200 microsec was detected in the light curve; variations on this timescale have not previously been observed in GRBs, and their explanation should be a stringent test of any GRB theory.

Bhat, P. N.↗

NASA NTRS1985

Search for gamma rays of energy 10(15) eV from Cygnus X-3

Finite flux of excess radiation of energy 10 to the 15th power has been reported by two groups from the direction of Cygnus X-3, with the characteristic periodicity of 4.8 hrs. Samorski and Stamm find that the muon content of the showers generated by this excess radiation is about 77% of that in normal cosmic ray showers, whereas the expectation for gamma ray showers is less than 10%. It is thus difficult to understand the nature of the radiation arriving from the direction of Cygnus X-3. Samorski and Stamm measured the muon densities close to the core (approx. 10 m), where contamination due to other components is severe. Even though this does not explain the high ratio of muon densities, measurements should be carried out away from the core to establish the nature of the radiation. In order to establish the signal from Cygnus X-3 and its muon content with better statistical significance, an extensive air shower array, specifically designed for this purpose was operated at Kolar Gold Fields (longitude: 78 deg .3 E; latitude: + 12 deg .95; atmospheric depth: 920 q/square centimeters) since September, 1984. The details of the array and the accuracy of arrival direction measurements are discussed.

Bhat, P. N.↗

NASA NTRS1985

Microburst of TeV gamma rays from the Crab pulsar

Data on Crab pulsar from atmospheric Cerenkov array at Ooty have shown emission of TeV gamma rays in the form of microbursts. These are a series of events which are unusually closely spaced in time with time separations of less than 1.5 milliseconds. The phasogram of events in the bursts when analyzed with the Crab pulsar period shows significant peaks. Data further show that the signal is at the same absolute phase as the radio peak. Monte Carlo calculations show that the probability of peaks being due to chance is very small.

Vishwanath, P. R.↗

NASA NTRS1985

Secondary periodicities of microbursts of TeV gamma rays from the Crab pulsar

Observations were made during the past several years on the Crab pulsar using the Ooty atmospheric Cerenkov array with the aim of detecting possible emission of ultra high energy gamma rays by the pulsar. During the course of these observations, it was found that the Crab pulsar emits TeV gamma rays in bursts of short duration. The microbursts of TeV gamma rays from the Crab pulsar, which were seen in the data of at least three years, also reveal interesting secondary periodicities. It was noticed at first that some bursts could be connected with the others that occurred during the same night or during the next two nights with integral number of cycles of periods 43 + or - 1 minute. Ten possible periods in the vicinity of 43 minutes were determined for all the combinations of bursts for each year. The best values of periods thus obtained were different from year to year. But when, instead of the real time, the number of Crab cycles elapsed between the bursts was used as the unit of time, two values of burst periods - 77460 and 77770 Crab cycles - were found to be significant in the data of at least two years. A Monte Carlo simulation using 1500 trial periods chosen randomly within + or - 5 minutes of the original burst period did not reveal any value of the period as significant.

Vishwanath, P. R.↗

NASA NTRS1985

Pulsed emission of TeV gamma rays from Vela pulsar

The Ooty atmospheric Cerenkov array, consisting of 10 parabolic mirrors of 0.9 m diameter and 8 of 1.5 m diameter, was used for observations on the Vela pulsar to see if it emits gamma rays in the TeV energy range. During the winter of 1984-85, the array was split into two parts: (1) consisting wholly of the smaller mirrors, and (2) wholly of the bigger mirrors. The two arrays were operated at two different sites to distinguish a marginally significant genuine pulsar signal from spurious signals produced trivially by chance fluctuations in the background rates. All the mirrors were pointed at the celestial object to track it for durations of the order of 1 to 6 hours during clear moonless nights. The event time data is analyzed to detect a possible pulsed emission of TeV gamma rays using the contemporaneous pulsar elements on the basis of their radio observations on the Vela pulsar. Results from the analyses of observations made during the winters of 1982-83 and 1984-85 on steady pulsed emission and on possible transient emission is presented.

Bhat, P. N.↗