29 research outputs found
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
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
Observations on TeV gamma rays from Geminga and PSR 0950+08
The Geminga (2 CG 195+04) which exhibits a periodicity with a period of 59 to 60 s in its emission of X-rays, GeV gamma rays and TeV gamma rays was studied. During the winter of 1984 to 1985, this object was observed to see if it emits TeV gamma rays with a periodicity approx 60 s. The observations were carried out at two different sites separated by 11 Km with the Ooty Atmospheric Cerenkov Array split into two parts. Data were collected during clear moonless nights for a total duration of 15.3 hours spread over 2 months. Since the first time derivative of period is believed to be large and uncertain. The total data are subdivided into segments of duration not more than 3 days each to steer clear of the effects of P in the phase analysis. If TeV gamma ray signals are seen in each of these segments, it is possible to derive P from observed data
EGRET Gamma-Ray Observations of the Crab P2/P1 Ratio
Recent observations of the Crab pulsar by the Energetic Gamma-Ray Experiment
Telescope (EGRET) on the Compton Gamma Ray Observatory show that the
high-energy gamma-ray light curve has changed little over the lifetime of the
instrument. Previous data collected by SAS-2 and COS-B in the years 1972-82,
along with earlier EGRET data, suggested a 14 year sinusoidal variation in the
flux ratio between the first and second peaks. The new data from EGRET indicate
that the flux ratio is constant.Comment: 6 pages, 3 figures. To be published in Ap
Very High-Energy Gamma-Ray Observations of PSR B1509-58 with the CANGAROO 3.8m Telescope
The gamma-ray pulsar PSR B1509-58 and its surrounding nebulae have been
observed with the CANGAROO 3.8m imaging atmospheric Cherenkov telescope. The
observations were performed from 1996 to 1998 in Woomera, South Australia,
under different instrumental conditions with estimated threshold energies of
4.5 TeV (1996), 1.9 TeV (1997) and 2.5 TeV (1998) at zenith angles of ~30 deg.
Although no strong evidence of the gamma-ray emission was found, the lowest
energy threshold data of 1997 showed a marginal excess of gamma-ray--like
events at the 4.1 sigma significance level. The corresponding gamma-ray flux is
calculated to be (2.9 +/- 0.7) * 10^{-12}cm^{-2}s^{-1} above 1.9 TeV. The
observations of 1996 and 1998 yielded only upper limits (99.5% confidence
level) of 1.9 * 10^{-12}cm^{-2}s^{-1} above 4.5 TeV and 2.0 *
10^{-12}cm^{-2}s^{-1} above 2.5 TeV, respectively. Assuming that the 1997
excess is due to Very High-Energy (VHE) gamma-ray emission from the pulsar
nebula, our result, when combined with the X-ray observations, leads to a value
of the magnetic field strength ~5 micro G. This is consistent with the
equipartition value previously estimated in the X-ray nebula surrounding the
pulsar. No significant periodicity at the 150ms pulsar period has been found in
any of the three years' data. The flux upper limits set from our observations
are one order of magnitude below previously reported detections of pulsed TeV
emission.Comment: Accepted to publication in Astrophys. Journal, 25 pages, 2 figure
Fermi Large Area Telescope Observations of Gamma-ray Pulsars PSR J1057-5226, J1709-4429, and J1952+3252
The Fermi Large Area Telescope (LAT) data have confirmed the pulsed emission
from all six high-confidence gamma-ray pulsars previously known from the EGRET
observations. We report results obtained from the analysis of 13 months of LAT
data for three of these pulsars (PSR J1057-5226, PSR J1709-4429, and PSR
J1952+3252) each of which had some unique feature among the EGRET pulsars. The
excellent sensitivity of LAT allows more detailed analysis of the evolution of
the pulse profile with energy and also of the variation of the spectral shape
with phase. We measure the cutoff energy of the pulsed emission from these
pulsars for the first time and provide a more complete picture of the emission
mechanism. The results confirm some, but not all, of the features seen in the
EGRET data.Comment: Accepted for publication in ApJ. 45 pages, 12 figures, 11 tables.
Corresponding authors: O. Celik, F. Gargano, T. Reposeur, D.J. Thompso