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

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

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

The Infrared to Gamma-Ray Pulse Shape of the Crab Nebula Pulsar

We analyze the pulse shape of the Crab Nebula pulsar in the near-infrared, optical, ultraviolet, X-ray, and gamma-ray bands, including previously unpublished ROSAT HRI observations. We show that, in addition to the previously known trend for the fluences of the Bridge and Peak 2 to increase with energy relative to the fluence of Peak 1, there is a small but statistically significant trend for both to decrease with energy relative to Peak 1 over the near-infrared range. We find that the phase separation between the two peaks of the pulse profile decreases nearly continuously as a function of energy over 7 decades of energy. We show that the peaks' full-width half-maxima are significantly variable over this energy range, but without any clear pattern to the variability. We find that the differences between the energy dependences of the leading and trailing edge half-width half-maxima of both peaks found by Eikenberry et al. (1996a) also continue over 7 decades of energy. We show that the cusped shape of Peak 2 reverses direction between the infrared/optical and X-ray/gamma-ray bands, while the cusped shape of Peak 1 shows weak evidence of reversing direction between the X-ray and gamma-ray bands. Finally, we find that many of the pulse shape parameters show maxima or minima at energies of 0.5-1 eV, implying that an important change in the pulsar emission is occuring near this energy. Many of these complex phenomena are not predicted by current pulsar emission models, and offer new challenges for the development of such models.Comment: 32 pages; 9 figure

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

Search for TeV gamma rays from extragalactic sources and the Galactic Center

The Ooty atmospheric Cerenkov array was used in 1982-83 to see if the galactic center and the extra-galactic sources 3C 273, Centaurus A emit gamma rays at TeV energies. The ON/OFF method was used wherein the source was tracked for 16 minutes and immediately afterwards, a background region in the same zenith angle range was tracked for the next 16 minutes. Data were taken for approx 15 hours (55 scans) on 3C273, approx 7 hours (26 scans) on Cen A and 5 hours (19 scans) on galactic center. A preliminary analysis involving direct comparison of total rates on and off the source shows no significant excess from any of these orbits. Results of detailed analysis using different energy thresholds are presented

Magnetic Photon Splitting: Computations of Proper-time Rates and Spectra

The splitting of photons in the presence of an intense magnetic field has recently found astrophysical applications in polar cap models of gamma-ray pulsars and in magnetar scenarios for soft gamma repeaters. Numerical computation of the polarization-dependent rates of this third order QED process for arbitrary field strengths and energies below pair creation threshold is difficult: thus early analyses focused on analytic developments and simpler asymptotic forms. The recent astrophysical interest spurred the use of the S-matrix approach by Mentzel, Berg and Wunner to determine splitting rates. In this paper, we present numerical computations of a full proper-time expression for the rate of splitting that was obtained by Stoneham, and is exact up to the pair creation threshold. While the numerical results derived here are in accord with the earlier asymptotic forms due to Adler, our computed rates still differ by as much as factors of 3 from the S-matrix re-evaluation of Wilke and Wunner, reflecting the extreme difficulty of generating accurate S-matrix numerics for fields below about \teq{4.4\times 10^{13}}Gauss. We find that our proper-time rates appear very accurate, and exceed Adler's asymptotic specializations significantly only for photon energies just below pair threshold and for supercritical fields, but always by less than a factor of around 2.6. We also provide a useful analytic series expansion for the scattering amplitude valid at low energies.Comment: 13 pages, AASTeX format, including 3 eps figures, ApJ in pres

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

Inundation mapping – a study based on December 2004 Tsunami Hazard along Chennai coast, Southeast India

Tsunami impact study has been undertaken along Chennai coast starting from Pulicat to Kovalam. The study area Chennai coast is mainly devoted to prepare large scale action plan maps on tsunami inundation incorporating land use details derived from satellite data along with cadastral data using a GIS tool. Under tsunami inundation mapping along Chennai coast an integrated approach was adopted to prepare thematic maps on land use/land cover and coastal geomorphology using multispectral remote sensing data. The RTK dGPS instruments are used to collect elevation contour data at 0.5 m intervals for the Chennai coast. The GIS tool has been used to incorporate the elevation data, tsunami inundation markings obtained immediately after tsunami and thematic maps derived from remote sensing data. The outcome of this study provides an important clue on variations in tsunami inundation along Chennai coast, which is mainly controlled by local geomorphologic set-up, coastal zone elevation including coastal erosion protection measures and near shore bathymetry. This study highlights the information regarding most vulnerable areas of tsunami and also provides indication to demarcate suitable sites for rehabilitation