Gamma-Ray Sources Observed by Cos-B

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DETECTION OF A FEATURE AT 0.44 MEV IN THE CRAB PULSAR SPECTRUM WITH FIGARO-II - A REDSHIFTED POSITRON-ANNIHILATION LINE

The balloon-borne experiment FIGARO II was launched from the base of Trapani-Milo on 1990 July 9 at 0433 UT and observed the Crab pulsar for about 5 hr. The light curve of the signal from PSR 0531 + 21 folded with the radio period shows clearly the known double-peak structure. The spectrum of the second peak, the dominant structure in the hard X-rays and low-energy gamma rays, is characterized by a feature which, if interpreted as an emission line, gives the intensity of (0.86 +/- 0.33) 10(-4) photons cm-2 s-1 at the energy of 0.44 +/- 0.01 MeV at a confidence level of 99.6%. If this feature is the signature of positron annihilation close to the neutron star surface, redshifted by the intense gravitational field, it is possible to estimate the mass-to-radius ratio of the Crab neutron star and the positron production rate from one of the polar caps. The former is equal to 0.087 M. km-1 and the latter 8.2 x 10(39) e+ s-1 for a pencil beam pattern, in reasonable agreement with some heuristic estimates based on the number of energetic particles radiating the optical pulses

The pulse shape and the spectrum of PSR B0531+21 (Crab pulsar) in the low-energy gamma rays observed with FIGARO II

The FIGARO II experiment observed the Crab pulsar in the energy range 0.15-4 MeV during two transmediterranean flights, on 1990 July 9 and on 1986 July 11. A detailed analysis of the pulse profiles shows that the profile in the energy band 0.37-0.51 MeV is characterized, in addition to the main ones, by the presence in the Interpeak region of two other peaks, which could be associated with the 0.44 MeV line reported by Massaro et al. (1991). Spectral analysis confirms that the Ip spectrum, harder than those of P1 and P2, should progressively steepen with increasing energy. We conclude that the observed Crab emission can be due to the superposition of two components and that the one dominant in the Ip is associated with the pair production in the magnetosphere

OBSERVATION OF THE X-RAY PULSAR-A 0535+26 WITH THE FIGARO-II EXPERIMENT

The FIGARO II experiment observed the transient X-ray pulsar A0535+26 in the 0.15-4 MeV range, only 11.1 +/- 4 days after an expected outburst. We found evidence for periodicity at 103.2 sec close to the extrapolated value. The light-curve and the spectral shape of this low energy gamma-ray emission are presented here

OBSERVATION OF A0535+26 AT ENERGIES ABOVE 150 KEV WITH THE FIGARO-II EXPERIMENT

The transient pulsating X-ray source A0535 + 26 was observed by the FIGARO II gamma-ray experiment on 1990 July 9. The periodogram of about 6 hr of data shows only one significant signal (3.5 sigma) at the period of 103.2 s, very close to the expected one. The folded light curve is characterized by a double-peak structure and a narrow dip; It is similar to that at lower energies. The pulsed flux is (8.6 +/- 2.3) x 10(-6) photons cm-2 s-1 keV-1 in the 148-260 keV band. We also find evidence of a low-energy cutoff below 167 keV

PHASE DISTRIBUTION OF THE 0.44 MEV FEATURE IN THE CRAB PULSAR SPECTRUM

On 1990 July 9, the balloon-borne experiment FIGARO II (0.15 - 4 MeV) observed the Crab region. The light-curve of PSR 0531+212 shows, in addition to the well-known two-peak structures, evidence for extra structures at absolute radio phases 0.1, 0.3, and 0.45 around the energy 0.44 MeV. The phase-resolved spectroscopy confirms that an excess, compatible with a line-like feature, is present at these phase positions

THE RADIO-GAMMA TIME-DELAY OF THE CRAB PULSAR

Gamma-ray observations of the pulsar of the Crab nebula, PSR0531+21, have been performed in the low energy range (0.15-4.0 MeV) with FIGARO II, a large area balloon borne NaI(Tl) detector, during two flights performed on 1986 July 11 and 1990 July 9. A Kernel estimator built from the phases of the individual gamma-ray arrival times has allowed an accurate derivation of the radio-gamma time delay from those short duration gamma-ray observations. The gamma-ray pulse is found ahead of the radio pulse by 600 +/- 145 mu s and 375 +/- 148 mu s for the 1986 and 1990 observations respectively. Both radio-gamma delays could be attributed to variability of the interstellar dispersion since dispersion measures are available from radio measurements respectively two months before the 1986 flight and six days after the 1990 flight. An alternative explanation, particularly from the 1990 observation, could be that maximum gamma-ray and radio emissions originate from spatially different regions of the magnetosphere, distant by about 100 km