Detection of a feature at 0.44 MeV in the Crab pulsar spectrum with FIGARO II - A redshifted positron annihilation line?

Search for Redshifted Annihilation Radiation from the Crab Pulsar

“…In no case did we Ðnd a statistically signiÐcant signal. It is not impossible that this is a line whose intensity varies in time so that the 3 p value of Massaro et al (1991) and our lower limitÈsmaller by a factor of nearly 4Èare both correct. Some hints of a long time variation in the relative strengths of c-ray intensities from the main pulse and the interpulse of the Crab pulsar were suggested by a comparison between COS B data and CGRO EGRET data (Nolan et al 1993).…”

“…We not only looked at emission in the three "" classic ÏÏ regions of the pulsar phase (peak 1, peak 2, and the bridge region) but also in the phase bins used by Massaro et al (1991). We also used the total Ñux from the pulsar and tried to Ðnd an enhancement by choosing those phase bins in the light curve for the central energy range (near 440 keV) that showed a positive deviation above the average light-curve shape.…”

“…Besides a light curve over the entire energy range, we also produced a light curve that just used the central energy range D425È475 keV to search for any enhancement compared to the light curve for the entire D400È500 keV energy range. We then produced spectra from peak 1, peak 2, the bridge region, the total pulsed emission, the phase bins used by the FIGARO group (Massaro et al 1991 ; denoted FIGARO peak 2), plus the region deÐned by an enhancement (which we denote as "" best ÏÏ) determined by comparison of the two light curves shown in Figures 1 and 2. By "" best ÏÏ we mean the portion of the light curve in Figure 2 that we judged (by eye) to be stronger at the same phase bins than the light curve in Figure 1.…”

“…On the basis of both a previously reported 3 p detection of a redshifted annihilation line (Massaro et al 1991) and the hypothesis that such a line was not incompatible with certain magnetospheric accelerator models (see Zhu & Ruderman 1997), we made observations of the Crab pulsar with the CGRO Oriented Scintillation Spectrometer Experiment (OSSE). This experiment is the most sensitive reported to date, and it places signiÐcant constraints on models in which positrons Ñow down toward the neutron star surface.…”

The pulse-phase-resolved spectroscopy of the Crab pulsar with SPI/INTEGRAL

“…Neither "Pulsed" nor "Unpulsed" mean spectra do not demonstrate emission line features. The 3σ upper limits on flux from the possible 440 keV red shifted positron annihilation line (see also [6], [7], [8]) are 3 × 10 −4 and 6 × 10 −4 ph/s/cm 2 , in "Pulsed" and "Unpulsed" emission respectively. These upper limits are higher than the line intensity that was observed previously with other instruments ( [6], [7]).…”