3. A compact remnant?
In contrast to type Ia supernovae core-collapse supernovae are expected to leave a neutron star or a black hole initially close to the explosion center. Here we restrict the discussion to a neutron star. If borne with a significant kick-velocity the neutron star will travel a distance from the centre given by the kick-velocity. Kick-velocities as large as 1000 km s-1 have been reported for pulsars, and for RX J0852.0-4622 with a mean expansion velocity of v = 5000 km s-1 any putative neutron star should be within a radius of about 12´ around the center. The ROSAT all-sky survey data of this area have been searched for point sources and two candidate sources have been found. Excess emission has been detected at RA(2000) = 8 52´ 3", DEC(2000) = 18´ 36", which is off-set from the explosion center by 3.4´. With the nominal value of v and t derived above the separation corresponds to 283 km s-1 for the transverse component of the kick-velocity or a proper motion of 0.3" yr-1. The center of the explosion has been determined by the circle matching best the SNR outer boundary. The uncertainty in the center position is estimated to be about 1.5´. This point-like source and the implications concerning a compact remnant have already been reported and discussed (Aschenbach, 1998). Here we report excess emission from a second point-like source inside the suspected area at RA(2000) = 8 51´ 58", DEC(2000) = 21´ 33". This source has been detected in the low energy ROSAT image of the Vela SNR created from the counts which have been recorded in the central 40´ diameter field of the PSPC. Compared to the full field of 2o this procedure improves the spatial resolution considerably and thereby the sensitivity of detecting point sources above the diffuse background. The 17 source counts per 40"40" pixel exceed the mean background level of 4.4 counts (40"40")-1 by 6-. The source count rate is 0.12 counts s-1. No spectrum and no information about interstellar absorption is available. But the flux can be used to estimate the size of the X-ray emitting area as a function of temperature for a black-body with the interstellar absorption as parameter, which is shown in Fig. 6.
If the source is a black-body radiating neutron star of 10 km radius with emission from the entire surface, there is an upper limit of = 1.2106 K imposed by the 5- upper limit of N = 1022 cm-2. But more realistical is a much lower value of N 1020 cm-2, which is typical for the southeastern section of the Vela SNR. Then, Fig. 6 implies that 3105 K. If just a fraction of the full surface is radiating the temperature may be slightly higher by about a factor of two, for instance, if the area of the radiating spot is about 1% of the full neutron star surface area. For a full size area radiating neutron star, which is just 700 yrs old, would be surprisingly low. Furthermore, if the supernova left a neutron star somewhere else the black-body surface temperature would be even lower because of a lower X-ray count rate, unless the column density to the putative neutron star should be even higher.
© European Southern Observatory (ESO) 1999
Online publication: October 14, 1999