An Unusual Pulsar with a Rising Spectrum

The preliminary ALFA pulsar surveys started on August 1st, 2004. In the first day of observations, a new 67.8-ms pulsar, PSR J1928+1746, was discovered: this was the first new pulsar found with ALFA. It turned out to be an interesting object. Subsequent timing analysis has shown that this is a rather young, energetic pulsar, the characteristic age is only about 80 kyr. An S-band pointing soon revealed that the pulsar is equally bright at that frequency, indicating an unusually flat spectrum, unlike that of almost all other pulsars. This was soon confirmed with observations at C-band and X-band, that detected the pulsar with signal-to-noise ratios comparable to those of the 1420-MHz observations. Strangely, the pulsar could not be detected in a 430-MHz observation. This pulsar's apparently unusual spectrum might not be so unusual: 1400-MHz surveys are biased towards finding this kind of object, but the pulsars found in these surveys were not consistently observed at higher frequencies. The lack of follow-up at higher frequencies was, historically, due to the lack of good receivers operating at those frequencies. The recent detections of PSR J1928+1746 at 5 and 9 GHz demonstrate that the Arecibo radiotelescope and its receivers can now achieve excellent sensitivities across the telescope's whole 10-GHz frequency range. This might reveal a previously unsuspected population of pulsars that remain bright at frequencies of several GHz, and change the established idea that the vast majority of pulsars are steep spectrum sources.

Pulse profile of PSR J1928+1746 at several frequencies

Pulse profile of PSR J1928+1746 at five different radio frequencies.

Another interesting characteristic of this pulsar is that it is a possible counterpart to an unidentified EGRET gamma-ray source, 3EGJ1928+1733. It is a better candidate than a previously suggested counterpart, PSR J1930+1852. This pulsar is a prime example of a good candidate for GLAST follow-up, hopefully the ALFA pulsar surveys will find many such objects. See Cordes et at. 2006 for details.

Position of PSR J1928+1746 in the gamma-ray sky

Grey-scale map for the probability of the location of 3EGJ1928+1733, an unidentified gamma-ray source catalogued by EGRET. The dots indicate the positions of two young, energetic radio pulsars.