Isolated Neutron Stars and Pulsars

SGRs and AXPs

The last several years has seen major breakthroughs in our understanding of both Soft Gamma-ray Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs). These objects are thought to be young neutron stars, they have spin periods ~7 s, spin down rapidly, and have X-ray luminosities significantly larger than their inferred spin down luminosities. SGRs generate intense bursts during sporadically occurring active periods. The X-ray emission and bursting behavior has been explained in the context of an internal energy source, most likely a super-strong magnetic field (Thompson & Duncan 1995, MNRAS 275, 255)

Kouveliotou et al. (1998, Nature 393, 235) discovered in the RXTE data a ~7-s pulsation associated with SGR 1806-20. The magnetic field of a neutron star can be indirectly derived by measuring the period derivative, assuming the pulsar spin-down is dominated by magnetic dipole radiation. By retrieving ASCA archival data from the HEASARC, Kouveliotou et al. were able to measure the pulse period at an earlier epoch and derive the period derivative. The inferred magnetic field of ~1015G provided the first solid evidence for the existence of the magnetars. This result prompted a rush to search for similar periods and derivatives in other SGRs. This "instant and timely" result would not have been possible without the ease of access of the HEASARC data. This result highlights the use of archives to follow up the discovery of a new phenomenon, that had been missed by previous observers.

The similarities of the pulse period and spin-down properties of the AXPs to the SGRs has led to the suspicion that the two groups are related (Thompson & Duncan 1996, ApJ 473, 332). AXPs may have magnetic fields that lie between those of the magnetars and regular pulsars. This was contrary to the earlier view that these were accretion driven systems. The accretion model for AXPs was based partly on the work of Baykal & Swank (1996, ApJ 460, 470) who had used archival ROSAT data to track the spin history of the AXP 1E2259+586 and found it to fluctuate in a manner similar to accreting systems. The puzzle deepened further with the discovery of a 4s radio pulsar PSR J1814-1744, whose spin down suggests a 6 x 1013 G field. Pivovaroff et al. (2000, ApJ in press) note that its location in the P -- P-dot diagram is similar to the parameters of the AXPs, in particular 1E2259+586. It lies close to the transition point where the pair production essential for the radio pulsar mechanism is suppressed by photon-splitting in the superstrong magnetic field (Baring & Harding 1998, ApJ 507, L55). The spin-down luminosity is a factor of 10 higher than 1E2259+586 so, in the magnetar model, it should be a bright X-ray source. Pivovaroff et al. used the HEASARC ASCA and ROSAT archival data to search for X-ray emission from this radio pulsar and set upper limits well below that expected based on other AXPs. This archival research suggests that the reason AXPs are bright X-ray sources, rather than radio pulsars, must depend on more than simply the magnetic field strength. There must be a distinct evolutionary path for the AXPs. A solution will probably have to await the more powerful spectroscopic and imaging capabilities of the new X-ray observatories. When the next breakthrough comes, the archival data will be waiting at the HEASARC for quick follow up.

Unidentified Gamma-Ray Sources

The nature of the high energy gamma ray sources found along the galactic plane remain a mystery (Gehrels et al. 2000, Nature 404, 363). A handful of these are identified with young pulsars suggesting that others may be the same class. Because of the large positional errors, the identification of these sources requires a multi-wavelength approach. An example of this process is 2EG J1049-5847, whose error circle includes the radio pulsar PSR B1046-58. Kaspi et al. (2000, ApJ 528, 445) phase-folded the archival EGRET data at the radio pulsar ephemeris and detected a modulation with a chance probability of ~10-4. However, additional support for this association was desirable because of the difficulty in interpreting the pulse significance. Pivovaroff et al. (2000, ApJ 528, 436) extracted from the HEASARC archive the ASCA data for this field and found an unusual source which spatially coincides with the radio pulsar and the much larger EGRET error circle. The X-ray source is not pulsed. Kaspi et al. suggest that this is a synchrotron nebula. If the association between PSR B1046-58 and 2EG J1049-5847 is correct, then this is only the eighth pulsar to show evidence for high-energy gamma-ray pulsation.


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