Appendix: Anomalous X-ray pulsars

There is growing evidence that anomalous X-ray pulsars are magnetars, a type of objects with dipolar magnetic fields much stronger than the critical magnetic field. Their common properties are:

(1) their spin periods are distributed in a narrow range (5-12 s), contrary to those of high-mass X-ray binary pulsars, which cover a much wider range;
(2) they have very soft X-ray spectra with a photon index about 3 or larger;
(3) their X-ray luminosity is relatively low (10³⁵-10³⁶  $\mbox{${\rm erg~s}^{-1}$ }$), compared to that of HMXB pulsars;
(4) their X-ray flux shows little variability on timescales from months to years (they are not transient systems);
(5) they have a relatively stable spin period evolution, with long intervals of nearly constant spin-down;
(6) they are located in the galactic plane with the galactic latitude $\mid b\mid~\leq1^\circ$;
(7) their optical counterparts, it present, are very faint of implying that at least they cannot have a massive companion; and
(8) a few of them are possibly associated with supernova remnants (SNRs).

In this appendix we present a simple table of these anomalous X-ray pulsars (Table 2). Further identification is needed for some AXP candidates, although we have included them in the table. Of these, AX J1845-0258 and RX J1838.4-0301 have no detected meaningful period derivatives due to the limited time span of the data, while the enigmatic source RX J072.4-3125 has a period of 8.39 s, but its X-ray luminosity is too low for an AXP.