A Search for New Galactic Magnetars in Archival Chandra and Xmm-newton Observations

We present constraints on the number of Galactic magnetars, which we have established by searching for sources with periodic variability in 506 archival Chandra observations and 441 archival XMMNewton observations of the Galactic plane (|b|<5◦). Our search revealed four sources with periodic variability on time scales of 200–5000 s, all of which are probably accreting white dwarfs. We identify 7 of 12 known Galactic magnetars, but find no new examples with periods between 5 and 20 s. We convert this non-detection into limits on the total number of Galactic magnetars by computing the fraction of the young Galactic stellar population that was included in our survey. We find that easily-detectable magnetars, modeled after persistent anomalous X-ray pulsars (e.g., either with X-ray luminosities LX=3×10 erg s [0.5–10.0 keV] and fractional rms amplitudes Arms=70%, or LX=10 erg s and Arms=12%), could have been identified in ≈5% of the Galactic spiral arms by mass. If we assume that there are 3 previously-known examples within our random survey, then there are 59 −32 in the Galaxy. Barely-detectable magnetars (LX = 3 × 10 erg s [0.5–10.0 keV] and Arms=15%) could have been identified throughout ≈0.4% of the spiral arms. The lack of new examples implies that <540 exist in the Galaxy (90% confidence). Similar constraints are found by considering the detectability of transient magnetars in outburst by current and past X-ray missions. For assumed lifetimes of 10 yr, we find that the birth rate of magnetars could range between 0.003 and 0.06 yr. Therefore, the birth rate of magnetars is at least 10% of that for normal radio pulsars. The magnetar birth rate could exceed that of radio pulsars, unless the lifetimes of transient magnetars are &10 yr. Obtaining better constraints will require wide-field X-ray or radio searches for transient X-ray pulsars similar to XTE J1810–197, AX J1845.0–0250, CXOU J164710.2–455216, and 1E 1547.0-5408. Subject headings: stars, neutron — stars, statistics — X-rays, stars