2008ApJ...680..639M

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 XMM-Newton observations of the Galactic plane (|b|<5°). Our search revealed four sources with periodic variability on timescales 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 nondetection into limits on the total number of Galactic magnetars by computing the fraction of the young Galactic stellar population that our survey covered. We find that easily detectable magnetars, modeled after persistent anomalous X-ray pulsars (e.g., with L_X=10³⁵ ergs/s [0.5-10.0 keV] and A_rms=12%), could have been identified in ~5% of the Galactic spiral arms by mass. If we assume that three previously known examples randomly fall within our survey, then there are 59⁺⁹²_–32 in the Galaxy. Barely detectable magnetars (L_X=3x10³³ ergs/s and A_rms=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. For assumed lifetimes of 10⁴ yr, the birth rate of magnetars is between 0.003 and 0.06/yr. Therefore, the birth rate of magnetars is at least 10% of that for normal radio pulsars, and could exceed that value, unless transient magnetars are active for ≳10⁵ yr.