Swift BAT (2005-2026) and MAXI (2009-2026) light curves show no new X-ray period from 2.1 to 1000 days that both monitors confirm; a 72.3-day ISS comb affects about half of MAXI sources

We searched the public daily light curves of the Swift BAT transient monitor (1,055 sources, 15-50 keV, 2005-2026) and of MAXI GSC (452 sources, 2-20 keV, 2009-2026) for periods from 2.1 to 1000 days. We used a semi-weighted Lomb-Scargle periodogram with a running-median red-noise normalisation and a 4-harmonic statistic. We calibrated it with 106 BAT blank-sky positions (none above S = 6.6), shuffled source pairs and 8,640 injected sinusoids. Three independent tests were applied: BAT x MAXI coincidence for 360 sources (368 confirmed peaks, 1.1 per cent false rate), BAT split-half persistence, and a BAT out-of-sample test (discovery in one decade, prediction of period and phase in the other; 170 of 431 peaks confirmed, 0.5 per cent false rate). All confirmed periods are catalogued orbital or superorbital periods, their harmonics, aliases, window side peaks or orbit-superorbit sidebands in 28-42 sources. The recurrence of MAXI J0903-531 (59.6-59.8 d in BAT 2021-2026, against a catalogued 57-d orbit) was already reported. A list of 15 weak BAT-only candidates, for example IGR J17200-3116 at 12.57 d, did not pass the out-of-sample test. For quiet sources, a sinusoid with a semi-amplitude of 2 mCrab at 2.2-100 d is recovered in 86-91 per cent of cases. Recovery is much lower for long periods and strongly variable sources. MAXI light curves contain a 72.3-day comb from the nodal precession of the ISS: 45-46 per cent of all MAXI sources have S1 > 3 at 72.3 d or 36.2 d, and harmonics are significant down to 3.46 d. MAXI also shows ISS sidebands of real periods (Vela X-1 at 7.98 d, Cen X-3 at 2.16 d) and orbits of neighbours up to 1.7 deg away (4U 1700-37 in GX 349+2). BAT shows no common line. Limitations: periods below 2.1 d were not searched, the red-noise model is approximate, quasi-periodic signals lose significance, and source identification depends on catalogues and a short literature list.

X-ray binaries · orbital periods · superorbital periods · Swift BAT · MAXI · periodogram · instrumental artifacts · ISS precession · source confusion · null result