Firm detection of 7-year X-ray periodicity from X Persei

Abstract

We report on the detection of long-term X-ray periodicity from the Be/X-ray binary pulsar X Persei. Based on over 23 years of X-ray data observed using RXTE/ASM, Swift/BAT and MAXI/GSC, we confirmed that X Persei exhibits quasi-periodic X-ray flares with a period of ∼7 years. The recurrence timescale corresponds to approximately 10 times its binary orbital period of 250 days. Spectral and hardness ratio changes were not detected along with long-term periodic activity. If we interpret the observed 7 year periodicity of X-ray band flux as a superorbital modulation, then this would be the first observation among the Be/X-ray binaries.

X-ray polarimetry of X-ray pulsar X Persei: another orthogonal rotator?

“…The pulsar mo v es around its optical companion, star HD 24534, along a moderately eccentric ( e = 0.11 ± 0.02) wide orbit with a period of P orb = 250.3 ± 0.6 d, orbital inclination i orb ≈ 30 • , a projected semimajor axis of the NS of a x sin i = 454 ± 4 lt-s, and the mass function í et al 2001 ). Additional quasi-periodic variations of the observed flux from the source with a period of about seven years were reported by Nakajima et al ( 2019 ). Recently, the most accurate distance to X Persei of 600 ± 13 pc was determined from the Gaia Early Data Release 3 (Bailer-Jones et al 2021 ).…”