X-rays observations of PSR B1259-63 do not show an increased X-ray activity at apastron
ATel #15942; Alessio Marino (ICE-CSIC, IEEC, Barcelona), Laura Driessen (University of Sydney), Emil Lenc (CSIRO), Nanda Rea (ICE-CSIC, IEEC, Barcelona), Denys Malishev (University of Tuebingen), Masha Chernyakova (Dublin City University), David Kaplan (University of Wisconsin-Milwaukee), Tara Murphy (University of Sydney, OzGrav), Brian M. Gaensler (University of Toronto), Gregory R. Sivakoff (University of Alberta), on behalf of the VAST collaboration
on 13 Mar 2023; 10:51 UT
Credential Certification: Alessio Marino (marino@ice.csic.es)
Subjects: X-ray, Binary, Variables, Pulsar
PSR B1259-63/LS 2883 is one of the few TeV binaries known and consists of a pulsar orbiting around a massive (10 solar masses) companion star of spectral class late-Oe or early-Be. During its long orbital period (3.4 years, Shannon et al., 2014), the system shows increased multi-wavelength activity (radio, X-rays and gamma-rays) around periastron, while it is visible as a radio pulsar at apastron. An unexpected radio brightening has been observed and reported on March 1 2023 (Driessen et al. 2023, Atel #15920) using ASKAP telescope observations, according to which the source doubled its radio (887.5 MHz) emission in 4 months. Currently, the source is at apastron, so that any increased radio brightness can not be easily connected to the periastron interaction between the pulsar and the wind/disk of the companion star, but scintillation was still a viable interpretation. A somewhat similar radio variability was reported for the pulsed emission (Lower & Johnston 2023, Atel #15923), based Parkes radio telescope observations (704-4032 MHz). However, thanks to the Parkes long-term monitoring it was possible to interpret this variability as scintillation.
In order to exclude the interaction with the companion wind or material ejected from the system, we investigated whether a similar brightening was occurring also at higher wavelengths. We observed the source with Swift/XRT on March 7th, 2023 (ObsID 00030966131). The observation was performed in PC mode and had an exposure of 3.4 ks. The source is clearly detected at a XRT count-rate of 0.009+/-0.001 cts/s (1-10 keV flux of about (10+/-3)x10^{-13} erg/s/cm^2), i.e. almost ten times brighter than the background. XMM-Newton also observed PSR B1259-63 on March 5th, 2023 and measured a source flux of (8.8+/-0.4)x10 ^{-13} erg/s/cm^2. The measured XRT count-rate is sensibly lower than the one reported for the source at the periastron passage, i.e. between 0.5 and 2 cts/s (Tam et al., 2018). The flux measured by both XMM-Newton and Switf/XRT is instead compatible or slightly lower than the apastron flux measured in previous works, e.g. (0.5-3)x10^{-12} erg/s/cm^2 in the 1-10 keV range (Chernyakova et al. 2006).
We conclude that PSR B1259-63 is not showing any X-ray excess. This result seems to rule out any X-ray brightening, which would be expected e.g. in the case of the interaction between the pulsar and blobs of matter cluttering the orbital plane. We conclude that such a radio increase was indeed due to interstellar scintillation. ASKAP observations of the field are ongoing as part of the Variables and Slow Transients (VAST) survey, and will keep monitoring the source radio variability.
