Radio brightening of PSR B1259-63/LS 2883 near its 2022 apastron

ATel #15920; L. N. Driessen (University of Sydney), E. Lenc (CSIRO), D. Kaplan (University of Wisconsin-Milwaukee), T. Murphy (University of Sydney, ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)), B. M. Gaensler (Dunlap Institute for Astronomy & Astrophysics, University of Toronto), G. R. Sivakoff (University of Alberta)
on 1 Mar 2023; 05:41 UT
Credential Certification: Gregory R Sivakoff (sivakoff@ualberta.ca)

Tweet

We report a radio brightening of the binary system PSR B1259-63/LS 2883 near its 2022 apastron at 887.5 MHz using ASKAP telescope observations performed as part of the Variables and Slow Transients (VAST) survey. The flux density of the binary system has almost doubled from 2022 November 19 to 2023 February 23.

The ASKAP telescope has observed the PSR B1259-63/LS 2883 system eight times between May 2019 and February 2023. Each observation was taken at a central frequency of 887.5 MHz with a bandwidth of 288 MHz and an integration time of 15 minutes. The first observation was in 2019 as part of the Rapid ASKAP Continuum Survey (RACS-low, Hale et al. 2021, PASA, 38, 58). The subsequent seven observations were part of the VAST survey program (Murphy et al. 2021, PASA, 38, 54). The observation times and corresponding PSR B1259-63/LS 2883 flux densities are as follows:

Observation Start  Orbital Phase  Flux Density 
2019-05-07T08:33   0.47897        6.60 +/- 0.09 mJy 
2022-11-19T00:59   0.52400        3.73 +/- 0.54 mJy 
2022-12-06T23:02   0.53849        4.08 +/- 0.67 mJy 
2022-12-20T22:59   0.54981        4.61 +/- 0.47 mJy 
2023-01-20T19:58   0.57476        5.55 +/- 0.62 mJy 
2023-02-03T19:04   0.58606        5.80 +/- 0.49 mJy 
2023-02-18T18:10   0.59816        6.45 +/- 0.61 mJy 
2023-02-23T17:49   0.60219        7.05 +/- 0.57 mJy

Stacking the VAST images (2022-11-19 to 2023-02-18), we measure a circular polarisation fraction of ~18%.

PSR B1259-63/LS 2883 is a highly eccentric (e~0.87) binary with an orbital period of 1236.72 days (Shannon et al. 2014, MNRAS, 437, 3255). The pulsed emission from the pulsar has a flux density of 2-6 mJy at 2.5 GHz, with a 1.4-4.8 GHz spectral index (α) that can vary from α~0 to -0.9, where flux density at frequency ν is proportional to ν^α (Johnston et al. 2005, MNRAS 358 1069). For ~100 days around periastron, the pulsed radio emission is eclipsed by the stellar wind and circumstellar disk of the companion star, LS 2883. LS 2883 is a late-Oe/early-Be star. The radio continuum flux density of the system increases significantly for 3-4 weeks either side of the periastron passage of the pulsar, and can brighten up to nearly 40 mJy at ~5 GHz (Chernyakova et al. 2021, Universe, 7, 242). The most recent periastron passages were on 2017 September 22 and 2021 February 09, and the next upcoming periastron passage will be on 2024 June 30 (Chernyakova et al. 2021, Universe, 7, 242). The recent ASKAP VAST observations are closer to apastron than periastron, which means that the increased radio brightness is not due to the periastron interaction between the pulsar and the wind/disk of the companion star. Rather, the circular polarization suggests that this is an increase in the pulsed flux, potentially due to interstellar scintillation.

Further observations as part of VAST are planned. Additional monitoring observations are encouraged.