SRG/eROSITA detection of a bright flare of the millisecond pulsar PSR J1023+0038

ATel #13765; Ole Koenig, Joern Wilms, Ingo Kreykenbohm, Philipp Weber (all ECAP/FAU), David Bogensberger, Arne Rau, Andrea Merloni, Chandreyee Maitra, Stefania Carpano (all MPE), Long Ji (IAAT)
on 27 May 2020; 19:16 UT
Credential Certification: Joern Wilms (j.wilms@sternwarte.uni-erlangen.de)

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On MJD 58991.58 (UTC 2020-05-22 13:53) the eROSITA instrument on-board the Russian/German Spektrum-Roentgen-Gamma (SRG) mission detected a bright flare of the transitional millisecond pulsar PSR J1023+0038 (Archibald et al., 2009, Science 324, 1411).

The flare was discovered by the eROSITA Near-Real Time Analysis (NRTA) transient search performed during the ongoing all-sky survey (eRASS1). PSR J1023+0038 was visible for 45.9 s at a count rate of 15.9±1.2 counts/s. The spectral shape can be described by an absorbed powerlaw with a photon index of 1.55±0.13 and a fixed N_H of 1.7x10²⁰ cm^-2, at an 0.3-10 keV flux of 3.6^+0.7_-0.6 x 10^-11 erg cm^-2 s^-1. This corresponds to a luminosity of 8.1^+1.4_-1.2 x 10³³ erg/s, assuming a distance of 1.368 kpc (Deller et al., 2012, ApJ 756, L25), putting the source into its high luminosity mode (Baglio et al., 2019, A&A 631, 104; Tendulkar et al., 2014, ApJ 791, 77 and references therein). We did not perform a pile-up correction, thus the count rate and derived luminosity in the flare should be seen as lower limit.

eROSITA also observed the source before and after the flare, separated by roughly 4 hours each. The observed 0.3-10 keV fluxes and luminosities were significantly lower, putting the source in the low luminosity mode (statistical errors at 90% confidence level):

MJD=58991.41, 5.1±0.6 counts/s, Flux=12.3^+2.1_-1.9 x 10^-12 erg cm^-2 s^-1, L_x=(2.7±0.5)x10³³ erg/s
MJD=58991.75, 2.2±0.6 counts/s, Flux=5⁺⁴_-2 x 10^-12 erg cm^-2 s^-1, L_x=1.1^+0.9_-0.4 x 10³³ erg/s

The source was also detected at the edge of the FOV at MJD 58991.25 and MJD 58991.91. Fits yield a similarly low luminosity ~10³²-10³³ erg/s but with large systematic uncertainty.

The fluxes and spectral shape of the non-flare observations are consistent with a ~1 ks Swift XRT observation (ObsID 00033012203) taken on MJD 58949.41 = 2020-04-10 when the source had a 0.3-10 keV flux of 10x10^-12 erg cm^-2 s^-1, L_x=2.2x10³³ erg/s and the similar observation taken roughly one year earlier on MJD 58469.05 = 2018-12-17 (ObsID 00033012203).

The general source behavior indicates that the source is currently in its X-ray active state where an accretion disk is present (Stappers et al., 2013, ATel #5513; Campana et al., 2019, A&A 629, L8 and references therein), and that the source transitioned from low to high luminosity mode and back during the encircling eROSITA slews.