Further Swift-XRT observations of Swift J0840.7-3516
ATel #13495; P. A. Evans (U. Leicester), J. A. Kennea (PSU)
on 17 Feb 2020; 18:27 UT
Credential Certification: Phil Evans (pae9@star.le.ac.uk)
Subjects: X-ray, Transient
Swift-XRT has carried out daily observations of the transient Swift J0840.7-3516 (GCN #26982, ATEL #13452, #13456) since its detection by BAT. While the light curve showed strong periods of flaring (ATEL 13456, GCN Circ. 27000) up until ~400 ks post-trigger, there was an apparent underlying power-law decay with an index of 1.08 (±0.02).
From ~400 ks after the BAT trigger, the decay has stopped and the source has stabilised at ~0.006 ct/sec; corresponding to an observed 0.3-10 keV flux of 1.84 × 10-13 erg cm-2 s-1 (observed, 0.3—10 keV).
The spectrum has shown a significant degree of softening. In the earliest data taken immediately after the BAT trigger, a power-law fit showed a spectral photon index of 1.06 (+0.06, 0.04), with no evidence for absorption beyond the Galactic value of 4.27 × 1021 cm-2 (from Willingale et al., 2013). A spectrum formed only from the data since the decay ceased shows a photon index of 2.95 (+0.42, -0.26), and again, no absorption beyond Galactic.
3-σ upper limits from the RASS and XMM Slew survey equate to 0.3—10 keV flux limits of ~8 × 10-13 erg cm-2 s-1, assuming the current spectrum; i.e. the flux is now around a factor of 4 lower than those limits. This may indicate that the source has returned to a quiescent state.
Observations are ongoing.
