[vsnet-grb-info 25016] Further Swift-XRT observations of Swift J0840.7-3516

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  27115
SUBJECT: Further Swift-XRT observations of Swift J0840.7-3516 
DATE:    20/02/17 18:27:44 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

P.A. Evans (U. Leicester) and J.A. Kennea (PSU) report on behalf of the Swift-XRT team:

Swift-XRT has carried out daily observations of the transient Swift J0840.7-3516 (GCN Circ. 26982, ATELs 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 1.84e-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 4.27e21 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-sigma upper limits from the RASS and XMM Slew survey equate to 0.3-10 keV flux limits of ~8e-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.