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GCN Circular 27000

Subject
GRB 200205A = Swift J0840.7-3516, A newly discovered transient X-ray Pulsar
Date
2020-02-06T08:12:46Z (4 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
J. A. Kennea (Penn State), D. M. Palmer (LANL), M. H. Siegel (Penn State), F. E.
Marshall (NASA/GSFC), P. A. Evans (U Leicester), A. Y. Lien (GSFC/UMBC), K. L.
Page (U Leicester), K. K. Simpson (Penn State), S. Campana (INAF-OAB) and A.
Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift Observatory
Team:

Swift trigger 954304 was initialy reported as GRB 200205A (Evans et al., GCN
Circ. 26982). The source was localized by XRT, and did not match any previously
known cataloged object. MAXI also reported a detection of this source (Niwano et
al., GCN Circ. 26987). The enhanced XRT position was reported by Osborne et al
(GCN Circ. 26996).

Examination of the downlnked Swift data shows that the source light-curve is not
behaving like a GRB, the flux over the three orbits of Windowed Timing XRT data
show no signs of overall fading, however the light-curve is highly variable,
showing both short and longer term variations. The mean flux over the first
three windows is 2.2 x 10^-9 erg/s/cm^2 (0.5 - 10 keV). However, in subsequent
observations starting ~3.3 hours after the trigger, the source fades
significantly, with PC mode flux measured at 3.7 x 10^-11 erg/s/cm^2 (0.5 - 10
keV).

A Z^2_1 period search of the WT event data reveals a possible detection of
coherent pulsations at P=8.96s, suggesting that this source is an accreting
pulsar, although longer periods (e.g. ~55s) cannot be ruled out due to aliasing
and the relatively short exposure this periodicity is based upon (754s). A
longer/more sensitive observation is needed in order to determine the
periodicity of this transient.

We note that the fitted spectrum of this source is hard, with a photon index of
1.06 +/- 0.05, which is typical of accreting pulsars.

Due to the presence of a bright star in the field, UVOT only took data in the UV
 filters. The preliminary UVOT position is: 

RA (J2000) = 08:40:40.96 = 130.17068 (deg.) 
Dec (J2000) = -35:16:25.0 = -35.27362 (deg.) 

with an estimated uncertainty of 0.47 arc sec. (radius, 90% confidence).

Preliminary detections using the UVOT photometric system (Breeveld et al. 2011,
 AIP Conf. Proc. 1358, 373) for the early exposures are:


Filter          T_start(s)   T_stop(s)      Exp(s)           Mag 
w2              139          389          246        17.09 +/- 0.09 
w2              4611         6293         661        16.69 +/- 0.07 

Follow-up observations are encouraged in order to determine the source type. 

This circular, a cross-posting of ATEL 13452, is an official product of the Swift team.
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