Swift J0840.7-3516 (AKA GRB 200205A): A newly discovered transient X-ray Pulsar

ATel #13452; 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:
on 5 Feb 2020; 22:38 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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At 06:35:51UT Swift BAT triggered on a transient source, originally reported as GRB 200205A (GCN #26982). The source was localized by XRT, and did not match any previously known cataloged object. MAXI reported a detection of this source at 06:54UT (GCN #26987).

Using 2012 s of XRT Photon Counting mode data and 5 UVOT images, we find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 130.17006, -35.27366 which is equivalent to:

RA (J2000): 08 40 40.81
Dec (J2000): -35 16 25.2

with an uncertainty of 1.9 arcsec (radius, 90% confidence).

Examination of three orbits of 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 uvw2 and uvm2 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 encouragd in order to determine the source type.