K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), E. Ambrosi
(INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (ASDC), A.
Tohuvavohu (U. Toronto), B. Sbarufatti (PSU), D.N. Burrows (PSU) and
P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of the
Fermi/LAT-detected burst GRB 200412B (Longo et al. GCN Circ. 27557) in
a series of observations tiled on the sky. The total exposure time is
4.9 ks, distributed over 7 tiles; the maximum exposure at a single sky
location was 1.5 ks. The data were collected between T0+20.1 ks and
T0+31.7 ks, and are entirely in Photon Counting (PC) mode.
Two uncatalogued X-ray sources are detected, of which one ("Source 2")
is above the RASS limit, and is therefore likely the GRB afterglow.
Using 671 s of PC mode data and 2 UVOT images, we find an enhanced XRT
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 278.31339, +62.53253 which is
RA (J2000): 18h 33m 15.21s
Dec(J2000): +62d 31' 57.1"
with an uncertainty of 2.5 arcsec (radius, 90% confidence). This
position is 30.1 arcmin from the Fermi/LAT position.
The light curve can be modelled with a power-law decay with a decay
index of alpha=1.9 (+1.3, -1.2).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.8 (+0.6, -0.5). The
best-fitting absorption column is 1.6 (+1.8, -1.0) x 10^21 cm^-2,
consistent with the Galactic value of 5.1 x 10^20 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum is 3.1 x 10^-11 (3.8 x
10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 1.6 (+1.8, -1.0) x 10^21 cm^-2
Galactic foreground: 5.1 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index: 1.8 (+0.6, -0.5)
If the light curve continues to decay with a power-law decay index of
1.9, the count rate at T+24 hours will be 0.015 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.7 x
10^-13 (5.8 x 10^-13) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis of the likely afterglow
are at https://www.swift.ac.uk/xrt_products/TILED_GRB00089/Source2.php.
The results of the full analysis of the tiled XRT observations are
available at https://www.swift.ac.uk/xrt_products/TILED_GRB00089.
This circular is an official product of the Swift-XRT team.