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GCN Circular 27561

GRB 200412B: Swift-XRT afterglow detection
2020-04-12T23:35:46Z (4 years ago)
Boris Sbarufatti at PSU <>
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
equivalent to:

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
The results of the full analysis of the tiled XRT observations are
available at

This circular is an official product of the Swift-XRT team.
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