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GCN Circular 24689

Subject
GRB 190530A: Swift-XRT afterglow detection
Date
2019-05-31T04:14:12Z (5 years ago)
From
Aaron Tohuvavohu at PSU/Swift <auc444@psu.edu>
A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), A. Tohuvavohu (PSU), S.
J. LaPorte (PSU), J.A. Kennea (PSU), J.P. Osborne (U. Leicester), K.L.
Page (U. Leicester), A.P. Beardmore (U. Leicester), M.G. Bernadini
(INAF-OAB) 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 190530A (Longo et al. GCN Circ. 24679),
collecting 3.1 ks of Photon Counting (PC) mode data between T0+33.8 ks
and T0+50.4 ks. 

Four uncatalogued X-ray sources are detected, of which one ("Source 5")
is above the RASS limit, and is therefore likely the GRB afterglow.
This source is coincident with the location of the optical counterpart
candidate reported by MASTER (Lipunov et al., GCN 24680). Using 2727 s
of PC mode data and 6 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 = 120.53242, +35.47947 which is equivalent
to:

RA (J2000): 08h 02m 07.78s
Dec(J2000): +35d 28' 46.1"

with an uncertainty of 1.4 arcsec (radius, 90% confidence). This
position is 11.3 arcmin from the Fermi/LAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.2 (+/-0.4).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.70 (+/-0.11). The
best-fitting absorption column is  1.1 (+0.4, -0.3) x 10^21 cm^-2, in
excess of 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 3.6 x 10^-11 (4.1 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.1 (+0.4, -0.3) x 10^21 cm^-2
Galactic foreground: 5.1 x 10^20 cm^-2
Excess significance: 3.1 sigma
Photon index:	     1.70 (+/-0.11)

If the light curve continues to decay with a power-law decay index of
2.2, the count rate at T+24 hours will be 0.13 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.7 x
10^-12 (5.5 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00020893.
The results of the full analysis of the XRT observations are available
at http://www.swift.ac.uk/ToO_GRBs/00020893.

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