Swift/XRT follow-up observation of SGR J1935+2154
ATel #8449; F. Coti Zelati (U. Insubria, U. Amsterdam, INAF-OAB), S. Campana (INAF-OAB), G. L. Israel (INAF-OA Roma), N. Rea (U. Amsterdam, CSIC-IEEC), S. Mereghetti, P. Esposito (IASF-Milano), A. Tiengo (IUSS Pavia)
on 22 Dec 2015; 18:36 UT
Credential Certification: Sergio Campana (campana@merate.mi.astro.it)
Subjects: X-ray, Neutron Star, Soft Gamma-ray Repeater, Transient, Pulsar
Following the detection of a 50-ms burst with Integral from a position consistent with that of the soft gamma-ray repeater SGR J1935+2154 (GCN #18711, #18712), Swift/XRT observed the field of the source starting on 2015 December 21 at 16:49:53 UTC and for a total exposure time of 3959 s.
The XRT was operated in photon counting mode. Seventy-four photons were collected over the 0.3-10 keV energy range within a circular region centered on the most accurate Chandra position (R.A.=293.7317 deg, DEC.=21.8966 deg) and within a radius of 20 pixels (47.2 arcsec). The source net count rate was (1.7+/-0.2)E-2 s-1.
For the spectral analysis we grouped the background-subtracted spectra to have one count in each spectral bin and adopted the Cash statistic (Cash 1979, ApJ, 228, 939). We described the photoelectric absorption through the TBabs model in the XSPEC spectral fitting package.
Modelling the 0.3-10 keV spectrum with an absorbed blackbody model yields the following best-fit parameters (errors at the 1sigma confidence level for a single parameter of interest):
- absorption column density N_H = (2+/-1)E22 cm-2,
- blackbody temperature kT=0.56+/-0.08 keV,
- absorbed 1-10 keV flux of (7+/-1)E-13 erg/cm2/s.
Alternatively, fitting the data with an absorbed power-law model gives
- N_H = (4+/-1)E22 cm-2,
- photon index Gamma=3.9+/-0.8,
- absorbed 1-10 keV flux of (8+/-1)E-13 erg/cm2/s.
