Swift/XRT and optical follow-up of the new X-ray transient SRGA J174800.5-280826
ATel #18050; G. K. Jaisawal (DTU Space) and A. Sanna (UniCA)
on 16 Sep 2026; 22:16 UT
Credential Certification: Gaurava Kumar Jaisawal (gaurava.jaisawal@gmail.com)
Subjects: Optical, X-ray, Transient
We report on a Swift X-Ray Telescope (XRT) observation of the new X-ray transient SRGA J174800.5-280826, discovered with the SRG/ART-XC telescope (ATel #17966) and subsequently observed with NuSTAR (ATel #18002).
We monitored the transient with the Swift/XRT for a net exposure of approximately 1.3 ks on 2026 September 13. We detected the source at a standard position of RA = 267.0023 deg, Dec = −28.1451 deg (J2000), corresponding to RA = 17h 48m 00.56s and Dec = −28d 08m 42.3s. The estimated 90% confidence error radius is 4.0 arcsec. The XRT position is approximately 16 arcsec from the position reported by ART-XC (ATel #17966).
We extracted the XRT spectrum over the 0.5–10 keV energy range and modeled it with an absorbed power-law model using Cash statistics. The resulting fit gives an equivalent hydrogen column density of 2.4+1.7−1.3 × 1023 cm−2, with a photon index of 1.0+1.5−1.2. The Cash statistic is 16 for 24 degrees of freedom. Uncertainties correspond to a 90% confidence level. The unabsorbed 0.5–10 keV flux is approximately 7 × 10−11 erg cm−2 s−1. Extrapolating the best-fitting model to the 4–12 keV band gives an unabsorbed flux of 6.5+2.4−1.7 × 10−11 erg cm−2 s−1. Assuming a distance of 8 kpc, this corresponds to a luminosity of 5.0+1.9−1.3 × 1035 erg s−1, nearly twice the luminosity inferred from the ART-XC and NuSTAR measurements, suggesting that the source may have brightened.
We also obtained optical imaging of the field on 2026 September 8 in the V, R, and I bands using the ALFOSC instrument on the Nordic Optical Telescope. We detected no optical counterpart within the XRT positional uncertainty down to a limiting magnitude of approximately 21 mag. The X-ray transient lies very close to the Galactic plane. Strong X-ray absorption, together with high foreground extinction along this line of sight, may hinder the identification of an optical counterpart.
