NuSTAR observations of the new X-ray transient SRGA J174800.5-280826
ATel #18002; A. Sanna (UniCA), A. Beri, Arvind Balasubramanian (IIA, Bangalore), G. Illiano (ICE-CSIC), M. C. Baglio (INAF-OAB), L. Tao (IHEP), G. L. Huang (IHEP), Q. C. Zhao (IHEP), I. Mereminskiy, S. Molkov, A. Lutovinov, A. Semena (IKI RAS)
on 24 Aug 2026; 04:50 UT
Credential Certification: Andrea Sanna (andrea.sanna@dsf.unica.it)
Subjects: X-ray, Transient
We report on NuSTAR follow-up observations of the new X-ray transient SRGA J174800.5-280826, recently discovered by the SRG ART-XC telescope (ATel #17966). NuSTAR observed SRGA J174800.5-280826 on 2026 August 19 starting at 14:19:49 UTC (MJD (TDB) = 61271.597). The source was detected with both FPMA and FPMB, with a net exposure of approximately 57 ks and a mean net count rate of 2.2 counts s-1. The light curve shows no strong variability during the observation, and no type-I X-ray bursts were detected. We searched for coherent pulsations and quasi-periodic oscillations in the frequency range 0.1-1500 Hz, but no significant signals were detected.
The source and background spectra were extracted following standard NuSTAR procedures, accounting for stray-light contributions in the region. Due to the high-energy background, the spectral analysis was restricted to the 3-35 keV energy range. The FPMA and FPMB spectra were fitted simultaneously using an absorbed thermal Comptonization model (constant × TBabs × nthComp), adopting Wilms abundances and Verner cross sections. The model provides a good description of the data with a χ2 of 1049 with 1043 degrees of freedom. We measure an absorption column density of NH = 1.9 × 1023 cm-2, which is significantly higher than the expected Galactic value along the line of sight (NH,Gal ∼ 1.5 × 1022 cm-2). However, given the limited sensitivity to soft X-rays below 3 keV, further observations are required to investigate the origin of the excess absorption. We inferred a Comptonization photon index of Γ = 1.75, while the electron temperature was fixed at kTe = 20 keV, as the NuSTAR spectrum does not provide sufficient sensitivity to constrain this parameter independently. The cross-calibration constant between FPMB and FPMA is 0.94.
The absorbed NuSTAR flux is F3-35 keV = 9.4 × 10-11 erg cm-2 s-1, with a corresponding flux in the ART-XC band of F4-12 keV = 3.4 × 10-11 erg cm-2 s-1. Assuming a distance of 8 kpc, the measured NuSTAR 4-12 keV flux corresponds to a luminosity of approximately 2.6 × 1035 erg s-1, slightly higher than the value reported by ART-XC, but still consistent with the variability observed during the discovery observations.
The corresponding unabsorbed fluxes are 1.1 × 10-10 erg cm-2 s-1 and 4.6 × 10-11 erg cm-2 s-1 in the 3-35 keV and the 4-12 keV bands, respectively.
The NuSTAR observation does not reveal signatures that would identify the nature of the compact object. The observed hard X-ray spectrum and luminosity are broadly consistent with accreting neutron-star and black-hole systems but do not uniquely classify the source. Given its position close to the Galactic plane and toward the Galactic Bulge, the source is likely associated with a Galactic X-ray binary population. If located at a distance of 8 kpc, the inferred luminosity is relatively low compared with typical black-hole transients observed during hard states, although low-luminosity black-hole accretion cannot be excluded. However, the non-detection of type-I X-ray bursts or coherent pulsations during the NuSTAR observation does not rule out an accreting neutron star, as these signatures may not be detectable in a single observation.
We thank the NuSTAR team for their rapid response in scheduling these Director's Discretionary Time observations, enabling prompt follow-up of this newly discovered transient. Continued X-ray and multiwavelength monitoring is encouraged to characterise the evolution of SRGA J174800.5-280826 and determine the nature of the compact object.
