Einstein Probe detection of renewed X-ray activity from the intermittent accreting millisecond pulsar SAX J1748.9-2021
ATel #18011; F. Coti Zelati, A. Marino, G. Illiano, Y. L. Wang (ICE-CSIC), J. H. Wu (GZHU), H. N. Yang, R. D. Liang, H. W. Pan (NAO, CAS), N. Rea (ICE, CSIC), H. Feng, L. Tao (IHEP, CAS), H. Sun, W. Yuan, C. Jin (NAO, CAS), J. U. Ness (ESA)
on 26 Aug 2026; 08:03 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Francesco Coti Zelati (cotizelati@ice.csic.es)
Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar
The intermittent accreting millisecond X-ray pulsar SAX J1748.9-2021, located in the Galactic globular cluster NGC 6440, has undergone six recorded outbursts since its discovery (in 1998, 2001, 2005, 2010, 2015, and a faint event in 2017 lasting only around 13 days, significantly shorter than the typically month-long previous outbursts; e.g., Marino et al. 2019). A further outburst from NGC 6440 in 2021 was tentatively associated with SAX J1748.9-2021 based on a type-I X-ray burst and a marginal detection of pulsations near its 442-Hz spin frequency (Pike et al. 2021, ATel #15048; #15095), although the source identification remained uncertain.
The Wide-field X-ray Telescope (WXT) onboard the Einstein Probe (EP) satellite recently detected renewed activity from the source, with an unabsorbed 0.5-4 keV flux of (2.4 ± 0.5) ✕ 10-11 erg cm-2 s-1 on 2026 August 21, rising to (9 ± 1) ✕ 10-11 erg cm-2 s-1 on 2026 August 25.
A first EP Follow-up X-ray Telescope (FXT) observation was performed on 2026 August 24 for an exposure of 6 ks. The 0.5-10 keV spectrum is well described by an absorbed model comprising a multicolour disk blackbody and a thermal Comptonization component, yielding a hydrogen absorption column density of nH = (0.87 ± 0.08)✕ 1022 cm-2 (consistent with values derived during previous outbursts), an inner disk temperature of 0.11 ± 0.02 keV, and a photon index of 2.2 ± 0.3. The unabsorbed 0.5-10 keV flux is (3.0 ± 0.1) × 10-10 erg cm-2 s-1, corresponding to a 0.5-10 keV luminosity of ∼2.6 × 1036 erg s-1 at a distance of 8.5 kpc. Positive residuals are present in the 6-7 keV energy range, consistent with the broad Fe-K reflection feature detected during the 2015 XMM-Newton observation of the source (Pintore et al. 2016).
No significant flux variability or type-I X-ray bursts were observed during the observation. No coherent pulsations at the known 442-Hz spin frequency or its harmonics were detected, which is unsurprising given their intermittent nature in this source (Altamirano et al. 2008; Sanna et al. 2016).
Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.
