Einstein Probe confirms a new outburst of the Accreting Millisecond Pulsar SAX J1808.4-3658

ATel #17324; B. T. Wang (YNAO, CAS), A. Marino (ICE-CSIC), G. B. Zhang (YNAO, CAS), F. Coti Zelati (ICE-CSIC), Y. L. Wang (NAOC, ICE-CSIC), J. Mao, X. Hou (YNAO, CAS), D. M. Russell, K. Alabarta (NYU Abu Dhabi), N. Rea (ICE-CSIC), H. Feng, L. Tao (IHEP), W. D. Zhang, H. W. Pan, W. Yuan (NAOC) on behalf of the Einstein Probe (EP) team
on 7 Aug 2025; 16:25 UT
Credential Certification: Francesco Coti Zelati (cotizelati@ice.csic.es)

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On 2025 August 6, XB-NEWS reported a new outburst from the accreting millisecond X-ray pulsar SAX J1808.4-3658 in the optical band (ATel #17323). We performed a rapid follow-up observation with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP), starting on August 6, 18:35 UTC (MJD 60893.77431), collecting 2.9 ks of exposure. A source is detected within the FXT field of view at a position of R.A. = 272.1138 deg and DEC = -36.9788 deg (J2000) with 10 arcsec uncertainty (radius, 90% C.L. statistical and systematic), 2.8 arcsec away from the known position of the optical counterpart. We have checked the light curve and did not find clear Type I burst features.

The average 0.5-10 keV spectrum can be fitted with an absorbed power law (chisq/d.o.f=35.26/37) with a Galactic hydrogen column density of 2.64(-0.09,+0.10)e21 cm^-2 and a photon index of 2.40 (-0.29,+0.31). The derived unabsorbed 0.5-10 keV flux is 4.30 (-0.35, +0.43)e-12 erg/s/cm^2. Assuming a distance of 3.5 kpc (Galloway et al., 2006), the corresponding X-ray luminosity is 6e33 erg/s. The uncertainties are at the 90% confidence level for the above parameters.

While the source is still too faint to be detected by the Wide X-ray Telescope (WXT), i.e., the All-Sky monitor onboard Einstein Probe, we have performed a day-by-day stacking of all the observations covering the position of SAX J1808.4-3658 since August 4th. WXT observed the source for a total of about 7 ks on August 4th, 17 ks on August 5th and 11 ks on August 6th. No significant source at the coordinates of SAX J1808.4-3658 was detected on August 4th, leading to a 3-sigma upper limit on the count rate of 9e-03 cts/s, compatible with the Swift/XRT upper limit reported for the source on the day before (ATel #17323). A source is instead marginally detected on August 5th and August 6th, with total count-rates of (5.3+/-1.7)e-03 and (1.1+/0.3)e-02, and SNR of 3.2 and 4.1 sigma, respectively. These results confirm the ongoing rise in the X-rays.

Further EP/FXT observations of this source are underway. Additional multiwavelength follow-up is encouraged.

We thank the Einstein Probe Science Operations team for a speedy scheduling of this observation. 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. 2025, Science objectives of the Einstein Probe mission).

References:

Galloway, D. K., & Cumming, A. 2006, ApJ, 652, 559

Yuan, W., Dai, L., Feng, H., et al. 2025, SCPMA, 68, 239501