XB-NEWS detects a new outburst of SAX J1808.4-3658

ATel #17323; David M. Russell, Kevin Alabarta, Payaswini Saikia, Sandeep K. Rout, D. M. Bramich (NYU Abu Dhabi), M. Cristina Baglio, Giulia Illiano (INAF-OAB), T. Russell (INAF-IASF Palermo), and Fraser Lewis (Faulkes Telescope Project & The Schools' Observatory, LJMU)
on 6 Aug 2025; 21:05 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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SAX J1808.4-3658 is a neutron star low-mass X-ray binary (LMXB) and accreting millisecond X-ray pulsar. It has an outburst roughly once every 2-4 years, the last of which was in 2022. The 2022 outburst started on 19th August (MJD 59810), detected first by MAXI and NICER at X-ray energies, followed by radio with MeerKAT and optical with the Las Cumbres Observatory (LCO) network (ATel #15559, #15563, #15584, #15603). An X-ray fade followed by reflares were reported from NICER data (#15647, Illiano et al. 2023), and XMM-Newton and Hubble Space Telescope (HST) observations of the reflares were reported in Ballocco et al. 2025.

Here we report the recent optical brightening of SAX J1808.4-3658, indicating a new outburst is underway. We have been monitoring the source with the 1m and 2m telescopes of the LCO network, as part of a monitoring campaign of ~50 low-mass X-ray binaries (Lewis et al. 2008). LCO images are processed and reduced, and magnitudes are extracted and calibrated using a real-time data analysis pipeline, the "X-ray Binary New Early Warning System" (XB-NEWS; see Russell et al. 2019, Goodwin et al. 2020, Alabarta et al., in prep, and ATel #13451 for details).

The source has remained in quiescence since the end of 2022, with magnitudes in the range g' = 21.1 +/- 0.3, r' = 20.6 +/- 0.3, i' = 20.3 +/- 0.2; z_s = 20.2 +/- 0.4, V = 20.6 +/- 0.4. On 3rd August the magnitudes appeared slightly brighter; g' = 20.39 +/- 0.06, r' = 20.01 +/- 0.04, i' = 19.85 +/- 0.05, z_s = 19.70 +/- 0.05 (MJD 60890.35), and separately, V = 20.30 +/- 0.17, i' = 19.92 +/- 0.11 (MJD 60890.78). This was after a few weeks gap; the previous points, which were at the quiescent level, were on 16th July (MJD 60872.67). The slight brightening on 3rd August was only ~0.3-0.4 mag above the mean quiescent level (marginally significant), and could potentially be a precursor to an outburst (similar to that seen in 2019; Goodwin et al. 2020) or some low-level activity. The next observations were on 6th August, and confirm a new outburst has started, with magnitudes of i' = 17.96 +/- 0.05 (MJD 60893.18), z_s = 18.15 +/- 0.12 (MJD 60893.44), V = 18.02 +/- 0.05 and i' = 18.17 +/- 0.05 (MJD 60893.58). This is > 2 mag brighter than quiescence. Multi-wavelength observations are encouraged to study the rise of the outburst. A link to the light curve, with a comparison to the 2019 and 2022 outbursts, is below.

After the tentative brightening on 3rd August, we requested a Swift/XRT observation of SAX J1808.4-3658, which was obtained the same day (MJD 60890.79). Using the cleaned event file in XIMAGE, we measure a count rate of (1.24 +/- 1.40)e-3 c/s, consistent with a non-detection due to the low signal-to-noise (S/N = 0.92). From this, we derive a 3sigma upper limit of 9.7e-3 c/s in the 0.3-10 keV band. The non-detection suggests that any X-ray emission at this stage was still below the Swift/XRT sensitivity threshold.

LCO light curves of SAX J1808.4-3658