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Subject: ATel  17323
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Date: Thu,  7 Aug 2025 03:05:54 -0400 (EDT)

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ATEL #17323							     ATEL #17323

Title:	XB-NEWS detects a new outburst of SAX J1808.4-3658
Author:	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 &amp; The Schools&#39; Observatory, LJMU)
Queries:	dr124@nyu.edu
Posted:	6 Aug 2025; 21:05 UT
Subjects:Optical, X-ray, Neutron Star, Transient, Pulsar

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, <a href="https://ui.adsabs.harvard.edu/abs/2023ApJ...942L..40I/abstract">Illiano
et al. 2023</a>), and XMM-Newton and Hubble Space Telescope (HST) observations
of the reflares were reported in <a href="https://arxiv.org/abs/2506.19556">Ballocco
et al. 2025</a>.

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 (<a href="http://arxiv.org/abs/0712.2751">Lewis
et al. 2008</a>). 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 <a href="https://ui.adsabs.harvard.edu/abs/2019AN....340..278R/abstract">Russell
et al. 2019</a>, <a href="https://ui.adsabs.harvard.edu/abs/2020MNRAS.498.3429G/abstract">Goodwin
et al. 2020</a>, 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: https://xbnews.abudhabi.nyu.edu/static/data/website.plots/SAXJ1808-lc-Aug2025.png
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