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To: stefano@bicar.it
From: atel@astronomerstelegram.org
Subject: ATel 17326, 17327
Message-Id: <20250811071111.0F94D288078C@server.astronomerstelegram.org>
Date: Mon, 11 Aug 2025 03:11:11 -0400 (EDT)

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Posted: Within the last 24 hours
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ATEL #17326							     ATEL #17326

Title:	Swift/XRT follow-up observations of SAX J1808.4-3658
Author:	Caterina Ballocco (INAF-OAR), Alessandro Papitto (INAF-OAR),
		Arianna Miraval Zanon (ASI), Giulia Illiano (INAF-OAB), Filippo Ambrosino
		(INAF-OAR)
Queries:	caterina.ballocco@inaf.it
Posted:	10 Aug 2025; 18:54 UT
Subjects:X-ray, Binary, Transient, Pulsar

On 2025 August 6, XB-NEWS reported a new outburst from the accreting millisecond
X-ray pulsar SAX J1808.4-3658 from a brightening observed in the optical
band (ATel #17323). Shortly after, the EP/FXT telescope confirmed the event
in X-rays, reporting a flux of 4.30 (-0.35, +0.43)&#10005; 10-12erg
cm-2 s-1 in the 0.5-10 keV band (ATel #17324).

On 2025 August 9 at 15:04:57 UTC, we performed a Swift/XRT target of opportunity
observation in Photon Counting (PC) mode with an exposure time of 0.82
ks. The average XRT count rate during the observation was 39.4+/-0.7 counts/s.

The 0.3-10 keV X-ray spectrum could be fitted with an absorbed power-law
model with an hydrogen column density of 7.5 (+2.5, -2.3) &#10005; 1020
cm-2 and a photon index of 1.50+/-0.08. The corresponding unabsorbed
flux in the 0.3-10 keV band is 1.93 (+0.09, -0.08) &#10005; 10-9erg
cm-2 s-1, which is equivalent to a luminosity of
~ 2.8&#10005; 1036erg s-1 assuming a distance of
3.5 kpc (<a href=”https://ui.adsabs.harvard.edu/abs/2006ApJ...652..559G/abstract”>
Galloway et al., 2006</a>). This luminosity is comparable to the peak values
observed during previous outbursts of the source (~1036erg s-1;
see e.g. <a href=”https://ui.adsabs.harvard.edu/abs/2023ApJ...942L..40I/abstract
”>Illiano et al. 2023</a>). This ToO observation is the first of an approved
10-days Swift monitoring program (1 ks/day) and we will continue to monitor
the outburst evolution.

We would like to thank the Swift team, Observatory Duty Scientists and
the PI for promptly scheduling and making these observations possible.

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Password Certification: Alessandro Papitto (alessandro.papitto@inaf.it)
https://www.astronomerstelegram.org/?read=17326
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ATEL #17327							     ATEL #17327

Title:	XB-NEWS detects a new outburst of GX 339-4
Author:	Payaswini Saikia, David M. Russell, Kevin Alabarta, D. M.
		Bramich, Sandeep K. Rout (NYU Abu Dhabi), M. Cristina Baglio (INAF-
		OAB), Thomas Russell (INAF- Palermo) and Fraser Lewis (Faulkes Telescope
		Project &amp; The Schools’ Observatory, LJMU)
Queries:	ps164@nyu.edu
Posted:	11 Aug 2025; 00:01 UT
Subjects:Optical, Binary, Black Hole, Transient

GX 339–4 is a well-known black hole low-mass X-ray binary that undergoes
recurrent outbursts approximately every 1-2 years. The most recent outburst
prior to this report started in 2023 September (MJD 60203.43), detected
in the optical with the Las Cumbres Observatory (LCO) network (ATel #16260)
and subsequently confirmed in X-rays with Swift/XRT (ATel #16260), MAXI/GSC
(ATel #16302), NuSTAR (ATel #16425), and AstroSat (ATel #16570). Infrared
observations with REM (ATel #16460) and radio detections with MeerKAT (ATel
#16421) and ATCA (ATel #16621) were also obtained.

We report the onset of a new outburst from GX 339-4, detected in our latest
observations as part of an ongoing optical monitoring with the 1 m and
2 m telescopes of the LCO network. This monitoring is part of a long-term
program targeting around 50 low-mass X-ray binaries (<a href=”http://arxiv.org/abs/0712.2751”>Lewis
et al. 2008</a>). LCO images are processed in real time and photometrically
calibrated using 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> and <a href="https://ui.adsabs.harvard.edu/abs/2020MNRAS.498.3429G/abstract">Goodwin
et al. 2020</a> for details).

The source has remained in quiescence since around 2025 March 1 (MJD 60735).
Our latest LCO observations, obtained on 2025 August 9 (MJD 60896), yield
optical magnitudes of g' = 18.28+/-0.03, V = 17.86+/-0.02, r' = 17.57+/-0.01,
R = 16.99+/-0.014, and i' = 17.02+/-0.02. For comparison, the most recent
LCO measurements prior to this were consistent with quiescence: on July
12 (MJD 60868), R = 18.93+/-0.05 and V = 19.69+/-0.08; and on July 16 (MJD
60872), r' = 19.36+/-0.03, g' = 20.55+/-0.13, and i' = 18.71+/-0.028. These
values mark the final quiescent state available prior to the current optical
rise. The recent data show a brightening of approximately 1.7-2.3 magnitudes
across all bands relative to quiescence, with an average increase of about
1.9 magnitudes, confirming the onset of the outburst. We encourage coordinated
multi-wavelength observations to track the rise of the outburst.

Optical light-curve of GX 339-4: https://xbnews.abudhabi.nyu.edu/static/data/website.plots/quick.results.gx339_2025Aug.png
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Password Certification: Payaswini Saikia (ps164@nyu.edu)
https://www.astronomerstelegram.org/?read=17327
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