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From: atel@astronomerstelegram.org
Subject: ATel 17722
Message-Id: <20260314070319.0EFB328806BE@server.astronomerstelegram.org>
Date: Sat, 14 Mar 2026 03:03:19 -0400 (EDT)

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Posted: Within the last 24 hours
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ATEL #17722							     ATEL #17722

Title:	NuSTAR confirms ongoing X-ray outburst from Be/X-ray binary
		pulsar GS 1843+00
Author:	G. K. Jaisawal (DTU Space) and G. Vasilopoulos (NKUA)
Queries:	gaurava@space.dtu.dk
Posted:	13 Mar 2026; 09:48 UT
Subjects:X-ray, Binary, Neutron Star, Transient, Pulsar

GS 1843+00 (Ginga 1843+009) is a Be/X-ray binary hosting a ~29.5 s accreting
X-ray pulsar (Koyama et al. 1990; Israel et al. 2001). Since 2010, the
source has remained inactive. On 2026 February 27 (MJD 61098), MAXI/GSC
reported renewed X-ray activity from the direction of GS 1843+00 (ATel
#17706). Public MAXI data suggest that the brightening began around 2026
February 19 (MJD 61090), with the daily-averaged 2-20 keV flux reaching
~50 mCrab on 2026 March 11.

We observed GS 1843+00 with NuSTAR between 2026 March 10 22:32:49 and March
11 20:06:20 UTC for a total effective exposure of ~36 ks. The source was
clearly detected at the known position of GS 1843+00, confirming the ongoing
outburst from the system. The NuSTAR 3-79 keV light curve shows an average
count rate of ~56 counts per second per focal plane module. Timing analysis
of the barycenter-corrected light curve reveals coherent X-ray pulsations
with a best-fit spin period of 29.8346(1) s. The pulse profile is complex
and shows significant energy dependence, evolving from softer to harder
X-ray bands. The pulsed fraction increases from ~3% in the soft band to
<20% toward higher energies in the 3-80 keV range.

The broadband 3-79 keV NuSTAR spectrum can be described by an absorbed
Comptonization model (comptt) with an additional blackbody component and
an iron emission line near 6.4 keV. Preliminary best-fit parameters are:
blackbody kT = 1.23 &plusmn; 0.02 keV, blackbody norm = 13.4 &plusmn; 0.5;
Comptonization seed photon temperature T0 = 3.35 &plusmn; 0.03
keV, electron temperature kTe = 10.9 &plusmn; 0.2 keV, optical
depth = 3.77 &plusmn; 0.02, and reduced &chi;2 = 1.13 for 2613
degrees of freedom. Uncertainties are quoted at the 90% confidence level.

The unabsorbed 3-80 keV flux is (5.84 &plusmn; 0.02) &times; 10-9
erg cm-2 s-1. Assuming a distance of 10 kpc (Piraino
et al. 2000), this corresponds to an X-ray luminosity of 7 &times; 1037
erg s-1, indicating that the source is currently accreting near
the critical luminosity regime typical of bright Be/X-ray binary outbursts.
These results confirm that GS 1843+00 has entered a renewed active accretion
phase after a long period of quiescence. Detailed spectral and timing analyses
of the NuSTAR observation are ongoing.

We thank the NuSTAR PI, Fiona Harrison, for approving the DDT observation
and the NuSTAR SOC for promptly scheduling and executing the observation.

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Password Certification: Gaurava Kumar Jaisawal (gaurava.jaisawal@gmail.com)
https://www.astronomerstelegram.org/?read=17722
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