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From: atel@astronomerstelegram.org
Subject: ATel  17760
Message-Id: <20260422070314.E578E288079A@server.astronomerstelegram.org>
Date: Wed, 22 Apr 2026 03:03:14 -0400 (EDT)

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ATEL #17760							     ATEL #17760

Title:	SVOM/ECLAIRs detection of a rare type-I X-ray burst from RX
		J1718.4-4029
Author:	M. Brunet (IRAP, France), F. Cangemi (APC, F), S. Guillot
		(IRAP, F), A. Coleiro (APC, F), L. Zhang (IHEP, China)
Queries:	marius.brunet@irap.omp.eu
Posted:	21 Apr 2026; 13:16 UT
Subjects:X-ray, Binary, Neutron Star

On Tuesday, April 19th 2026, at 01:02:07.9 UTC (T0), the SVOM/ECLAIRs (4-150
keV) coded-mask telescope detected a Type I burst via an offline search
with the event-by-event data downloaded through the X-band ground station.

The burst has been located at (R.A., Dec) = 259.574, -40.460 degrees =
(17 18 18, -40 27 36) (J2000), with a 90% statistical uncertainty of 7.9'
in radius. We associate the ECLAIRs trigger source with RX J1718.4-4029,
whose coordinates are located 2.3' away from the trigger location.  The
lightcurve showed a single peak of about 13.5s in the 4-20 keV band.

A preliminary spectral analysis indicates that the 4-20 keV spectrum, over
a 13.5 seconds time window starting at T0, is well fitted (chi2 = 4.2/7
dof) with a blackbody (bbodyrad) model with a temperature of kT = 1.99+/-0.11
keV (68% confidence level) and a radius of 9.8 +1.3/-1.0 km for a distance
of 9 kpc (in't Zand et al. 2005, A&A, 440, 287). The 4-20 keV flux is (1.62
+0.06/-0.16) e-08 erg/cm2/s (or 1.36 +0.05/-0.14 photons/cm2/s), corresponding
to a luminosity of about 1.6e38 erg/s, at 9.0 kpc. Assuming a distance
of 6.5 kpc (Kaptein et al. 2000, A&A, 358, L71), the radius is 7.1 +0.9/-0.7
km and the luminosity 8.2 e37 erg/s.

RX J1718.4-4029 has exhibited only a small number of X-ray bursts to date
and is considered a rare burster, likely due to its low persistent emission,
indicative of a low mass accretion rate (Campana et al. 2009, ApJ, 699,
1144).

The Space Variable Objects Monitor (SVOM) is a China-France joint mission
led by the Chinese National Space Administration (CNSA, China), National
Center for Space Studies (CNES, France), and the Chinese Academy of Sciences
(CAS, China), which is dedicated to observing gamma-ray bursts and other
transient phenomena in the energetic universe. ECLAIRs was developed jointly
by APC, CEA, CNES and IRAP.

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