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To: stefano@bicar.it
From: atel@astronomerstelegram.org
Subject: ATel  17626, 17627
Message-Id: <20260125071210.F373B28808C9@server.astronomerstelegram.org>
Date: Sun, 25 Jan 2026 02:12:10 -0500 (EST)

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Posted: Within the last 24 hours
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ATEL #17626							     ATEL #17626

Title:	Continued rise of the 2026 outburst of 1A 1118-61 observed
		with NuSTAR
Author:	Alexander Salganik, Sergey S. Tsygankov, Hua Xiao, Juri Poutanen
		(University of Turku)
Queries:	alsalganik@gmail.com
Posted:	24 Jan 2026; 15:05 UT
Subjects:X-ray, Neutron Star, Pulsar

A new outburst of the Be/X-ray pulsar 1A 1118-61 was detected by the Swift/BAT
telescope on 2026 January 17 (GCN #43435; GCN #43442; ATel #17608) and
subsequently confirmed by MAXI/GSC observations (ATel #17613). A target-of-opportunity
(TOO) observation with ART-XC on SRG began on 2026 January 19 at 13:01
UTC, the source was detected with a mean 4-12 keV flux of 2.5e-9 erg s-1
cm-2 (ATel #17616).  Prominent pulsations with a spin period of P_spin
= 408.9+/-0.1 s were also reported. Optical SALT spectroscopy obtained
on 2026 January 21 shows strong Balmer emission lines consistent with an
active Be disc during the current outburst (ATel #17620).

We subsequently triggered a NuSTAR TOO observation. At the epoch of the
NuSTAR observation on 2026 January 24 at 00:16 UTC, the 3-79 keV flux was
(2.294 +/- 0.003)e-8 erg s-1 cm-2, corresponding to a luminosity of (4.39+/-0.01)e+37
erg s-1, while in the 4-12 keV band the flux was (5.55 +/- 0.01)e-9 erg
s-1 cm-2, corresponding to a luminosity of (1.062 +/- 0.001)e+37 erg s-1,
assuming distance to the source 4 kpc (Coe & Payne 1985). This flux level
is approximately a factor of two higher than that reported in ATel #17616
based on SRG/ART-XC observations.

The broadband energy spectrum is well described by a thermal Comptonization
continuum modified by cyclotron absorption and an iron emission line, modeled
as compTT * gabs + gaussian. The inclusion of photoelectric absorption
is not required by the data, with the hydrogen column density remaining
unconstrained. The best-fit Comptonization parameters are a seed photon
temperature of T0 = 1.33 +/- 0.02 keV, an electron temperature of kT =
6.80 +/- 0.04 keV, and an optical depth of tau = 7.29 +/- 0.06.

A broad absorption-like feature is detected at 
E_cyc = 53.8+-0.4 keV, with a width of 
sigma_cyc = 10.0(+0.4, -0.3) keV and a line depth 0.71+/-0.05. This feature
is interpreted as a cyclotron resonance scattering feature and is consistent
with the cyclotron line energy previously reported for this source by Doroshenko
et al. (2010). 
An iron emission line is detected at 6.4 keV, with an equivalent width
of 0.09 +/- 0.01 keV.

We measure a pulsar spin period of 408.8 +/- 0.1 s, consistent with SRG/ART-XC
measurements  (ATel #17616). The pulse profile in the 3-79 keV energy range
is double-peaked, with a weaker secondary peak, and has a pulsed fraction
of 27.5 +/- 0.2%. The orbital parameters of the system are currently unknown
and no correction for orbital motion has been applied. We detect a previously
reported quasi-periodic oscillation (QPO) at 0.1 Hz in the power spectrum
(Maitra, Paul & Naik 2012).

We thank the NuSTAR team for accepting the TOO observation and for the
quick execution of the observing program.

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Password Certification: Alexander Salganik (alsalganik@gmail.com)
https://www.astronomerstelegram.org/?read=17626
==============================================================================
ATEL #17627							     ATEL #17627

Title:	Swift observations of the Be X-ray binary 1A 1118-61
Author:	Aru Beri (IIA, Bangalore)
Queries:	aru.beri8@gmail.com
Posted:	24 Jan 2026; 18:17 UT
Subjects:X-ray, Binary, Neutron Star

We report results from a Swift/XRT observation of the Be/X-ray pulsar 1A
1118-61 obtained on 2026 January 23 at 13:08:56 UTC as part of our monitoring
program following the onset of its recent outburst (GCN #43435; GCN #43442;
ATel #17608).

The source is clearly detected in the 1-10 keV energy range. The XRT spectrum
was fitted using an absorbed thermal Comptonization model. The fit provides
an acceptable description of the data with a chi-squared value 529 for
510 degrees of freedom.

The best-fit hydrogen column density is  (0.8 +/- 0.1) x 10^22 cm^-2. The
Comptonization component yields a seed photon temperature of = 1.22 +/-
0.06 keV. The electron temperature and optical depth were fixed at 7.3
keV and 10, respectively, due to limited constraints within the Swift/XRT
bandpass. All quoted uncertainties are at the 90% confidence level. The
observed 1-10 keV flux is ~ 4e-09 erg cm^-2 s^-1. The spectral fit parameters
are consistent with that reported in ATel #17626.

A timing analysis of the XRT light curve reveals coherent pulsations with
a period of about 409.24 s, consistent with the known spin period of the
system. No correction for orbital motion has been applied. The pulse profiles
show double-peaked behavior.

These Swift/XRT results confirm that 1A 1118-61 is in an active outburst
state. Continued Swift monitoring observations are planned.

We thank the Swift team for accepting the ToO request and for the quick
execution of the observing program.

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Password Certification: Aru Beri (aru.beri8@gmail.com)
https://www.astronomerstelegram.org/?read=17627
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