Continued rise of the 2026 outburst of 1A 1118-61 observed with NuSTAR

ATel #17626; Alexander Salganik, Sergey S. Tsygankov, Hua Xiao, Juri Poutanen (University of Turku)
on 24 Jan 2026; 15:05 UT
Credential Certification: Alexander Salganik (alsalganik@gmail.com)

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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.