NuSTAR observation of RX J0440.9+4431 near the peak of the 2022-2023 giant outburst

ATel #15874; Alexander Salganik (Saint Petersburg State University, IKI RAS, Moscow), Sergey S. Tsygankov (University of Turku), Alexander A. Lutovinov (IKI RAS, Moscow), Sergey V. Molkov (IKI RAS, Moscow)
on 26 Jan 2023; 09:30 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Alexander Lutovinov (aal@iki.rssi.ru)

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MAXI/GSC detected on Dec 29, 2022 that a transient Be/X-ray binary RX J0440.9+4431 (aka LS V +44 17) is going through a Type I outburst (ATel #15835), observed from the source every 155-days orbit. The source reached a peak flux in the beginning of January 2023, after that the flux began to drop. However, on Jan 13, 2023 unexpectedly, the source flux started to rise rapidly again, signaling a possible beginning of the first giant (Type II) outburst ever detected from RX J0440.9+4431 with the flux exceeding 1 Crab in the 15-50 keV range (ATel #15848).

A follow-up NuSTAR observation was performed on Jan 25, 2023. The measured flux in the energy range of 3-79 keV was (3.511+/-0.004)e-8 erg/s/cm^2, which corresponds to a luminosity of (2.500+/-0.002)e+37 erg/s, assuming a distance to the source of 2.4 kpc (Bailer-Jones et al., 2021, 2021AJ....161..147B). Pulsations with a period of 207.47+/-0.01 s were confidently detected in the light curve. The source continuum spectrum has a rather complex shape and can be described by two-component models, in particular, the compTT+compTT model with the seed photons temperature of T0=1.16+/-0.01 keV (for both components) and with plasma temperatures of T=4.9+/-0.1 and 11.6+/-0.2 keV for soft and hard compTT components, respectively. As was shown recently in several studies (see, e.g., Tsygankov et al. 2019, 2019MNRAS.483L.144T, Lutovinov et al. 2021, 2021ApJ...912...17L), a deficit of photons between these two spectral components may appear at low luminosities. Such a phenomenon was previously found for RX J0440.9+4431 around 30 keV and interpreted as a possible cyclotron absorption line when the source was about 20 times less bright.

We continue monitoring of RX J0440.9+4431 with different instruments at different luminosities to make final conclusions about its spectral and physical parameters.

We are grateful to the NuSTAR team for approving and rapid scheduling of the follow-up observation.