Low-luminosity accretion in Cep X-4 during the transition to quiescence observed by NuSTAR
ATel #16171; E. Sokolova-Lapa, N. Zalot, J. Stierhof, A. Zainab, J. Wilms (Remeis-Observatory & ECAP, FAU Erlangen-Nuernberg), R. Ballhausen (UMD & NASA-GSFC/CRESST), C. Malacaria (ISSI), P. Kretschmar, A. Rouco Escorial (ESA/ESAC), K. Pottschmidt (UMBC & NASA-GSFC/CRESST), F. Fuerst (Quasar SR for ESA/ESAC), Carlo Ferrigno (University of Geneva), P. Pradhan (ERAU, Prescott), Joel B. Coley (Howard University & NASA-GSFC/CRESST II) on behalf of the XMAG collaboration
on 3 Aug 2023; 16:35 UT
Credential Certification: Katja Pottschmidt (katja@umbc.edu)
Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar
Referred to by ATel #: 16189
We report on a NuSTAR observation of Be X-ray binary Cep X-4 (alias Ginga 2138+56) during its transition to quiescence after the recent outburst, at a state of much lower luminosity than was observed before. The outburst started around 2023 May 26 (MJD 60090; ATEL #16088). NuSTAR observed Cep X-4 on 2023 July 7 (MJD 60132) for 26.5 ks and on 2023 July 9 (MJD 60134) for 31.5 ks, at the end of the outburst decay. The average unabsorbed 3.5-50 keV flux of the source decreased by a factor of ~3 between the two observations, from ~1.7e-11 erg/cm2/s down to ~4.6e-12 erg/cm2/s. Assuming a distance of 7.2 kpc (Bailer-Jones et al 2021, ApJ 161 147), these fluxes correspond to 3.5-50 keV luminosities of 1.1e35 erg/s and 3.3e34 erg/s, respectively.
We detect pulsations with a period of about 66.37 s in both observations. The pulse profile shape resembles the previously observed structure at significantly higher luminosity (~1e36 erg/s, Vybornov et al. 2017, A&A, 601, A126) during the decline of the 2014 outburst (ATEL #6243, ATEL #6212). In a preliminary investigation of the spectral shape of the combined low-luminosity observations by NuSTAR (together with a snapshot from Swift/XRT), we find that the spectrum can be described by a continuum using the same spectral model as successfully used for higher luminosities (Vybornov et al. 2017), but with significantly different parameters, including a much higher, but steeper cutoff (Ecut=36+6-5 keV, Efold=4.8+2.2-1.6 keV) and a broad (sigma ~10 keV) Gaussian emission line (know as "10-keV" feature) centered at~20 keV. We find a similarly good fit with a model with an absorbed cutoff-power-law (cutoffpl) and a blackbody, however with an unusually high temperature of 5.05±0.20 keV. The spectra of Be X-ray binaries with highly magnetized neutron stars often show two-hump continua at low luminosities (~1e34 erg/s), which can be described by two Comptonized components (Comptt) of different temperatures and are typically attributed to the spectral formation in the overheated atmosphere of a neutron star (Sokolova-Lapa et al. 2021, A&A, 651, A12). We tested such a model, which also provides a good description of the data.
We find that in all cases the fit can be improved by inclusion of an absorption line (gabs) at ~29 keV, consistent with the previously claimed cyclotron line. However, the background contribution above ~25 keV is significant, therefore careful treatment of it is needed before inferring any further conclusions from the line parameters.
A flux excess below the cyclotron line (around 20 keV) and the presence of pulsations suggest a transition to low-luminosity accretion onto a neutron star and emission from the heated atmosphere, which was previously reported only for sources with higher or unknown magnetic fields and longer spin periods (e.g., Tsygankov et al. 2019, MNRAS, 487, L30). Work on understanding the background and detailed physics of the accretion regime is ongoing. We encourage continued hard X-ray monitoring of Be X-ray binaries outbursts during their decline and in quiescence.
We thank the NuSTAR Principal Investigator, Fiona Harrison, and the NuSTAR Science Operations Center staff for approving, planning, and scheduling of these Director's Discretionary Time observations.
