Detection of a cyclotron line in the Be X-ray pulsar IGR J06074$+$2205

Abstract

IGR J0607.4+2205 is a transient Be X-ray binary discovered two decades ago. IGR J0607.4+2205 underwent an outburst in 2023 during which it was observed twice with \textit{NuSTAR}. The main goal of this work is to model the broadband X-ray spectrum of IGR J0607.4+2205 during the outburst and to study the variations of the spectral and timing features at different intensities. We extracted the light curve and spectrum of the source from the two \textit{NuSTAR} observations carried out during the recent outburst in the energy range of 3$-$78 keV. We used the epoch folding technique to find pulsation from the source and to study the changes in emission characteristics from the source with energy across an order of magnitude variation in source luminosity. IGR J0607.4+2205 shows pulsations with a period of $\sim$347.6 s during both the observations, with a pulse fraction of $\geq$50\%. The broadband spectrum of the source was modelled using a power-law continuum with a high-energy cutoff. During the first observation, a cyclotron absorption line at $\sim$51 keV was also present in the source with an optical depth of $\sim$1.3. However, no cyclotron line feature was detected in the second observation when the source was an order of magnitude fainter. Additionally, soft excess was detected in the second observation, which was modelled with a black body component emerging from close to the neutron star (NS). We report the first ever detection of a cyclotron line in the broadband spectrum of IGR J0607.4+2205 centred at 51$\pm$1 keV. The magnetic field strength of the NS is estimated to be $\sim$4$\times$$10^{12}$ G from the centroid energy of the absorption line. A significant change is observed in the pulse profile with luminosity during the decay of the outburst, indicating an associated change in the beaming pattern.