NuSTAR observation of the SMC pulsar RX J0032.9-7348
ATel #16930; Birendra Chhotaray (PRL, Ahmedabad, India), Sachindra Naik (PRL, Ahmedabad, India), Gaurava K. Jaisawal (DTU Space, Denmark), Arghajit Jana (UDP, Santiago, Chile)
on 3 Dec 2024; 12:13 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Birendra Chhotaray (rsbirendra786@gmail.com)
Subjects: X-ray, Binary, Transient, Pulsar
Recently, an X-ray brightening of the high mass X-ray binary RX J0032.9-7348 was reported by the Einstein Probe mission on 27 October 2024 (ATel#16880). The source was detected at a flux level of ~7.95x10^-11 erg s^-1 cm^-2 in the 0.5-10 keV energy range. The source is likely associated with a Be/X-ray binary system (Kahabak & Pietsch 1996, ATel#16904). XMM-Newton also observed the source on 02 November 2024, where pulsed emission at a period of 7.02 s was discovered at a flux level of 4.2x10^-11 erg s^-1 cm^-2 in the 0.2-10 keV energy band (ATel#16901). Additionally, Swift SMC Survey (S-CUBED) found the X-ray brightening on 11 November 2024, with a flux level of ~2.77x10^-11 erg s^-1 cm^-2 in the 0.3-10.0 keV energy range (ATel#16900). We proposed for ToO observation with NuSTAR, and the ToO observation was performed on 17 November 2024. We obtained ~25 ks exposure data with NuSTAR.
NuSTAR observed the source on 17 November 2024 with an exposure of 25 ks. Our preliminary timing analysis involved identifying periodic signals in the NuSTAR light curve. In our preliminary timing analysis, we detected three significant peaks at periods of 7.02 s (0.1424 Hz), 3.51 s (0.2849 Hz), and 1.78 s (0.5617 Hz). We infer that 7.02 s (0.1424 Hz) corresponds to the fundamental frequency, with the other two peaks representing its harmonics. This confirms the discovery of pulsation by XMM-Newton.
We performed the spectral analysis in the 3-50 keV energy range with NuSTAR. The energy spectrum can be well-fitted using an absorbed cutoff power-law model. The obtained best-fit values of photon index and cutoff energy are gamma=1.06+/-0.10 and Ecut=17.14 (+3.74,-2.72) keV, respectively. During the fitting, the line-of-sight absorption column density was fixed at 2.25x10^21 cm^-2 (Galactic value, HI4PI). The uncertainties on the parameters are calculated within the 90 percent confidence range. The unabsorbed flux in the 3-50 keV range is estimated to be 2.1x10^-11 erg s^-1 cm^-2, corresponding to a luminosity of 9.2x10^36 erg s^-1 assuming a distance of 62 kpc). Currently, the pulsar's flux is one-fourth to one-half of the flux levels reported by Einstein Probe and XMM-Newton on 27 October and 02 November 2024, respectively. This indicates a decline in the outburst activity. However, the flux observed during the NuSTAR observation remains an order of magnitude higher than the ROSAT measurement. We are currently monitoring the source with NICER.
We thank the NuSTAR PI for approving our ToO request.
