Neutron Stars and Black Holes in the Small Magellanic Cloud: The SMC NuSTAR Legacy Survey

Abstract

We present a source catalog from the first deep hard X-ray (E > 10 keV) survey of the Small Magellanic Cloud (SMC), the NuSTAR Legacy Survey of the SMC. We observed three fields, for a total exposure time of 1 Ms, along the bar of this nearby star-forming galaxy. Fields were chosen for their young stellar and accreting binary populations. We detected 10 sources above a 3σ significance level (4-25 keV) and obtained upper limits on an additional 40 sources. We reached a 3σ limiting luminosity in the 4-25 keV band of ∼ 10 35 erg s −1 , allowing us to probe fainter X-ray binary (XRB) populations than has been possible with other extragalactic NuSTAR surveys. We used hard X-ray colors and luminosities to constrain the compact-object type, exploiting the spectral differences between accreting black holes and neutron stars at E > 10 keV. Several of our sources demonstrate variability consistent with previously observed behavior. We confirmed pulsations for seven pulsars in our 3σ sample. We present the first detection of pulsations from a Be-XRB, SXP305 (CXO J005215.4−73191), with an X-ray pulse period of 305.69 ± 0.16 seconds and a likely orbital period of ∼1160-1180 days. Bright sources ( 5 × 10 36 erg s −1 ) in our sample have compact-object classifications consistent with their previously reported types in the literature. Lower luminosity sources ( 5 × 10 36 erg s −1 ) have X-ray colors and luminosities consistent with multiple classifications. We raise questions about possible spectral differences at low luminosity between SMC pulsars and the Galactic pulsars used to create the diagnostic diagrams.

Disentangling the neighbouring pulsars SXP 15.3 and SXP 305

“…Table A1 shows the observations made by Chandra, NuSTAR and Suzaku. NuSTAR was the only one to successfully register strong pulsations from SXP 305 which was also reported by Lazzarini et al (2019) on 12th March 2017. The pulse profile for the same was a single broad peak.…”

“…The 15.3 s pulse period of SXP 15.3 was derived by Lamb et al (1999) through the analysis of ROSAT and BATSE data. The detection of the 305 s pulse period of SXP 305 has for a long time proved elusive, having only been detected recently using NuSTAR observations (Lazzarini et al 2019). In this paper we investigate the origin of recent outbursting behaviour coming from the combined field of both sources as seen from Swift XRT observations.…”

Optical and X-ray study of the peculiar high-mass X-ray binary XMMU J010331.7−730144

“…The recurring time-scale of the outbursts has a range ∼1150-1300 d and last for ∼700-750 d. This remarkable variability seen in the photometric light curves is believed to be an indicator of size and structural changes of the Be disc (Rajoelimanana, Charles & Udalski 2011). While similar patterns in the photometric variability have been seen in BeXB systems such as CXO J005215.4−731915 (Schurch et al 2011;Lazzarini et al 2019), the large amplitude of X0103 is atypical of the known systems. Fig.…”

“…1) are generally rare in BeXBs. Lazzarini et al (2019) suggested that the ∼1180 d peak separation of the OGLE outbursts in the BeXB system, CXO J005215.4-731915, to be attributable to the orbital period. This is in contrast to earlier analysis performed by Schurch et al (2011) on the same source, where it was suggested that the long-term periodicity is due to structural changes in the disc.…”

Optical and X-ray study of the peculiar high mass X-ray binary XMMU J010331.7-730144

“…This remarkable variability seen in the photometric lightcurves is believed to be an indicator of size and structural changes of the Be disc (Rajoelimanana et al 2011). While similar patterns in the photometric variability have been seen in BeXB systems such as CXO J005215.4−731915 (Schurch et al 2011;Lazzarini et al 2019), the large amplitude of X0103 is atypical of the known systems.…”