Abstract
Following the report of an X-ray outburst from CXOU J005245.0-722844 in the Small Magellanic Cloud (ATel #16631, #16633), NICER observed the source on the 28th and 29th of May 2024 for a total exposure of 4.9 ks. During the observation, the source count rate evolved from 84 to 46 counts per second in the 0.5-2 keV band. The 2-10 keV light curve showed an average rate of 0.6 counts per second, nearly consistent with the background. We conducted a preliminary search for periodic signals in the power spectrum, as well as an acceleration search, and did not find anything significant in the 0.5-2 and 2-10 keV bands. The 0.5-10 keV averaged power spectrum is consistent with white noise (i.e., is flat).
The NICER spectrum in the 0.6-2 keV range can be well-fitted with an absorbed blackbody model with two edges and a Gaussian emission line, using the data accumulated on 28th May 2024. The source spectrum is dominated by background above 2 keV, and could also be affected by SMC X-3, located within 5 arcmin of the source. We find a hydrogen column density N_H = (3.0 +/-0.2) x10^21 cm^-2, blackbody temperature kT = 91 +/-2 eV, and a blackbody normalization of (2.25 +/- 0.54)x10^6. The energies of the edge features were found to be 0.88+/-0.1 and 0.98+/-0.2 keV. The Gaussian emission line is detected at an energy of 1.18+/-0.4 keV, with a line width of 70+/-20 eV. Uncertainties on spectral parameters are quoted at the 90% confidence level. The 0.88 keV edge is identified as a signature of the absorption edge from OVIII. This is a typical feature observed in super-soft X-ray sources from white dwarf atmospheres (Shimura 2000, MNRAS, 315, 345). Considering a distance of 62 kpc to the Small Magellanic Cloud (SMC; Graczyk et al. 2014, ApJ, 780, 59), the blackbody emission radius is estimated to be 9300+/-1200 km.
We measured an unabsorbed flux of (3.0+/-0.2)x10^-10 erg/s/cm^2 on 28th May 2024 in 0.5-10 keV using the cflux model in XSPEC. The unabsorbed flux declined rapidly to (0.83+/-0.2)x10^-10 erg/s/cm^2 on 29th May 2024. This corresponds to an evolving source luminosity from 1.4x10^38 to 3.8x10^37 erg/s during NICER observations, assuming a distance of 62 kpc. Our measured flux with NICER is lower than the value found during the XRT observation on 27th May 2024 (ATel #16633), suggesting a rapid decline in the emission from CXOU J005245.0-722844. Based on the super-soft thermal emission and the presence of the OVIII edge, CXOU J005245.0-722844 likely hosts an accreting white dwarf. A massive optical star (2MASS J00524508-7228437) of spectral class O9/B0e is located within 1 arcsec of the source X-ray position measured by XRT at RA=13.18825 deg and Dec=-72.47907 deg (ATel #16633). This makes CXOU J005245.0-722844 the third candidate Be-white dwarf X-ray binary system in the SMC (refer to Kennea et al. 2021, MNRAS, 508, 781; Coe et al. 2020, MNRAS, 497, L50 for the other two sources).
NICER is a 0.2-12 keV X-ray telescope operating on the ISS. The NICER mission and portions of the NICER science team activities are funded by NASA.
The NICER spectrum in the 0.6-2 keV range can be well-fitted with an absorbed blackbody model with two edges and a Gaussian emission line, using the data accumulated on 28th May 2024. The source spectrum is dominated by background above 2 keV, and could also be affected by SMC X-3, located within 5 arcmin of the source. We find a hydrogen column density N_H = (3.0 +/-0.2) x10^21 cm^-2, blackbody temperature kT = 91 +/-2 eV, and a blackbody normalization of (2.25 +/- 0.54)x10^6. The energies of the edge features were found to be 0.88+/-0.1 and 0.98+/-0.2 keV. The Gaussian emission line is detected at an energy of 1.18+/-0.4 keV, with a line width of 70+/-20 eV. Uncertainties on spectral parameters are quoted at the 90% confidence level. The 0.88 keV edge is identified as a signature of the absorption edge from OVIII. This is a typical feature observed in super-soft X-ray sources from white dwarf atmospheres (Shimura 2000, MNRAS, 315, 345). Considering a distance of 62 kpc to the Small Magellanic Cloud (SMC; Graczyk et al. 2014, ApJ, 780, 59), the blackbody emission radius is estimated to be 9300+/-1200 km.
We measured an unabsorbed flux of (3.0+/-0.2)x10^-10 erg/s/cm^2 on 28th May 2024 in 0.5-10 keV using the cflux model in XSPEC. The unabsorbed flux declined rapidly to (0.83+/-0.2)x10^-10 erg/s/cm^2 on 29th May 2024. This corresponds to an evolving source luminosity from 1.4x10^38 to 3.8x10^37 erg/s during NICER observations, assuming a distance of 62 kpc. Our measured flux with NICER is lower than the value found during the XRT observation on 27th May 2024 (ATel #16633), suggesting a rapid decline in the emission from CXOU J005245.0-722844. Based on the super-soft thermal emission and the presence of the OVIII edge, CXOU J005245.0-722844 likely hosts an accreting white dwarf. A massive optical star (2MASS J00524508-7228437) of spectral class O9/B0e is located within 1 arcsec of the source X-ray position measured by XRT at RA=13.18825 deg and Dec=-72.47907 deg (ATel #16633). This makes CXOU J005245.0-722844 the third candidate Be-white dwarf X-ray binary system in the SMC (refer to Kennea et al. 2021, MNRAS, 508, 781; Coe et al. 2020, MNRAS, 497, L50 for the other two sources).
NICER is a 0.2-12 keV X-ray telescope operating on the ISS. The NICER mission and portions of the NICER science team activities are funded by NASA.
| Original language | English |
|---|---|
| Publication date | 30 May 2024 |
| Publication status | Published - 30 May 2024 |
| Series | The Astronomer's telegram |
|---|---|
| Number | ATel #16636 |
Keywords
- X-ray
- Binary
- Transient