Discovery of a strong emission line at 0.5 keV in MAXI J0556-332 with the RGS on XMM-Newton

ATel #3349; D. Maitra, M. T. Reynolds, J. M. Miller (U. of Michigan), John Raymond (CfA)
on 13 May 2011; 23:19 UT
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Credential Certification: Dipankar Maitra (maitra@astro.yale.edu)

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MAXI J0556-332 was observed twice during its ongoing outburst (see, e.g., ATEL #3102, #3103, #3104, #3106, #3110, #3112, #3116, #3119, #3327) with ESA's XMM-Newton observatory, triggered as target-of-opportunity (ToO) observations. The X-ray light curve morphology during these observations, as seen by EPIC-PN detector as well as the RGS, is complex and shows ocassional 'dipping' behavior (as previously noted, e.g. in ATEL #3106). The data obtained from the reflection grating spectrometers (RGS) onboard XMM during both of these observations (made on MJD 55608 and 55653, each of ~40 ksec duration) revealed a strong emission line at 0.5 keV. Preliminary joint fits to the RGS1 and RGS2 spectrum from the first 10 ksec of the first observation (when no dips were seen in the light curve) suggests a Gaussian line profile and a power law continuum. The best-fit line energy is 500.0±0.5 eV and the line width (sigma) is 4.1±0.6 eV. The equivalent width of the line is 5.01±0.40 eV. The line is also present in the second observation as well as during the dipping period of the first observation.

In the absence of any other significantly prominent line(s) in the X-ray spectra, the identification of this line is uncertain. The energy of the line center is consistent with the Lyman-alpha transition of N VII in rest frame. If the line is indeed from N VII, this would require the donor to have an extremely high N/O abundance (>70 based on our preliminary XSTAR modeling), based on the weakness of oxygen lines of similar charge states. A hot subdwarf sdB or sdO type donor appears to be a strong candidate if the 0.5 keV line is indeed due to nitrogen. This would make the system an unique laboratory to study binary evolution. The archival USNO B1.0 colors of the optical counterpart, after correcting for reddening due to a column density of N_H = 5x10²⁰ cm^-2, suggests a blackbody temperature of ~45,000 K or greater (if N_H is larger), which may support the hot subdwarf scenario.

Another intriguing possibility, assuming solar-abundance plasma, is that the observed line is a gravitationally redshifted (z=0.25) H-like O VIII line originating from the surface of the neutron star (assuming canonical values for a neutron star, i.e. 1.4 solar mass and 10 km radius). Detailed analysis of the data is in progress (Maitra et al. 2011, in prep.).

It is a pleasure to thank Norbert Schartel and the XMM planning team for carrying out the ToO observations.