NuSTAR detection of an absorption line in the Type I burst spectrum of GRS 1741.9-2853

We report on two NuSTAR observations of GRS 1741.9-2853, a faint neutron star LMXB burster located 10‧ away from the Galactic center. We caught the source serendipitously as it emerged from quiescence (since July 2010). A bright H-triggered mixed H/He thermonuclear Type I burst with mild photospheric radius expansion (PRE) is present during the second observation. Assuming an H mass fraction X = 0.7 in the atmosphere and a neutron star mass M = 1.4M⊙, we determine a new distance for this source at 6.3±0.5 kpc, but we note that it should be regarded as a lower limit. We searched for oscillations during the burst and the outburst but found none. A narrow absorption line at 5.49 (+0.08 -0.10 ) keV is detected during the PRE phase of the burst at the 99.5% confidence level, which most likely formed in the wind above the photosphere of the neutron star. We speculate that the line is formed by resonant Kα transition from H-like Cr gravitationally redshifted by a factor 1 + z = 1.08, corresponding to a radius range of 29.0 - 41.4 km for a mass range of 1.4 - 2.0 M⊙.