NuSTAR observation during the fading outburst of the new BeXRB MAXI J0901-531

Following the discovery of the new X-ray transient MAXI J0901-531 (ATel #14555) and follow-up in soft X-rays (ATel #14557, #14558, #14559) and at optical wavelengths (ATel #14564, #14568, #14569), we requested a DDT observation with the Nuclear Spectroscopic Telescope Array (NuSTAR), which was performed on 2021 May 03 at 03:08 UTC with a total exposure of ~23.3 ks.

The pointing direction was the Swift localization from ATel #14557. The outburst has faded significantly from the time of detection to the NuSTAR observation, to a flux level that is about 40x lower. The spectrum can be modeled well with a simple absorbed cutoff power-law ("cutoffpl" in xspec/isis).

As the absorption column density could not be constrained we fixed it to N_H = 4.1 x 10^21 cm^-2 as estimated by NICER. From a preliminary analysis of the NuSTAR quicklook data, we derive the following spectral parameters with 90% confidence intervals: photon index = 1.25 (+0.19/-0.14), folding energy = 27 (+18/-7) keV and an unabsorbed 3-50 keV flux of 1.35+/-0.1 10^-11 erg/s/cm^2.

This best fit resulted in a reduced Cash statistic of 1.24 for 199 degrees of freedom. We note the significantly softer photon index compared to previous NICER and Swift observations, which, however, could only cover the spectrum below ~10 keV. The inclusion of a 0.61 keV blackbody component ("bbodyrad" in xspec/isis), as observed by NICER, lowers the photon index slightly to 1.08 (+0.17/-0.29) with a folding energy of 27 (+18/-7) keV, but the blackbody norm itself is only weakly constrained to < 8.

Further, we applied epoch folding to the barycenter corrected NuSTAR light-curve, resulting in a pulse period of 14.053+/-0.002 s, in agreement with previous NICER observations. We thank Fiona Harrison for approving the DDT observation. We thank Karl Forster and the NuSTAR SOC/MOC for its planning and execution.