NuSTAR + XMM-Newton monitoring of the neutron star transient AX J1745.6-2901

Creators: Ponti, G.; Bianchi, S.; Muños-Darias, T.; Mori, K.; De, K.; Rau, A.; De Marco, B.; Hailey, C.; Tomsick, J.; Madsen, K. K.; Clavel, M.; Rahoui, F.; Lal, D. V.; Roy, S.; Stern, D.

Abstract

AX J1745.6-2901 is a high-inclination (eclipsing) transient neutron star (NS) low-mass X-ray binary showcasing intense ionized Fe K absorption. We present here the analysis of 11 XMM-Newton and 15 NuSTAR new data sets (obtained between 2013 and 2016), therefore tripling the number of observations of AX J1745.6-2901 in outburst. Thanks to simultaneous XMM-Newton and NuSTAR spectra, we greatly improve on the fitting of the X-ray continuum. During the soft state, the emission can be described by a disc blackbody (kT ∼ 1.1–1.2 keV and inner disc radius rDBB ∼ 14 km), plus hot (kT ∼ 2.2–3.0 keV) blackbody radiation with a small emitting radius (rBB ∼ 0.5 − 0.8 km) likely associated with the boundary layer or NS surface, plus a faint Comptonization component. Imprinted on the spectra are clear absorption features created by both neutral and ionized matter. Additionally, positive residuals suggestive of an emission Fe K α disc line and consistent with relativistic ionized reflection are present during the soft state, while such residuals are not significant during the hard state. The hard-state spectra are characterized by a hard (Γ ∼ 1.9–2.1) power law, showing no evidence for a high energy cut-off (kTe > 60–140 keV) and implying a small optical depth (τ < 1.6). The new observations confirm the previously witnessed trend of exhibiting strong Fe K absorption in the soft state that significantly weakens during the hard state. Optical (GROND) and radio (GMRT) observations suggest for AX J1745.6-2901 a standard broad-band spectral energy distribution as typically observed in accreting NSs.

Additional Information

© 2017 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2017 September 18. Received 2017 September 18; in original form 2017 May 13. Published: 22 September 2017. The authors wish to thank Jan-Uwe Ness, Karl Foster, Ignacio de la Calle and the rest of the XMM-Newton and NuSTAR scheduling teams for the support that made the coordinated observations possible. We would like to thank the referee for the helpful comments and careful reading of the paper. The GC XMM-Newton monitoring project is supported by the Bundesministerium für Wirtschaft und Technologie/Deutsches Zentrum für Luft- und Raumfahrt (BMWI/DLR, FKZ 50 OR 1408 and FKZ 50 OR 1408) and the Max Planck Society. SB acknowledges financial support from the Italian Space Agency under grant ASI-INAF I/037/12/0. TMD acknowledges support via a Ramón y Cajal Fellowship (RYC-2015-18148). BDM acknowledges support from the European Union's Horizon 2020 research and innovation programme and the Polish National Science Centre grant Polonez 2016/21/P/ST9/04025. These results are based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. This work made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory and funded by the National Aeronautics and Space Administration. We thank the NuSTAR Operations, Software and Calibration teams for support with the execution and analysis of these observations. This research has made use of the NUSTARDAS jointly developed by the ASI Science Data Centre (ASDC, Italy) and the California Institute of Technology (USA). We thank the staff of the GMRT who have made these observations possible. The GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. Part of the funding for GROND (both hardware as well as personnel) was generously granted from the Leibniz-Prize to Prof. G. Hasinger (DFG grant HA 1850/28-1).

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