Volume 622, February 2019
|Number of page(s)||21|
|Section||Stellar structure and evolution|
|Published online||21 February 2019|
Prolonged sub-luminous state of the new transitional pulsar candidate CXOU J110926.4−650224
Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Barcelona, Spain
2 Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
3 INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone (Roma), Italy
4 INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
5 Southern African Large Telescope Foundation, PO Box 9 7935 Observatory, Cape Town South Africa
6 Department of Physics, University of the Free State, PO Box 339 9300 Bloemfontein, South Africa
7 Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
8 Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
9 Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Spain
10 Dipartimento di Fisica e Chimica, Università di Palermo, Via Archirafi 36, 90123 Palermo, Italy
11 Department of Astronomy, University of Geneva, Chemin d’Ecogia 16, 1290 Versoix, Switzerland
12 Warsaw University Astronomical Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
13 INAF – Osservatorio Astronomico di Brera, via Emilio Bianchi 46, 23807 Merate (LC), Italy
14 XMM-Newton Science Operations Centre, ESAC/ESA, Camino Bajo del Castillo s/n, Urb. Villafranca del Castillo, 28691 Villanueva de la Cañada, Madrid, Spain
15 Department of Quantum Physics and Astrophysics & Institute of Cosmos Sciences, University of Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain
Accepted: 26 December 2018
We report on a multi-wavelength study of the unclassified X-ray source CXOU J110926.4−650224 (J1109). We identified the optical counterpart as a blue star with a magnitude of ∼20.1 (3300–10500 Å). The optical emission was variable on timescales from hundreds to thousands of seconds. The spectrum showed prominent emission lines with variable profiles at different epochs. Simultaneous XMM-Newton and NuSTAR observations revealed a bimodal distribution of the X-ray count rates on timescales as short as tens of seconds, as well as sporadic flaring activity. The average broad-band (0.3–79 keV) spectrum was adequately described by an absorbed power law model with photon index of Γ = 1.63 ± 0.01 (at 1σ c.l.), and the X-ray luminosity was (2.16 ± 0.04) × 1034 erg s−1 for a distance of 4 kpc. Based on observations with different instruments, the X-ray luminosity has remained relatively steady over the past ∼15 years. J1109 is spatially associated with the gamma-ray source FL8Y J1109.8−6500, which was detected with Fermi at an average luminosity of (1.5 ± 0.2) × 1034 erg s−1 (assuming the distance of J1109) over the 0.1–300 GeV energy band between 2008 and 2016. The source was undetected during ATCA radio observations that were simultaneous with NuSTAR, down to a 3σ flux upper limit of 18 μJy beam−1 (at 7.25 GHz). We show that the phenomenological properties of J1109 point to a binary transitional pulsar candidate currently in a sub-luminous accretion disk state, and that the upper limits derived for the radio emission are consistent with the expected radio luminosity for accreting neutron stars at similar X-ray luminosities.
Key words: accretion, accretion disks / methods: data analysis / stars: neutron / methods: observational / X-rays: binaries / X-rays: individuals: CXOU J110926.4−650224
© ESO 2019
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