Prolonged sub-luminous state of the new transitional pulsar candidate CXOU J110926.4−650224

¹ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Barcelona, Spain
e-mail: cotizelati@ice.csic.es
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

Received: 12 December 2018
Accepted: 26 December 2018

Abstract

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.