XMM-Newton and INTEGRAL view of the hard state of EXO 1745−248 during its 2015 outburst

¹ Università degli Studi di Palermo, Dipartimento di Fisica e Chimica, via Archirafi 36, 90123 Palermo, Italy
e-mail: marco.matranga01@unipa.it
² INAF, Osservatorio Astronomico di Roma, via di Frascati 33, 00044 Monte Porzio Catone (Roma) , Italy
³ ISDC Data Centre for Astrophysics, Chemin d’Ecogia 16, 1290 Versoix, Switzerland
⁴ Institut de Ciéncies de l’Espai (IEEC-CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Barcelona, Spain
⁵ Università degli Studi di Cagliari, Dipartimento di Fisica, SP Monserrato-Sestu KM 0.7, 09042 Monserrato, Italy
⁶ Osservatorio Astronomico di Capodimonte, via Moiarello 16, 80131 Napoli, Italy
⁷ European Space Astronomy Centre (ESA/ESAC), Science Operations Department, 28691 Villanueva de la Cañada Madrid, Spain
⁸ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands

Received: 16 September 2016
Accepted: 20 March 2017

Abstract

Context. Transient low-mass X-ray binaries (LMXBs) often show outbursts that typically last a few weeks and are characterized by a high X-ray luminosity (L_x ≈ 10³⁶−10³⁸ erg s^-1), while most of the time they are found in X-ray quiescence (L_X ≈ 10³¹−10³³ erg s^-1). The source EXO 1745−248 is one of them.

Aims. The broad-band coverage and sensitivity of the instrument on board XMM-Newton and INTEGRAL offers the opportunity of characterizing the hard X-ray spectrum during the outburst of EXO 1745−248.

Methods. We report on quasi-simultaneous XMM-Newton and INTEGRAL observations of the X-ray transient EXO 1745−248 located in the globular cluster Terzan 5, performed ten days after the beginning of the outburst (on 2015 March 16) of the source between March and June 2015. The source was caught in a hard state, emitting a 0.8−100 keV luminosity of ≃ 10³⁷ erg s^-1.

Results. The spectral continuum was dominated by thermal Comptonization of seed photons with temperature kT_in ≃ 1.3 keV, by a cloud with a moderate optical depth τ ≃ 2, and with an electron temperature of kT_e ≃ 40 keV. A weaker soft thermal component at temperature kT_th ≃ 0.6−0.7 keV and compatible with a fraction of the neutron star radius was also detected. A rich emission line spectrum was observed by the EPIC-pn on board XMM-Newton; features at energies compatible with K-α transitions of ionized sulfur, argon, calcium, and iron were detected, with a broadness compatible with either thermal Compton broadening or Doppler broadening in the inner parts of an accretion disk truncated at 20 ± 6 gravitational radii from the neutron star. Strikingly, at least one narrow emission line ascribed to neutral or mildly ionized iron is needed to model the prominent emission complex detected between 5.5 and 7.5 keV. The different ionization state and broadness suggest an origin in a region located farther from the neutron star than where the other emission lines are produced. Seven consecutive type I bursts were detected during the XMM-Newton observation, none of which showed hints of photospheric radius expansion. A thorough search for coherent pulsations from the EPIC-pn light curve did not result in any significant detection. Upper limits ranging from a few to 15% on the signal amplitude were set, depending on the unknown spin and orbital parameters of the system.