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

Abstract

Context. Transient low-mass X-ray binaries (LMXBs) often show outbursts lasting typically a few-weeks and characterized by a high X-ray luminosity (L x ≈ 10 36 − 10 38 erg s −1 ), while for most of the time they are found in X-ray quiescence (L X ≈ 10 31 − 10 33 erg s −1 ). EXO 1745-248 is one of them. Aims. The broad-band coverage, and the sensitivity of instrumet on board of XMM-Newton and INTEGRAL, offers the opportunity to characterize the hard X-ray spectrum during EXO 1745-248 outburst. Methods. In this paper 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 16th) shown by the source between March and June 2015. The source was caught in a hard state, emitting a 0.8-100 keV luminosity of ≃ 10 37 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 moderate optical depth τ ≃ 2 and electron temperature 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.

Rapid X-ray variability properties during the unusual very hard state in neutron-star low-mass X-ray binaries

“…Matranga et al 2017), although we do find a small decrease in strength at the lowest and highest frequencies. The decrease at high frequencies was not explicitly mentioned by Matranga et al (2017), although it can be seen from their Figure 6 as well.…”

“…Similarly to Matranga et al (2017) we found that the PDS of EXO 1745-248 (green curve in left panel of Figure 4) is relatively flat over the full frequency range (in the νPν representation, which is different than what was used by 9 When this PDS of 1RXS J180408.9-342058 is fitted with a broken power-law model plus a Lorentzian, the frequencies of those components are 0.72 +0.03 −0.01 Hz and 4.1±0.1 Hz, respectively. These values are fully consistent with the relation found by Wijnands & van der Klis (1999a) for these components.…”

“…Since this system is an AMXP this mechanism might indeed be present, although it is unclear why only this AMXP would show this behaviour and none of the other AMXPs. 1RXS J180408.9-342058 and EXO 1745-248 do not show obvious pulsations in the XMM-Newton data (see, e.g., the detailed pulse search performed for EXO 1745-248 by Matranga et al 2017) and no pulsations could be seen in the RXTE data of EXO 1745-248 either (Altamirano et al 2017) 12 . Although pulsations could be still present in those two sources, it is striking to note that when a particular AMXP exhibits millisecond X-ray pulsations, those oscillations always show up very readily in its PDS (i.e., so far all AMXPs were discovered when making simple PDS without the need for sophisticated pulse search methods).…”

“…In contrast, the high frequency PDS (> 100 Hz) of 1RXS J180408.9-342058 and EXO 1745-248 did not show any sign at all for pulsations. This strongly suggest that both sources do not pulsate when they are accreting (at least they are 12 The upper limits on the signal amplitude on any pulsations in EXO 1745-248 were between a few percent and 15 percent, depending on the method used and the assumptions made on the spin and orbital parameters of the source (Matranga et al 2017). A similar pulse search for 1RXS J180408.9-342058 would likely result in comparable upper limits but such a detailed study is beyond the scope of our paper.…”

A broadband spectral analysis of 4U 1702-429 using XMM-Newton and BeppoSAX data

“…Typical values of the reflection amplitude Ω/2π for NS-LMXB atoll sources are within 0.2-0.3 (Matranga et al 2017b;Di Salvo et al 2015;Egron et al 2013;D'Aì et al 2010). However, smaller reflection fractions are also observed for other systems, such as Ser X-1 (Matranga et al 2017a;Cackett et al 2010) and 4U 1820-30 (Cackett et al 2010) and for AMSP SAX J1748.9-2021 .…”