IGR J18483-0311: an accreting X-ray pulsar observed by INTEGRAL

Abstract

Context. IGR J18483−0311 is a poorly known transient hard X-ray source discovered by INTEGRAL during observations of the Galactic Center region performed between 23-28 April 2003. Aims. To detect new outbursts from IGR J18483−0311 using INTEGRAL and archival Swift XRT observations and finally to characterize the nature of this source using the optical/near−infrared (NIR) information available through catalogue searches. Methods. We performed an analysis of light curves and spectra of INTEGRAL and archival Swift XRT data as well as of optical/NIR catalogues. Results. We report on 5 newly discovered outbursts from IGR J18483−0311 detected by INTEGRAL. For two of them it was possible to constrain a duration of the order of a few days. The strongest outburst reached a peak flux of ∼120 mCrab (20-100 keV); its broad band JEM-X/ISGRI spectrum (3-50 keV) is best fitted by an absorbed cutoff power law with Γ=1.4±0.3, cutoff energy of 22 +7.5 −4.5 keV and N H =9 +5 −4 ×10 22 cm −2 . Timing analysis of INTEGRAL data allowed us to identify periodicities of 18.52 days and 21.0526 seconds which are likely the orbital period of the system and the spin period of the X-ray pulsar respectively. Swift XRT observations of IGR J18483−0311 provided a very accurate source position which strongly indicates a highly reddened star in the USNO-B1.0 and 2MASS catalogues as its possible optical/NIR counterpart. Conclusions. The X-ray spectral shape, the periods of 18.52 days and 21.0526 seconds, the high intrinsic absorption, the location in the direction of the Scutum spiral arm and the highly reddened optical object as possible counterpart, all favour the hypothesis that IGR J18483−0311 is a HMXB with a neutron star as compact companion. The system is most likely a Be X-ray binary, but a Supergiant Fast X-ray Transient nature can not be entirely excluded.

Transient outburst mechanisms in Supergiant Fast X-ray Transients

“…This is naturally provided by a second wind component crossing the neutron star orbit. We note that periodicities in the outburst recurrence begin to be found also in other SFXTs: IGR J18483-0311 displays a recurrence periodicity of ∼18.5 days (Sguera et al, 2007b); this source was at first classified as a Be X-ray transient, but very recently optically identified the companion as a blue supergiant, thus the source is actually a SFXT; IGR J08408-4503 displays an X-ray activity (multiple flare duration and Xray outburst recurrence) which is compatible with an orbital period of 35 days (see Romano et al 2009 for details).…”

“…This property seems to be in contrast with the hypothesis suggested by Bozzo et al (2008) that SFXTs are magnetars. Moreover, Bozzo et al (2008) suggest that, to get the large dynamic ranges observed in SFXTs, these transients should host neutron stars with long spin periods (of the order of 1000 s or larger), which is not observed in SFXTs, to date: indeed, four SFXTs are X-ray pulsars with much shorter spin periods: IGR J11215-5952 (187 s, Swank et al 2007), AX J1841.0-0536 (4.7 s, Bamba et al 2001), IGR J18483-0311 (21 s, Sguera et al 2007b), IGR J16465-4507 (228 s, Lutovinov et al 2005). The large dynamic range between the quiescence and the maximum luminosity at the peak of the flares can, in any case, be explained already if the supergiant winds are clumpy with density contrasts as large as 10 5 (e.g.…”

High-mass X-ray binaries in the Milky Way

“…INTEGRAL data also showed that IGR J18483-0311 sporadically displays a few days-long X-ray active states (∼3 days), during which fast flares with typical timescales of a few hours can be observed (Krimm et al, 2011;Romano et al, 2010b;Ducci et al, 2013a). Pulsations with a period of ∼21 s were first reported by Sguera et al (2007). Giunta et al (2009) discussed the possible detection of pulsations during the low X-ray intensity states of the source.…”

Unveiling the nature of INTEGRAL objects through optical spectroscopy

“…Several approaches using X-ray data analysis have shown their effectiveness in identifying the nature of some of these new INTEGRAL sources, such as X-ray timing (e.g., Walter et al 2006;Sguera et al 2007;Del Santo et al 2007;La Parola et al 2010) or spectroscopy and imaging (see for instance Rodriguez et al 2010;Malizia et al 2010;Tomsick et al 2012, and references therein). A&A 556, A120 (2013) Alternatively and also effectively, cross-correlation with soft X-ray catalogues and consequent optical spectroscopy on thereby selected candidates allows the determination of the nature and the main multiwavelength characteristics of unidentified or poorly studied hard X-ray objects.…”