INTEGRAL, XMM-Newton and ESO/NTT identification of AX J1749.1-2733: an obscured and probably distant Be/X-ray binary

Abstract

Context: AX J1749.1-2733 is an unclassified transient X-ray source discovered during surveys by ASCA in 1993-1999. The transient behaviour and the short and bright flares of the source have led to the idea that it is part of the recently revealed subclass of supergiant fast X-ray transients.
Aims: A multi-wavelength study in NIR, optical, X-rays, and hard X-rays of AX J1749.1-2733 is undertaken in order to determine its nature.
Methods: Public INTEGRAL data and our target of opportunity observation with XMM-Newton were used to study the high-energy source through timing and spectral analysis. Multi-wavelength observations in optical and NIR with the ESO/NTT telescope were also performed to search for the counterpart.
Results: AX J1749.1-2733 is a new high-mass X-ray binary pulsar with an orbital period of 185.5±1.1 days (or 185.5/f with f=2,3 or 4) and a spin period of 66 s, parameters typical of a Be/X-ray binary. The outbursts last 12 d. A spin-down of dot{P}=0.08 ± 0.02 s yr ^-1 is also observed, very likely due to the propeller effect. The most accurate X-ray position is RA (2000) =17^h49^m06.8^s and Dec =-27°32 arcmin32.5 arcsec (uncertainty 2 arcsec). The high-energy broad-band spectrum is well-fitted with an absorbed powerlaw and a high-energy cutoff with values N_H=20.1_-1.3^+1.5×10²² cm^-2, Γ=1.0_-0.3^+0.1, and E_cut=21_-3⁺⁵ keV. The only optical/NIR candidate counterpart within the X-ray error circle has magnitudes of R=21.9±0.1, I=20.92±0.09, J=17.42±0.03, H=16.71±0.02, and K_s=15.75±0.07, which points towards a Be star located far away (>8.5 kpc) and highly absorbed (N_H∼ 1.7×10²² cm^-2). The average 22-50 keV luminosity is 0.4-0.9×10³⁶ erg s^-1 during the long outbursts and 3×10³⁶ erg s^-1 during the bright flare that occurred on MJD 52891 for an assumed distance of 8.5 kpc.

Based on observations made with 1) INTEGRAL, an ESA project with instruments and science data centre funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain), Czech Republic and Poland, and with the participation of Russia and the USA; 2) XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA; and 3) ESO Telescopes at the La Silla or Paranal Observatories under programme ID 079.D-0432(A).