2008A&A...489..657Z

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. 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. 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. 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 ∼66s, parameters typical of a Be/X-ray binary. The outbursts last ∼12d. A spin-down of {dot}(P)=0.08 ±0.02 s/yr is also observed, very likely due to the propeller effect. The most accurate X-ray position is RA(2000)=17:49:06.8 and Dec=27:32:32.5 (uncertainty 2''). 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^+1.5x10²²cm^–2, Γ=1.0_–0.3^+0.1, and E_cut=21_–3⁺⁵keV. 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 (>8.5kpc) and highly absorbed (N_H∼1.7x10²²cm^–2). The average 22-50keV luminosity is 0.4-0.9x10³⁶erg/s during the long outbursts and 3x10³⁶erg/s during the bright flare that occurred on MJD 52891 for an assumed distance of 8.5kpc.