The new intermediate long-bursting source XTE J1701-407

Aims: XTE J1701-407 is a newly discovered X-ray transient source. We investigate its flux variability and study the intermediate long- and short-bursts discovered by Swift on July 17, and 27, 2008, respectively.
Methods: Only one intermediate long-burst, of duration ≈18 min and ten days later a short burst, have been recorded from XTE J1701-407. We analyzed the public available data from Swift and RXTE, and compared the observed properties of the intermediate long-burst with theoretical ignition condition and light curves to investigate the possible nuclear-burning processes.
Results: The intermediate long-burst may have exhibited a photospheric radius expansion, allowing us to derive the source distance at 6.2 kpc, assuming the empirically derived Eddington luminosity for pure helium. The intermediate long-burst decay was described most accurately by using two exponential functions with e-folding times of τ₁ = 40±3 s and τ₂ = 221±9 s. The bursts occurred at a persistent luminosity of L_per = 8.3×10³⁶ erg s^-1 (≈2.2% of the Eddington luminosity). For the intermediate long-burst, the mass accretion rate per unit area onto the neutron star was dot{m}≈4×10³ g cm^-2 s^-1, and the total energy released was E_burst≈3.5×10⁴⁰ erg. This corresponds to an ignition column depth of y_ign≈1.8×10⁹ g cm^-2, for a pure helium burning. We find that the energetics of this burst can be modeled in different ways, as (i) pure helium ignition, as the result of either pure helium accretion or depletion of hydrogen by steady burning during accumulation; or (ii) as ignition of a thick layer of hydrogen-rich material in a source of low metallicity. However, comparison of the burst duration with model light curves suggests that hydrogen burning plays an important role during the burst, and therefore that this source is a low accretion-rate burster with a low metallicity in the accreted material.