We report on the discovery of mHz quasi-periodic oscillations ( QPOs ) from the high mass X-ray binary ( HMXB ) IGR J19140+0951 , during a 40 ks XMM-Newton observation performed in 2015 , which caught the source in its faintest state ever observed .
At the start of the observation , IGR J19140+0951 was at a low flux of 2 \times 10 ^ { -12 } erg cm ^ { -2 } s ^ { -1 } ( 2-10 keV ; L _ { X } =3 \times 10 ^ { 33 } erg s ^ { -1 } at 3.6 kpc ) , then its emission rised reaching a flux \sim 10 times higher , in a flare-like activity .
The investigation of the power spectrum reveals the presence of QPOs , detected only in the second part of the observation , with a strong peak at a frequency of 1.46 \pm { 0.07 } mHz , together with higher harmonics .
The X–ray spectrum is highly absorbed ( N _ { H } = 10 ^ { 23 } cm ^ { -2 } ) , well fitted by a power-law with a photon index in the range 1.2-1.8 .
The re-analysis of a Chandra archival observation shows a modulation at \sim 0.17 \pm { 0.05 } mHz , very likely the neutron star spin period ( although a QPO can not be excluded ) .
We discuss the origin of the 1.46 mHz QPO in the framework of both disc-fed and wind-fed HMXBs , favouring the quasi-spherical accretion scenario .
The low flux observed by XMM-Newton leads to about three orders of magnitude the source dynamic range , overlapping with the one observed from Supergiant Fast X–ray Transients ( SFXTs ) .
However , since its duty cycle is not as low as in SFXTs , IGR J19140+0951 is an intermediate system between persistent supergiant HMXBs and SFXTs , suggesting a smooth transition between these two sub-classes .