Stevens, A. L., Uttley, P., Altamirano, D., Arzoumanian, Z., Bult, P., Cackett, E. M., Fabian, A. C., Gendreau, K. C., Ha, K. Q., Homan, J., Ingram, A. R., Kara, E., Kellogg, J., Ludlam, R. M., Miller, J. M., Neilsen, J., Pasham, D. R., Remillard, R. A., Steiner, J. F. and Eijnden, J. Van Den (2018) A NICER discovery of a low-frequency quasi-periodic oscillation in the soft-intermediate state of MAXI J1535-571. Astrophysical Journal Letters, 865 (2), [L15]. (doi:10.3847/2041-8213/aae1a4).
Abstract
We present the discovery of a low-frequency ≈5.7 Hz quasi-periodic oscillation (QPO) feature in observations of the black hole X-ray binary MAXI J1535-571 in its soft-intermediate state, obtained in 2017 September-October by the Neutron Star Interior Composition Explorer. The feature is relatively broad (compared to other low-frequency QPOs; quality factor Q ≈ 2) and weak (1.9% rms in 3-10 keV), and is accompanied by a weak harmonic and low-amplitude broadband noise. These characteristics identify it as a weak Type A/B QPO, similar to ones previously identified in the soft-intermediate state of the transient black hole X-ray binary XTE J1550-564. The lag-energy spectrum of the QPO shows increasing soft lags toward lower energies, approaching 50 ms at 1 keV (with respect to a 3-10 keV continuum). This large phase shift has similar amplitude but opposite sign to that seen in Rossi X-Ray Timing Explorer data for a Type B QPO from the transient black hole X-ray binary GX 339-4. Previous phase-resolved spectroscopy analysis of the Type B QPO in GX 339-4 pointed toward a precessing jet-like corona illuminating the accretion disk as the origin of the QPO signal. We suggest that this QPO in MAXI J1535-571 may have the same origin, with the different lag sign depending on the scale height of the emitting region and the observer inclination angle.
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