Discovery of a quasi-periodic oscillation in the X-ray pulsar 1A 1118-615: correlated spectral and aperiodic variability

Abstract

Aims. Our goal is to investigate the X-ray timing and spectral variability of the high-mass X-ray binary 1A 1118-615 during a type-II outburst.Methods. We performed a detailed color, spectral, and timing analysis of a giant outburst from 1A 1118-615 using RXTE data. Results. We report the discovery of a variable quasi-periodic oscillation (QPO) in the power spectral density of 1A 1118-615, with a centroid frequency of ∼0.08 Hz. The centroid frequency of the QPO correlates with the X-ray flux, as expected according to the most accredited models for QPO production. For energies above ∼4 keV, the QPO rms variability decreases as the energy increases. Pulse profiles display an energy dependence with a two-peak profile at lower energies and a single peak at higher energies. From the spectral analysis we confirm the presence of a cyclotron absorption feature at ∼60 keV, the highest value measured for an X-ray pulsar. We find that the spectral parameters (photon index, cutoff energy, iron fluorescence line strength) display a marked dependence with flux. We detect two different levels of neutral hydrogen column density, possibly owing to to the Be companion activity. We report for the first time a correlation between the timing and spectral parameters in an X-ray pulsar. All correlations found between spectral/timing parameters and X-ray flux are present up to a flux of ∼6 × 10 −9 erg cm −2 s −1 , when a saturation level is reached. We propose that the observed saturation corresponds to the minimum extent of the neutron star magnetosphere. We estimate the magnetic field of the neutron star from two independent ways, using results from spectral (cyclotron line energy) and timing (QPO frequency) analysis, obtaining consistent values of ∼7−8 × 10 12 G. Results from the comprehensive spectral and timing analysis are discussed in comparison with other X-ray pulsars.

Timing and broad-band spectroscopy of 1A 1118−61 with Suzaku

“…The energy dependence of the pulse profiles of the Be HMXB pulsar 1A 1118−61 during its 2009 outburst shows a multipeaked structure below ∼10 keV, and a single asymmetric peak thereafter. These features have been reported previously using RXTE observations by Doroshenko et al (2010), Devasia et al (2011a) and Nespoli & Reig (2011). Owing to the better energy resolution, and the broader energy coverage, especially below 3 keV, the Suzaku data show a more detailed and clearer picture of the energy dependence of the pulse profiles.…”

“…In addition, we have detected a soft‐excess peak (in the 0.3–2 keV range) at the main dip of the pulse profiles for both observations. Broad‐band spectral analysis of the RXTE observations has been performed by Doroshenko et al (2010), Devasia et al (2011a) and Nespoli & Reig (2011) who reported the presence of a CRSF at ∼55 keV. Suchy et al (2011) performed broad‐band spectral analysis of the same Suzaku observations using partial covering cut‐off power‐law and thermal Comptonization models and confirmed the presence of the CRSF feature at ∼55 keV, discussing also the possibility of the presence of a first harmonic at ∼110 keV.…”

RXTE-PCA observations of 1A 1118−61: timing and spectral studies during an outburst

“…The QPO discovery mentioned in this paper was presented at the 38th COSPAR Scientific Assembly, 2010 July, Bremen, Germany. After initial submission of this paper, discovery of the same has also been reported by Nespoli & Reig (2011).…”

High-mass X-ray binaries in the Milky Way

“…Three type II outbursts have been detected in 38 years featuring correlated X-rays and Hα fluxes (Coe et al, 1994b). A cyclotron absorption feature at 55 keV (Doroshenko et al, 2010b) and QPOs (Nespoli and Reig, 2011) have been detected leading to the magnetic field estimations of (7 − 8) × 10 12 G. The companion is a O9.5IV-Ve star located at 3-7 kpc (Janot-Pacheco et al, 1981).…”