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Discovery of 0.19 Hz QPO in MAXI J1409-619

ATEL # 3082; Kaur, R. (Univ. of Amsterdam); Casella, P. (Univ. of Southampton); Linares, M. (MIT); Altamirano, D,; Patruno, A,; Armas-Padilla, M,; Cavecchi, Y,; Degenaar, N,; Russell, D,; Kalamkar, M,; Klis, M. Van Der,; Watts, A,; Wijnands, R, (Univ. of Amsterdam); Rea, N. (CSIC-IEEC)
on 18 Dec 2010; 14:49 UT
Password Certification: Ramanpreet Kaur (raman.pk@gmail.com)

Subjects: X-ray, Binaries, Neutron Stars, Pulsars, Transients

The RXTE has been monitoring the new transient X-ray source MAXI J1409-619 since its discovery on October 20, 2010 (Yamaoka et al 2010, ATEL # 2959) . We analyzed the observations of this source performed on December 11, 2010 during the current re-brightening of the source (Ueno et al 2010, ATEL # 3067). The observation lasted for a total exposure time of about 20 ks. We detect a quasi-periodic oscillation (QPO) with a centroid frequency 0.192 +/- 0.006 Hz and with a width of 0.029 +/- 0.013 Hz. The QPO is detected with a significance of 5.0 sigma (single trial).

We also detect two harmonics of the QPO in the power density spectrum of MAXI J1409-619 at a frequency of 0.422 +/- 0.012 Hz and 0.621 +/- 0.018 Hz, and with a width of 0.047 +/- 0.036 Hz and 0.076 +/- 0.070 Hz, respectively. Both the harmonics are detected with a significance of 5.5 and 3.6 sigma, respectively. Visual inspection of other long observations taken after this observation also shows the presence of the same QPO and its harmonics.

MAXI J1409-619 has been identified as a high mass X-ray pulsar (HMXB), with P_spin = 506 s (Yamamoto et al. 2010, ATEL # 3070). The QPO_frequency/Spin_frequency ratio of 96 makes the source the HMXB with the fastest known QPO, relatively to the spin (other HMXB QPOs detected so far can be checked in James et al. 2010, MNRAS, 407, 285, table-3). As the QPO frequency is much larger than the spin frequency, the Keplerian and the beat frequency models are applicable and would give similar values of the radius at which the 0.19 Hz QPO is produced. Assuming a neutron star of mass 1.4 solar masses, we calculate the radius of the QPO production region to be ~ 5000 km. During this observation, the unabsorbed X-ray flux of MAXI J1409-619 in the 2-10 keV band is 2.9 e-10 erg/s/cm^2 which amounts to an X-ray luminosity of 7.3 e+36 erg/s for a source distance of 14.5 kpc (Orlandini et al. 2010, astroph-1012.1218v1). Assuming that the QPO are produced at the inner edge of the accretion disk truncated by the magnetosphere, we calculate the neutron star surface strength to be 2.1 e+12 G assuming a dipole field and a 10 km neutron star radius, which is typical for accretion powered X-ray pulsars and is compatible with Orlandini et al. 2010.

The summed PCA spectrum in the 3-40 keV range is well fitted with a cut-off powerlaw model plus a narrow iron line. The best fit parameters for this model are power-law index = 0.90 +/- 0.02 and e-folding energy = 19.9 +/- 0.9 keV. The reduced chi square for this fit is 0.9 for 63 degrees of freedom.


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