Discovery of quasi-periodic oscillations in the new X-ray pulsar XTE J1858+034

ATel #11; B. Paul (Tata Institute of Fundamental Research)
on 19 Mar 1998; 16:59 UT
Credential Certification: B. Paul (bpaul@tifrvax.tifr.res.in)

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Discovery of quasi-periodic oscillations in the new X-ray pulsar XTE J1858+034

B. Paul
Department of Astronomy and Astrophysics
Tata Institute of Fundamental Research
Homi Bhabha Road, Mumbai 400 005, India

We have discovered low frequency quasi-periodic oscillations (QPO) at a frequency of 0.11 Hz in the newly discovered 221 s X-ray pulsar XTE J1858+034 (IAUC 6826, 6828). We have used public archival data of four observations of this source made with the PCA detectors of the RXTE during 1998, February 20 and 24. The QPO feature in the power density spectrum (PDS) is centered at 0.11 ± 0.01 Hz and the rms variability at the QPO frequency is ~6.5%. Apart from the gaussian QPO feature, the PDS, in the frequency range of 0.006-0.6 Hz is power-law type with an index of -0.95. The energy spectrum in 2-60 keV range has been analyzed for one of the observations and it is found to be very hard and has the following components :
a) black body component of temperature 3.5 ± 0.2 keV,
b) iron fluroscence line at 6.60 ± 0.07 keV of equivalent width 200 ± 40ev,
c) power-law component with photon spectral index of 1.56 ± 0.10 and
d) an ionized absorber.
Total incident flux in the 1.3 - 100 keV observation band of the PCA detectors, is 9 × 10^-10 erg cm^-2 s^-1.

This is the fifth X-ray pulsar after Cen X-3, EXO 2030+375, 4U 1626-67 and GRO J1744-28 in which QPOs have been detected. Assuming that the QPOs are produced as a result of some inhomogeneity at the magnetospheric boundary with the disk rotating at the Keplerian frequency (f_K), the Keplerian frequency f_K is derived to be 0.11 Hz . For a neutron star of mass 1.4 M_Sun, this indicates that the magnetosphere radius r_M is 7.3 × 10⁸ cm. From the X-ray luminosity of this pulsar for an assumed distance of r_kpc, the magnetic field of this pulsar is estimated to be about ~10¹² r_kpc Gauss, where r_kpc is the distance of the pulsar expressed in kiloparsec.

The Keplerian frequency of the disk at the magnetospheric radius is ~ 0.2 Hz in three of these five pulsars in which QPOs are observed. These are Cen X-3, EXO 23030+375 and 4U 1626-67. In the bursting pulsar GRO J1744-28 the magnetic field strength is estimated to be much smaller which is consistent with a smaller magnetosphere with Keplerian frequency of the disk at magnetosphere being 40 Hz. This new pulsar with lowest f_K appears to be the one having a largest magnetosphere.