Discovery of Kilohertz Quasi‐periodic Oscillations and Shifted Frequency Correlations in the Accreting Millisecond Pulsar XTE J1807−294

Abstract

We report the discovery of twin kilohertz quasi-periodic oscillations ( kHz QPOs) in the X-ray flux of XTE J1807À294, the fourth accreting millisecond pulsar (AMP). This is the second AMP exhibiting twin kHz QPOs. In contrast to the first case, SAX J1808.4À3658, the frequency separation Á between these kHz QPOs is consistent with the pulse frequency (190.6 Hz), not with half that value, confirming for the first time from pulsation measurements the inference, based on burst oscillations, that ''slow rotators'' (spin frequency less than 400 Hz) have Á approximately equal to the spin frequency. While the QPOs move in frequency together over a range of more than 200 Hz, Á remains constant with an average value of 205 AE 6 Hz. Variability components were found in the 5-130 Hz range, similar to those seen in other LMXBs. The correlations between the QPO and noise frequencies are also similar to those in other sources, but shifted by a factor of 1.59 in kHz QPO frequencies, similar to the factor of 1.45 shift found for SAX J1808.4À3658. Our results argue in favor of a spin-related formation mechanism for twin kHz QPOs and against a spin-related cause of the shift in the frequency correlations.

Forced oscillations in fluid tori and quasi‐periodic oscillations

“…In XTEJ1807-294, the peak separation is consistent with the spin frequency (Linares et al 2005), and there generally appears to be a trend whereby the "slow" neutron stars have twin peaks with ∆ν ≈ ν s and the "fast" ones show ∆ν ≈ ν s (with the transition at about 300 Hz).…”

The optical counterparts of accreting millisecond X-ray pulsars during quiescence

“…Campana et al (2003) observed the source during outburst with the XMM-Newton satellite at a luminosity level of about 2 × 10 36 erg s −1 (0.5−10 keV, assuming a distance of 8 kpc) with an X-ray featureless spectrum well fitted by an absorbed (N H ∼ 5 × 10 21 cm −2 ) blackbody plus hard Comptonization model like COMPTT (Titarchuk 1994). Kilohertz quasi-periodic oscilllations (kHz QPOs) have been detected in the X-ray flux of XTE J1807−294 during outburst with a frequency separation consistent with the 191 Hz pulse frequency (Linares et al 2005). From XMM-Newton outburst observations, Kirsch et al (2004) improved the determination of binary orbital parameters and estimated an X-ray mass function of the system of f X (M) = 1.6 × 10 −7 M , which implies (assuming a neutron star mass of 1.4 M and i = 90 • ) a minimum companion mass of 0.007 M .…”

The QPO phenomenon

“…At low frequencies where the relations are clearest, the match can be restored by multiplying the the upper and lower kHz QPO frequencies with ∼1.45, i.e., close to (but significantly different from) 1.5 . The millisecond pulsar XTE J1807-294 behaves in a similar way, but with a shift factor of ∼1.59 (Linares et al 2005). This suggests that the kHz QPOs form one group of correlated frequencies and the low frequency complex another, independent one.…”

“…In their new model a beat interaction also plays a role, but now it occurs not at the inner edge, but at a resonant radius relatively far out, in the disk, where a resonant wave travels around the star at ν spin . In another millisecond pulsar, XTE J1807-294, twin kHz QPOs were found that have ∆ν ≈ ν spin = 191 Hz (Linares et al 2005); so, it seems very likely, as had been suspected earlier from oscillation frequencies measured during X-ray bursts, that the spin frequencies can be either ∼ ∆ν or ∼ 2∆ν. Fig.…”