High Frequency Quasi Periodic Oscillations

High-frequency QPOs (HFQPOs; 40-450 Hz) have been detected in seven sources (5 black hole binaries and 2 black hole candidates). These oscillations are transient and subtle (a ~ 0.01), and they attract interest primarily because their frequencies are in the expected range for matter in orbit near the ISCO for a ~ 10 Mq black hole.

The entire sample of HFQPOs with strong detections (>4a) is shown in Figure 3. Three sources have exhibited single oscillations. The other four sources display pairs of HFQPOs with frequencies that scale in a 3:2 ratio. Most often, these pairs of QPOs are not detected simultaneously. The four sources are GRO J1655-40 (300, 450 Hz), XTE J1550-564 (184, 276 Hz), GRS 1915+105 (113, 168 Hz), and H 1743-322 (165, 241 Hz). GRS 1915+105 also has a second pair of HFQPOs with frequencies that are not in a 3:2 ratio (41, 67 Hz).

HFQPOs are of further interest because they do not shift freely in frequency in response to sizable luminosity changes (factors of 3-4). There is evidence of frequency shifts in the HFQPO at lower frequency (referring to the 3:2 pairing), but such variations are limited to 15%. This is an important difference between these black hole binary HFQPOs and the variable-frequency kHz QPOs seen in accreting neutron stars, where both peaks can shift in frequency by a factor of two. Overall, black hole binary HFQPOs appear to be a stable and identifying "voice-print" that may depend only on the mass and spin of the black hole.

All of the strong detections (>4a) above 100 Hz occur in the SPL state. In three of the sources that exhibit HFQPOs with a 3:2 frequency ratio, the 2vo QPO appears when the PL flux is very strong, whereas 3vo appears when the PL flux is weaker. Currently, there is no explanation for this result.

The commensurate frequencies of HFQPOs suggests that these oscillations are driven by some type of resonance condition. Abramowicz and Kluzniak (2001) proposed that orbiting blobs of accreting matter could generate the harmonic frequencies via a resonance between a pair of the coordinate frequencies given by GR. Earlier work had used GR coordinate frequencies and

GRS1915+105

I GRS1915+105 r41, 67 Hz

GRS1915+105

I GRS1915+105 r41, 67 Hz lO-5 r

10"e

10"e

lO"4 10-6 lO"8

E 4U1630-

- V 1

Hz

1

10 100 100100 100 1000

100 100010

'i XTEJ 1859+226 190 Hz

100 1000

100 100010 100 100010 Frequency (Hz)

100 100010

100 1000

Fig. 3. High-frequency quasi-periodic oscillations (HFQPOs) observed in black hole binary and black hole candidate systems. The traces in blue show power density spectra (PDSs) for the range 13-30 keV. Red traces indicate PDSs with a broader energy range, which may be either 2-30 or 6-30 keV. Reprinted with permission from Volume 44 of Annual Reviews of Astronomy & Astrophysics (RM06).

10 100 100100 100 1000

associated beat frequencies to explain fast QPOs in both neutron-star and black hole systems (Stella et al. 1999), but without invoking a resonance condition. Current work on resonances as a means of explaining HFQPOs includes more realistic models for fluid flow in the Kerr metric. Resonance models are considered in more detail in ยง6.2.

Was this article helpful?

0 0

Post a comment