QPOs in the time domain: an autocorrelation analysis

Context. Motivated by the recent proposal that one can obtain quasi-periodic oscillations (QPOs) by photon echoes manifesting as non-trivial features in the autocorrelation function (ACF), we study the ACFs of the light curves of three accreting black hole candidates and a neutron star already known to exhibit QPOs namely, GRS 1915+105, XTE J1550-564, XTE J1859+226 and Cygnus X-2. Aims. We present a comparative study of the timing properties of these systems in the frequency and time domain in search for similarities/differences that may provide clues to the physics underlying the QPO phenomenon. Methods. We compute and focus on the form of the ACFs in search of systematics or specific temporal properties at the time scales associated with the known QPO frequencies in comparison with the corresponding PDS. Results. Even within our small object sample we find both similarities as well as significant and subtle differences in the form of the ACFs both amongst black holes and between black holes and neutron stars to warrant a closer look at the QPO phenomenon in the time domain: the QPO features manifest as an oscillatory behavior of the ACF at lags near zero; the oscillation damps exponentially on time scales equal to the inverse QPO width to a level of a percent or so. In black holes this oscillatory behavior is preserved and easily discerned at much longer lags while this is not the case for the neutron star system Cyg X-2. The ACF of GRS 1915+105 provides an exception to this general behavior in that its decay is linear in time indicating an undamped oscillation of coherent phase. We present simple ad hoc models that reproduce these diverse time domain behaviors and we speculate that their origin is the phase coherence of the underlying oscillation. Conclusions. It appears plausible that time domain analyses, complementary to the more common frequency domain ones, could impose tighter constraints and provide clues for the driving mechanisms behind the QPO phenomenon.

[1]  D. Kazanas,et al.  QUASI-PERIODIC OSCILLATIONS FROM RANDOM X-RAY BURSTS AROUND ROTATING BLACK HOLES , 2009, 0901.2858.

[2]  D. Kazanas,et al.  Light Echoes in Kerr Geometry: A Source of High-Frequency QPOs from Random X-Ray Bursts , 2007, 0712.1084.

[3]  S. Kuznetsov,et al.  Correlations between X-Ray Spectral and Timing Characteristics in Cygnus X-2 , 2007, 0706.1595.

[4]  J. McClintock,et al.  X-Ray Properties of Black-Hole Binaries , 2006, astro-ph/0606352.

[5]  W. Lewin,et al.  Compact stellar X-ray sources , 2006 .

[6]  P. Uttley,et al.  Where are the X-ray quasi-periodic oscillations in active galaxies? , 2005 .

[7]  S. Vaughan,et al.  Where are the X-ray QPOs in active galaxies? , 2005, astro-ph/0506455.

[8]  L. Stella,et al.  A study of the low-frequency quasi-periodic oscillations in the X-ray light curves of the black hole candidate XTE J1859+226 , 2004, astro-ph/0407262.

[9]  J. Schnittman.  Interpreting the High-Frequency Quasi-periodic Oscillation Power Spectra of Accreting Black Holes , 2004, astro-ph/0407179.

[10]  J. Orosz,et al.  Evidence for Harmonic Relationships in the High-Frequency Quasi-periodic Oscillations of XTE J1550–564 and GRO J1655–40 , 2002, astro-ph/0202305.

[11]  M. Muno,et al.  Characterizing the Quasi-periodic Oscillation Behavior of the X-Ray Nova XTE J1550–564 , 2002 .

[12]  T. Strohmayer.  Discovery of a Second High-Frequency Quasi-periodic Oscillation from the Microquasar GRS 1915+105 , 2001, astro-ph/0105338.

[13]  R. Wijnands,et al.  The Complex Phase-Lag Behavior of the 3-12 Hz Quasi-Periodic Oscillations during the Very High State of XTE J1550–564 , 1999, The Astrophysical journal.

[14]  W. Cui.  Phase Lags of Quasi-periodic Oscillations in Microquasar GRS 1915+105 , 1999 .

[15]  D. Kazanas,et al.  Modeling the Time Variability of Accreting Compact Sources , 1999, astro-ph/9902186.

[16]  W. Cui,et al.  Probing the Structure of Accreting Compact Sources through X-Ray Time Lags and Spectra , 1998, astro-ph/9810143.

[17]  Jochen Greiner,et al.  RXTE Observations of QPOs in the Black Hole Candidate GRS 1915+105 , 1997 .

[18]  N. White,et al.  Quasi-periodic oscillations in the X-ray flux of Cyg X-2 , 1986, Nature.

[19]  Martin C. Weisskopf,et al.  On searches for pulsed emission with application to four globular cluster X-ray sources - NGC 1851, 6441, 6624, and 6712 , 1983 .