In
Extraordinary Activity in the BL Lacertae Object OJ 287
we use a continuous wavelet transform to analyze more than two decades
of data for the BL Lac object OJ 287 acquired as part of the University
of Michigan Radio Astronomy Observatory (UMRAO) variability program. We
find clear evidence for a persistent modulation of the total flux and
polarization with period ~1.66 yr and for another signal that dominates
activity in the 1980s with period ~1.12 yr. The relationship between these
two variations can be understood in terms of a "shock-in-jet" model, in which
the longer timescale periodicity is associated with an otherwise quiescent
jet and the shorter timescale activity is associated with the passage of
a shock. The different periodicities of these two components may reflect
different internal conditions of the two flow domains leading to different
wave speeds or different contractions of a single underlying periodicity
due to the different Doppler factors of the two flow components. We suggest
that the modulation arises from a wave driven by some asymmetric disturbance
close to the central engine. The periodic behavior in polarization exhibits
excursions in U that correspond to a direction ~45 deg from the VLBI
jet axis. This behavior is not explained by the random walk in the Q-U
plane that is expected from models in which a pattern of randomly aligned
magnetic field elements propagate across the visible portion of the flow
and suggests a small amplitude, cyclic variation in the flow direction in
that part of the flow that dominates centimeter wavelength emission.
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Quasi-periodic signals have yielded important constraints on the masses of
black holes in galactic X-ray binaries, and in
Wavelet Analysis of AGN X-Ray Time Series: A QPO in 3C 273?
we have extended this to active galactic nuclei (AGN). We have employed a
wavelet technique to analyze 19 observations of 10 AGN obtained with the
XMM-Newton EPIC-PN camera. We report the detection of a statistically
significant 3.3 kilosecond quasi-period in 3C 273. If this period
represents an orbital time scale originating near a last stable orbit of 3
Rs, it implies a central black hole mass of 7.3 x 106
Msun. For a maximally rotating black hole with a last stable
orbit of 0.6 Rs, a central black hole mass of 8.1 x 107
Msun is implied. Both of these estimates are substantially lower
than previous reverberation mapping results which place the central black
hole mass of 3C 273 at about 2.35 x 10 8 Msun. Assuming
that this reverberation mass is correct, the X-ray quasi-period must be
caused by a higher order oscillatory mode of the accretion disk.
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We have used a combination of wavelet and cross-wavelet
transforms to quantify the quasiperiodicity in the
total flux light curves of AGN monitored as part of the ( University
of Michigan Radio Astronomy Observatory ) program between 1980.0 and
2005.0. Statistically significant quasiperiodic variations have already been
reported for the sources OJ 287 and 0048-097. No other sources are found
to exhibit such clear quasiperiodicity, but ~20% of all sources exhibit
such variations at the 99% confidence level, for at least part of the
time spanned by the data. We interpret these flux variations as arising
from the propagation of the normal modes of a relativistic jet, leading
to a periodic variation in the lateral flow displacement, and thus to a
concomitant variation in local flow direction and Doppler boost. The need
to occasionally re-excite such modes, and small realignment of the overall
flow direction, might be significantly modulating the observed amplitude
of this behavior, giving it a somewhat transient appearance. This behavior
is normally `masked' by the larger amplitude, more random fluctuations
in the flow, evident on superficial inspection of light curves. This work
will be submitted to ApJ in Fall 2007.
| See Wiki Wavelets for a good introduction and lots of links. | For the application of spectral techniques, rather than wavelets, see: Spectral Methods |
