J/A+A/693/A318 AGN radio variability at 37GHz (Kankkunen+, 2025)
Long-term radio variability of active galactic nuclei at 37 GHz.
Kankkunen S., Tornikoski M., Hovatta T., Lahteenmaki A.
<Astron. Astrophys. 693, A318 (2025)>
=2025A&A...693A.318K 2025A&A...693A.318K (SIMBAD/NED BibCode)
ADC_Keywords: Active gal. nuclei ; QSOs ; Radio sources
Keywords: methods: data analysis - galaxies: active - quasars: general
Abstract:
We present the results of analysing the long-term radio variability of
active galactic nuclei at 37GHz using data of 123 sources observed in
the Aalto University Metsahovi Radio Observatory. Our aim was to
constrain the characteristic timescales of the studied sources and to
analyse whether up to 42 years of monitoring was enough to describe
their variability behaviour.
We used a periodogram to estimate the power spectral density of each
source. The power spectral density is used to analyse the power
content of a time series in the frequency domain, and it is a powerful
tool in describing the variability of active galactic nuclei. We were
interested in finding a bend frequency in the power spectrum, that is,
a frequency at which the slope β of the spectrum changes from a
non-zero value to zero. We fitted two models to the periodograms of
each source, namely the bending power law and the simple power law.
The bend frequency in the bending power law corresponds to a
characteristic timescale.
We were able to constrain a timescale for 11 out of 123 sources, with
an average characteristic timescale xb=1300 days and an average
power-law slope β=2.3. The results suggest that up to 42 years of
observations may not always be enough for obtaining a characteristic
timescale in the radio domain. This is likely caused by a combination
of both slow variability as well as sampling-induced effects. We also
compared the obtained timescales to 43GHz very long baseline
interferometry images. The maximum length of time a knot was visible
was often close to the obtained characteristic timescale. This
suggests a connection between the characteristic timescale and the jet
structure.
Description:
We analysed the long-term radio variability of 123 sources observed by
the Metsahovi Radio Observatory in 37GHz. We utilised the longest
monitoring periods in the 37 GHz band to date, maximum of which
extended to 42 years of observations with an average monitoring period
of 34.5 years. Our sample of 123 sources was also exceptionally large
for such analysis, allowing unique insights into the long-term radio
variability of AGNs.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 107 123 *Results from the bending power-law fit (Eq. 4),
simple power-law fit (Eq. 3), and bending
power-law fit using
βhigh=βbpl,best (Eq. 6)
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Note on tablea1.dat: All values are reported with their 90% confidence regions,
where the slope and timescale constraints for the bending power-law fits
(Eq. 4) are the minimum and maximum values obtained from some combination of
the two parameters within the bending power-law fit 90% confidence region.
A missing value in the table indicates a rejection confidence of over 90%.
The probed limits for the timescales were xb,min=100 days and
xb,max=7000 days, and the probed limits for the slopes were
βmin=1 and βmax=3.5.
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Byte-by-byte Description of file: tablea1.dat
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Bytes Format Unit Label Explanations
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1- 9 A9 --- Source Source name
12- 23 A12 --- OName Other name
25- 28 I4 --- N Total number of data points in the light curve
30- 37 A8 --- Type Type of object
40- 42 F3.1 --- betabpl ? Best-fit bending power-law slope
βbpl (1)
44- 46 F3.1 --- e_betabpl ? Best-fit bending power-law slope
βbpl error (lower value)
48- 50 F3.1 --- E_betabpl ? Best-fit bending power-law slope
βbpl error (upper value)
52- 55 I4 --- xb ? Best-fit bending power-law bend timescale (1)
57- 60 I4 --- e_xb ? Best-fit bending power-law bend timescale
error (lower value)
62- 65 I4 --- E_xb ? Best-fit bending power-law bend timescale
error (upper value)
67- 70 F4.2 --- pbpl ? Best-fit bending power-law slope p-value (1)
72- 74 F3.1 --- betaspl ? Best-fit simple power-law slope
76- 78 F3.1 --- e_betaspl ? Best-fit simple power-law slope error
(lower value)
80- 82 F3.1 --- E_betaspl ? Best-fit simple power-law slope error
(upper value)
84- 87 F4.2 --- pspl ? Best-fit p-value
89- 92 I4 --- xb1 ? Best-fit bending power-law bend timescale (2)
94- 97 I4 --- e_xb1 ? Best-fit bending power-law bend timescale
error (lower value)
99-102 I4 --- E_xb1 ? Best-fit bending power-law bend timescale
error (upper value)
104-107 F4.2 --- pbpl1 ? Best-fit bending power-law bend timescale
p-value (2)
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Note (1): Best-fit bending power-law slope βbpl and bend timescale xb
(Only the timescales for the 11 constrained sources should be considered
reliable. For the other sources more observations are needed.) using Eq. 4,
as well as the corresponding p-values (pbpl).
Note (2): For the case with βlow=1 using Eq. 6, we give the best-fit
bending power-law bend timescale (xb1) where βhigh=βbpl,best.
The corresponding p-values are given by pbpl1.
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 21-May-2025