J/A+A/694/A96 NESMP sources main variability characteristics (Marchili+, 2025)
Sun-related variability in the light curves of compact radio sources.
A new view on extreme scattering events
Marchili N., Witzel G., Aller M.F.
<Astron. Astrophys. 694, A96 (2025)>
=2025A&A...694A..96M 2025A&A...694A..96M (SIMBAD/NED BibCode)
ADC_Keywords: Active gal. nuclei ; Radio sources ; Sun
Keywords: scattering - methods: data analysis - Sun: heliosphere -
solar-terrestrial relations - quasars: general -
radio continuum: general
Abstract:
Compact radio sources can show remarkable flux density variations at
GHz frequencies on a wide range of timescales. The origin of the
variability is a mix of source-intrinsic mechanisms and propagation
effects, the latter being generally identified with scattering from
the interstellar medium. Some of the most extreme episodes of
variability, however, show characteristics that are not consistent
with any of the explanations commonly proposed.
An in-depth analysis of variability at radio frequencies has been
carried out on light curves from the impressive database of the US
Navy's extragalactic source monitoring program at the Green Bank
Interferometer (GBI) - a long-term project mainly aimed at the
investigation of extreme scattering events - complemented by UMRAO
light curves for selected sources. The purpose of the present work is
to identify events of flux density variations that appear to correlate
with the position of the Sun.
The 2GHz and 8GHz light curves observed in the framework of the GBI
monitoring program have been inspected in a search for one-year
periodic patterns in the data. Variations on timescales below one year
have been isolated through a de-trending algorithm and analysed,
looking for possible correlations with the Sun's position relative to
the sources.
Objects at ecliptic latitude below ∼20° show one-year periodic
drops in flux densities, centred close to the time of minimum solar
elongation; both interplanetary scintillation and instrumental effects
may contribute to these events. However, in some cases the drops
extend to much larger angular distances, affecting sources at high
ecliptic latitudes, and causing variability on timescales of months.
Three different kinds of such events have been identified in the data.
Their exact nature is not yet known.
In the present study we show that the variability of compact radio
sources is heavily influenced by effects that correlate with solar
angular distance; this unexpected contribution significantly alters
the sources' variability characteristics estimated at GHz frequencies.
In particular, we found that many extreme scattering events previously
identified in the GBI monitoring program are actually the consequence
of Sun-related effects; others occur simultaneously in several
objects, which excludes interstellar scattering as their possible
cause. These discoveries have a severe impact on our understanding of
extreme scattering events. Furthermore, Sun-related variability, given
its amplitude and timescale, can significantly alter results of
variability studies, which are very powerful tools for the
investigation of active galactic nuclei. Without a thorough
comprehension of the mechanisms that cause these variations, the
estimation of some essential information about the emitting regions,
such as their size and all the derived quantities, might be seriously
compromised.
Description:
Basic information and variability characteristics of the compact radio
sources monitored within the US Navy's extragalactic source monitoring
program. For each source we specify ecliptic latitude, time of minimum
solar elongation, average flux densities and standard deviations of
the 2 and 8GHz light curves, amplitude of the 2GHz dropout at
minimum solar elongation, SRV/TDV classification according to visual
inspection and automatic detection.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table3.dat 118 148 *Main variability characteristics of the sources
included in the US Navy's extragalactic source
monitoring program (NESMP), Lazio et al.,
2001ApJS..136..265L 2001ApJS..136..265L, Cat. J/ApJS/136/265
--------------------------------------------------------------------------------
Note on table3.dat: the X-ray binary system 1909+048 has been excluded because
of its very strong source-intrinsic variability component.
--------------------------------------------------------------------------------
See also:
J/ApJS/136/265 : Monitoring compact radio sources at 2.5+8.2GHz (Lazio+, 2001)
Byte-by-byte Description of file: table3.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 8 A8 --- Source Source name (HHMM+DDm)
12- 16 F5.1 deg ELAT Ecliptic latitude
21- 24 F4.2 a SEmin Time of the year in which solar elongation
reaches the minimum
28- 32 F5.2 Jy S2GHz Average flux density at 2GHz
37- 40 F4.2 Jy s_S2GHz Standard deviation of the 2 GHz flux density
43- 47 F5.2 Jy S8GHz Average flux density at 8GHz
52- 55 F4.2 Jy s_S8GHz Standard deviation of the 8GHz flux density
59- 63 F5.3 Jy 2Gdip ?=- Amplitude of the dip at minimum solar
elongation in the 2GHz light curve
67- 71 F5.3 Jy e_2Gdip ? Uncertainty in the 2GHz dip estimation
75- 78 A4 --- SRVV2 SRV classification at 2GHz according to
visual inspection
82- 85 A4 --- SRVV8 SRV classification at 8GHz according to
visual inspection
89 A1 --- TDVV [Y] TDV classification according to visual
inspection
93- 99 F7.5 --- AutoProb ? Probability that the number of SRV events
is consistent with an homogeneous
distribution of dips across the year
103-105 A3 --- SRVA2 SRV classification at 2 GHz according to
automatic detection
109-114 F6.4 --- AutoProbT ? Probability that the number of TDV events
is consistent with an homogeneous
distribution of dips across the year
118 A1 --- TDVA [Y] TDV classification according to automatic
detection
--------------------------------------------------------------------------------
Acknowledgements:
Nicola Marchili, nicola.marchili(at)gmail.com
(End) Patricia Vannier [CDS] 10-Dec-2024