J/A+A/684/A93 Position and photometric variability (Lambert+, 2024)
VLBI position variability of AGNs is inversely correlated with their photometric
variability.
Lambert S., Secrest N.J.
<Astron. Astrophys. 684, A93 (2024)>
=2024A&A...684A..93L 2024A&A...684A..93L (SIMBAD/NED BibCode)
ADC_Keywords: QSOs ; Interferometry ; Positional data
Keywords: techniques: interferometric - catalogs - reference systems -
quasars: general
Abstract:
The stability of the International Celestial Reference Frame (ICRF),
realized through geodetic very long baseline interferometry (VLBI)
positions of thousands of extragalactic objects, is dependent on the
individual positional stability of these objects. It has been recently
shown that the prevalence of offsets between the VLBI positions of
ICRF objects and their Gaia optical positions, which limit the
optical-radio reference frame tie, is inversely correlated with
optical photometric variability, suggesting that photometrically
variable objects may be more positionally stable. In this work, we
determine the relationship between VLBI position stability of ICRF
objects and optical-radio position offsets as well as optical
photometric variability.
We created multi-epoch geodetic VLBI solutions for a sample of 520
ICRF sources that have sufficient data to determine the variability in
their VLBI positions over time. We compared this position variability
with the fractional photometric variability provided by the Gaia
extragalactic source catalog, the Gaia-ICRF optical-radio position
offsets, the uncertainty-normalized position offsets, and optical
BP-RP color as well as with possible confounders such as optical
magnitude, VLBI/Gaia position error, and redshift. We determined the
relationship between VLBI position stability and gamma-ray detection
by the Fermi Large Area Telescope (LAT), and we determined how the
VLBI position and optical flux variabilities correlate with the
spectral classification of our sample, considering flat spectrum radio
quasars (FSRQs), quasi-stellar objects, BL Lacs, Seyfert, and
gigahertz-peaked spectrum radio sources or compact-steep-spectrum
radio sources.
We found that VLBI astrometric variability is (i) negatively
correlated with optical flux variability, (ii) positively correlated
with optical-radio offsets, (iii) negatively correlated with optical
color index BP-RP, and (iv) negatively correlated with gamma-ray
detection. We also found that the most positionally stable sources are
among the FSRQ and BL Lac classes. In other words, redder,
photometrically variable sources have the most stable VLBI positions,
the smallest optical-radio position offsets, and the highest rate of
gamma-ray detection, and these sources tend to be spectrally
classified as blazars.
Our results are consistent with the most positionally stable sources
being blazars, a class of object in which the jet is oriented close to
the line of sight and where relativistic beaming increases photometric
variability and minimizes the projected offset between the optical and
radio positions. Our study should therefore orient future geodetic
VLBI observing programs preferentially toward sources with high
photometric variability because these sources are predicted to have
better VLBI position stabilities and smaller optical-radio position
offsets, improving the stability of the celestial reference frame
axes.
Description:
The table contains the astrometric positional variability deduced from
VLBI time series computed from the full geodetic VLBI data base along
with the fractional variability taken from Gaia DR3. It also contains
the other quantities of interest used in the study computed by
ourselves from ICRF3 and Gaia catalogs (optical-radio offsets and
their normalized values, semi-major axes of the position error
ellipses in radio and optical) or taken from surveys (gamma detection,
NED's redshifts and spectral classes).
The table gathers the numerical quantities that allowed to conclude
about the inverse correlation between positional variability of AGN
and their photometric (optical) variability, as well as how these
quantity correlates with magnitude, color index, redshift, astrometric
precision, and gamma detection.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 98 520 Astrometric positional variability deduced from
VLBI time series computed from the full
geodetic VLBI data base along with the
fractional variability taken from Gaia DR3
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See also:
I/355 : Gaia DR3 Part 1. Main source (Gaia Collaboration, 2022)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 8 A8 --- IERS IERS B1950 name (HHMM+DDd)
12- 16 F5.3 mas Avar Positional variability from VLBI position
time series
20- 24 F5.3 --- Fvar Fractional variability from Gaia DR3
27- 32 F6.3 mas OR Optical-radio offset
35- 40 F6.3 --- NOR Normalized optical-radio offset
43- 48 F6.3 mag Gmag Gaia DR3 G magnitude
51- 56 F6.3 mag BP-RP Gaia DR3 BP-RP color index
61 I1 --- Class [0/7]?=- Activity Type (1)
66- 74 E9.4 ph/m2/s Gamma ?=- Gamma 1-100 GeV flux from Fermi-LAT 4FGL DR3
78- 82 F5.3 --- z ?=- Redshift from NED
86- 90 F5.3 mas PEVLBI Semi-major axis of the error ellipse from ICRF3
94- 98 F5.3 mas PEGAIA Semi-major axis of the error ellipse
from Gaia DR3
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Note (1): NED Homogenized Classification as follows:
0 = FSRQ
1 = QSO
2 = BL Lac
3 = Seyfert 1
4 = Seyfert 2
5 = GPS
6 = CSS
7 = other
- = undetermined
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Acknowledgements:
Sebastien Lambert, sebastien.lambert(at)obspm.fr
(End) Patricia Vannier [CDS] 07-Feb-2024