15 research outputs found
Deep VLBI Observations Challenge Previous Evidence of a Binary Supermassive Black Hole Residing in the Seyfert Galaxy NGC 7674
Previous Ku-band (15 GHz) imaging with data obtained from the Very Long
Baseline Array (VLBA) had shown two compact, sub-pc components at the location
of a presumed kpc-scale radio core in the Seyfert galaxy NGC 7674. It was then
presumed that these two unresolved and compact components were dual radio cores
corresponding to two supermassive black holes (SMBHs) accreting surrounding gas
and launching radio-bright relativistic jets. However, utilizing the original
VLBA dataset used to claim the detection of a binary SMBH, in addition to later
multi-epoch/multi-frequency datatsets obtained from both the VLBA and the
European VLBI Network, we find no evidence to support the presence of a binary
SMBH. We place stringent upper limits to the flux densities of any sub-pc-scale
radio cores which are at least an order of magnitude lower than the original
VLBI radio-core detections, directly challenging the original binary SMBH
detection claim. With this in mind, we discuss the possible reasons for the
non-detection of any VLBI radio cores in our imaging, the possibility of a
binary SMBH still residing in NGC 7674, and the prospect of future observations
shedding further light on the true nature of this active galactic nucleus.Comment: Accepted by Ap
Varstrometry for Off-nucleus and Dual Sub-kpc AGN (VODKA): Very Long Baseline Array Searches for Dual or Off-nucleus Quasars and Small-scale Jets
Dual and off-nucleus active supermassive black holes are expected to be
common in the hierarchical structure formation paradigm, but their
identification at parsec scales remains a challenge due to strict angular
resolution requirements. We conduct a systematic study using the Very Long
Baseline Array (VLBA) to examine 23 radio-bright candidate dual and off-nucleus
quasars. The targets are selected by a novel astrometric technique
("varstrometry") from Gaia, aiming to identify dual or off-nucleus quasars at
(sub)kilo-parsec scales. Among these quasars, 8 exhibit either multiple radio
components or significant (>3) positional offsets between the VLBA and
Gaia positions. The radio emission from the three candidates which exhibit
multiple radio components is likely to originate from small-scale jets based on
their morphology. Among the remaining five candidates with significant
VLBA-Gaia offsets, three are identified as potential dual quasars at parsec
scales, one is likely attributed to small-scale jets, and the origin of the
last candidate remains unclear. We explore alternative explanations for the
observed VLBA-Gaia offsets. We find no evidence for optical jets at kilo-parsec
scales, nor any contamination to Gaia astrometric noise from the host galaxy;
misaligned coordinate systems are unlikely to account for our offsets. Our
study highlights the promise of the varstrometry technique in discovering
candidate dual or off-nucleus quasars and emphasizes the need for further
confirmation and investigation to validate and understand these intriguing
candidates.Comment: 17 pages, 8 figures, 4 tables, submitted to ApJ, comments are welcom
Variability of extragalactic X-ray jets on kiloparsec scales
Unexpectedly strong X-ray emission from extragalactic radio jets on
kiloparsec scales has been one of the major discoveries of Chandra, the only
X-ray observatory capable of sub-arcsecond-scale imaging. The origin of this
X-ray emission, which appears as a second spectral component from that of the
radio emission, has been debated for over two decades. The most commonly
assumed mechanism is inverse Compton upscattering of the Cosmic Microwave
Background (IC-CMB) by very low-energy electrons in a still highly relativistic
jet. Under this mechanism, no variability in the X-ray emission is expected.
Here we report the detection of X-ray variability in the large-scale jet
population, using a novel statistical analysis of 53 jets with multiple Chandra
observations. Taken as a population, we find that the distribution of p-values
from a Poisson model is strongly inconsistent with steady emission, with a
global p-value of 1.96e-4 under a Kolmogorov-Smirnov test against the expected
Uniform (0,1) distribution. These results strongly imply that the dominant
mechanism of X-ray production in kpc-scale jets is synchrotron emission by a
second population of electrons reaching multi-TeV energies. X-ray variability
on the time-scale of months to a few years implies extremely small emitting
volumes much smaller than the cross-section of the jet.Comment: Published in Nature Astronomy 29 May 2023; Supplemental Information
and Excel File include
Circumnuclear Dust in AP Librae and the Source of Its VHE Emission
The broad high-energy spectral component in blazars is usually
attributed to various inverse Compton scattering processes in the
relativistic jet, but has not been clearly identified in most cases due
to degeneracies in physical models. AP Librae, a low-synchrotron-peaking
BL Lac object (LBL) detected in 2015 by H.E.S.S. at very high energies
(VHE; >0.5 TeV), has an extremely broad high-energy spectrum, covering
similar to 9 decades in energy. Standard synchrotron self-Compton models
generally fail to reproduce the VHE emission, which has led to the
suggestion that it might arise not from the blazar core, but on
kiloparsec scales from inverse Compton (IC) scattering of cosmic
microwave background (CMB) photons by a still-relativistic jet (IC/CMB).
IC/CMB models for the TeV emission of AP Librae in prior works have
implied a high level of infrared emission from the kiloparsec-scale jet.
With newly obtained Hubble Space Telescope (HST) imaging, we obtain a
deep upper limit on the kiloparsec-scale jet emission at 1.6 mu m, well
below the expected level. High-resolution Atacama Large
Millimeter/submillimeter Array imaging in bands 3-9 reveals a residual
dust-disk signature after core subtraction, with a clearly thermal
spectrum, and an extent (similar to 500 pc) that matches with a nonjet
residual emission seen after point-spread function subtraction in our
1.6 mu m HST imaging. We find that the unusually broad GeV and VHE
emission in AP Librae can be reproduced through the combined IC
scattering of photons from the CMB and the dust disk, respectively, by
electrons in both the blazar core and subkiloparsec jet