Time Domain Studies and Fundamental Physics

Left: stacked 42 epoch total intensity (contours) and linear polarization (color) of the VLBA imacge at 0.8mas resolution of the z=0.6 neutrino blazar PKS1424+340. The position angle of the electric vectors are shown as line segments. Right: the magnetic field direction in a linear integral convolution projection (Kovalev et al. 2025, A&A 700, L12).The broad frequency coverage, high sensitivity and resolution, and temporal flexibility keep the NRAO facilities at the forefront of studies of the most extreme energetic events in the Cosmos. Radio observations have long been critical to multi-messenger astronomy, and to understanding the origin of photons, particles, and neutrinos at the highest energies. Recent VLBA observations have illuminated the physical processes involved in the prominent neutrino emitting blazar Active Galactic Nuclei (AGN) at z = 0.6, PKS1424+240, in unprecedented detail. A stack of 42 epochs of VLBA observations at 15 GHz reveals the persistent, parsec-scale emission, with a morphology that implies that the object is viewed inside the jet cone, i.e. straight down the axis of its relativistic jet, with a viewing angle of <0.6o. This projection effectively maximizes Doppler boosting to values ∼30 and greatly boosts the electromagnetic and neutrino emission in the direction of the observer. The polarized emission implies a toroidal component in the magnetic field of the jet, suggesting a current-carrying jet that flows directly toward our line of sight. Blazars with very small jet viewing angles offer a solution to the Doppler factor crisis, i.e., to the longstanding mismatch between Doppler factors inferred from the low apparent jet speeds determined by Very Long Baseline Interferometry (VLBI) with higher speeds derived from observations of ultra-high energy photons (eg gamma-rays and beyond). Relativistic beaming clearly plays the critical role in the gamma-ray and neutrino emission of blazars.

Figure: Left: stacked 42 epoch total intensity (contours) and linear polarization (color) of the VLBA imacge at 0.8mas resolution of the z=0.6 neutrino blazar PKS1424+340. The position angle of the electric vectors are shown as line segments. Right: the magnetic field direction in a linear integral convolution projection (Kovalev et al. 2025, A&A 700, L12).

Upper limits (95% confidence) on the number density of FFOs for different mass ranges in the vicinity of 10 of the longest timed pulsars in the NANOgrav dataset. The combined upper limits on the FFO number density from all 68 pulsars are also shown with black squares (Dey et al. 2025 arXiv:2507.19475). The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) survey, anchored by the GBT, continues to break new ground in the study of gravitational waves from merging supermassive black holes. Most recently, NANOGrav has set limits on free-floating objects (FFO) in interstellar space— rogue planets, brown dwarfs, and large asteroids that are not gravitationally bound to any star. Such objects have been seen in microlensing surveys, as well as through the discovery of interstellar asteroids passing through our solar system on hyperbolic trajectories. Pulsar timing can potentially show signatures of such objects passing near the pulsar. The NANOGrav 15-year data set has set an upper limit to the number density of such encounters in our Galactic neighborhood. For example, the upper limit on the number density for Jupiter-mass FFOs is < 6x105 pc-3.

Figure: Upper limits (95% confidence) on the number density of FFOs for different mass ranges in the vicinity of 10 of the longest timed pulsars in the NANOgrav dataset. The combined upper limits on the FFO number density from all 68 pulsars are also shown with black squares (Dey et al. 2025 arXiv:2507.19475).

Two examples of offset AGN identified through cross correlation of the VLASS and the SDSS. In these cases, the radio AGN has an optical counterpart. Color frames are SDSS g+r+I and the grayscale image is the VLASS. The scale bars are 10’’ (Barrows &amp; Comerford 2025, arXiv:2509.09768). The VLA Sky Survey (VLASS) has become the gold-standard for high resolution, high sensitivity, synoptic surveys of the radio sky. A recent cross correlation of VLASS AGN candidates with galaxies in the Sloan Digital Sky Survey (SDSS) has made the most reliable measurement of the demographics of off-nuclear (i.e. outside the galaxy center), massive black holes (MBH, 105 to 109 M⊙) to date. These systems likely represent the remnants of galaxy mergers (major and minor), where the MBH may wander indefinitely within the merging galaxy halos. Tracing this population is key to understanding the efficiency of binary MBH formation (a key parameter in nanoHz gravitational wave statistics), and the rates at which MBHs are seeded in low-mass satellite galaxies. The cross correlation identified 328 offset AGN candidates, of which about 30% are likely chance projections. The offset AGN occupation fraction is positively correlated with host galaxy stellar mass, consistent with predictions that most offset MBH will reside in massive halos. This trend vanishes, and may reverse, at the lowest stellar masses, potentially reflecting the weaker host galaxy gravitational potentials. The results suggest a binary MBH formation rate of < 0.5 per merger.

Figure: Two examples of offset AGN identified through cross correlation of the VLASS and the SDSS. In these cases, the radio AGN has an optical counterpart. Color frames are SDSS g+r+I and the grayscale image is the VLASS. The scale bars are 10’’ (Barrows &amp; Comerford 2025, arXiv:2509.09768).

Left: VLA images of the off-nuclear TDE at z = 0.045 before and after the advent of the radio source. Right: Radio light curves showing the double peak and the unprecedented fast temporal evolution (Margutti et al. ApJ 992, L18).

Tidal disruption events (TDE) occur when a star falls into the gravitational potential of a massive black hole, being subsequently torn apart by the extreme tidal forces of a massive black hole. The debris forms an accretion disk, driving broadband electromagnetic emission, possibly a strong wind from the disk, and even a relativistic jet. TDEs provide information of MBH demographics, and the stellar populations in their vicinity, as well as physical processes in transient accretion disks and relativistic jet formation. The VLA and ALMA, with an assortment of other telescopes, have recently discovered the first off-nuclear TDE, indicating association with an MBH originating from the merger of a smaller satellite galaxy. The off-nuclear TDE exhibits double-peaked radio light curves and the fastest-evolving radio emission observed from a TDE to date. The observations suggest a self- and free-free absorbed synchrotron spectrum. The source may be explained by either a single outflow launched 80 days after the event, or including a second outflow launched 100 days later. The fast evolution and double peak may relate to the off-nuclear position and the local stellar and gaseous environment, as well as a potentially lower mass for the MBH (105 M⊙).

Figure: Left: VLA images of the off-nuclear TDE at z = 0.045 before and after the advent of the radio source. Right: Radio light curves showing the double peak and the unprecedented fast temporal evolution (Margutti et al. ApJ 992, L18).

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