Introduction
The ever-changing universe
Despite these decades of work, many mysteries still remain, including the origin of the hard energy distribution of particles in the nebula (Sironi and Spitkovsky, 2011; Bühler and R. Blandford, 2014). ZTF in particular has driven most of the growth in this area over the past few years.
Slow radio transients: A new window into the dynamic sky
The amplitude of scintillation modulation is a strong function of the size of the radio emitting region. With some additional assumptions, the shock energy, shock velocity, and density of the gas being impacted can also be constrained.
A quick summary of the VLA Sky Survey
Under these assumptions, the number of transients found in a survey should correspond to the probed volume: 𝑁trans ∝ 𝑉survey ∝ 𝐴survey × 𝑑max, where 𝐴survey is the total area covered by the survey and 𝑑max is the maximum distance to which a transient can be detected and identified. The area covered by the survey can simply be increased by observing more fields, at a cost that is linear with the observation time: 𝐴survey ∝ 𝑡obs.
Outline of this thesis
Full details of the search will be presented in a forthcoming paper (Dong et al. in prep). In our model, VT 1137-0337's current position is at the intersection of the orange shaded region (derived from limits on the initial spin-down time 𝜏0) and the blue shaded region (derived from our estimate of𝐸¤𝑑).
A systematic survey of luminous, extragalactic radio transients in
Abstract
The starbursts in our sample are likely dominated by supernovae interacting with the asymmetric and dense surrounding stellar gas ejected from the binary interaction in the ∼ decades to centuries before the explosion. These terminal explosions occur at a rate of up to ~0.3% of the supernova core collapse rate.
Introduction
Radio transient studies are also sensitive to those transients that were not classified by high energy studies as eligible for follow-up. Of these, 10 were not present in Bietenholz, Bartel, et al. 2021) compilation of supernovae with targeted follow-up.
Sample Selection
Altogether, we estimate that our sample contains ≳ 50% of extragalactic ∼mJy radio transients, decadal timescales, in the 𝑧 < 0.045 FIRST trace. Somewhat counter-intuitively, this places an even tighter upper limit on the size of the emitting region.
Transient Classification Scheme
Most of the spectra we considered were taken at the cores of the host galaxy. In rare cases, the available spectrum is off-nuclear and taken at the site of the transient.
Host Galaxy Properties
In Fig. 2.8 we show the stellar mass distributions for our sample compared to the SDSS galaxies. The starburst and TDE patterns are consistent with the SDSS stellar mass distribution within uncertainties.
Radio Properties
The velocity per galaxy is determined by normalizing the observed velocity for each transient class to the number of galaxies with the corresponding BPT classification. For the AGN flares category, we divide by the number of BPT AGN galaxies in SDSS within 200 Mpc.
Discussion
A final spectrum taken -5 years after explosion was consistent with host galaxy emission (Palliyaguru et al., 2019). The double radio peak suggests that there is structure in the radial density profile of the CSM, with the second peak possibly due to a density enhancement at ~1017 cm (Palliyaguru et al., 2019).
Summary and Conclusions
Six of the off-nuclear transitions in our sample have optical counterparts identified either in archival surveys (3 events) or in our subsequent observations (3 events). With a careful enough analysis, the forward evolution of the transient (eg, the increase in size of the emitting region) can be modeled using standard synchrotron theory.
Luminosity Function and Rates Methodology
This is as expected if the observed burst duration (∼15s) is shorter than the MAXI transit duration (∼40s). One contextual clue to the origin of VT 1137-0337 comes from the properties of the host galaxy and the local region surrounding the radio transient.
VT 1210+4956: A merger-driven explosion
Abstract
The radio emission is consistent with a supernova bursting into a dense shell of material that could have been ejected by the binary interaction in the centuries before the explosion. The combination of an early relativistic jet and a dense late-time interaction is consistent with expectations for a merger-driven explosion.
Main Text
This energetic accretion is predicted to launch a jet and cause a fusion-driven explosion (Roger A. Chevalier, 2012; Schrøder et al., 2020; Soker, 2017). We performed a systematic blind search for radio transients in the Very Large Array Sky Survey (VLASS; Lacy et al., 2020).
Materials and Methods
When it reaches the core, theory predicts the formation of an accretion disk and launch of a jet (Roger A. Chevalier, 2012; Schrøder et al., 2020). A steep density profile is thought to be required for fusion during the dynamic inspiratory phase (Taam and Sandquist, 2000; Ivanova et al., 2013).
Transient Identification in the VLA Sky Survey
These binaries can then evolve into doubly compact object systems, with orbits close enough to merge within the lifetime of the Universe, thereby contributing to gravitational wave events (Tauris et al., 2017; Taam and Sandquist, 2000; Ivanova et al. , 2013). In our sample we find an excess of transient candidates within 2 exp of their host galaxies, where the 𝑟 exp of a galaxy is considered the median of all detected bands in Data Release 15 of the Sloan Digital Sky Survey (SDSS) (Aguado et al ., 2019).
Optical Follow-up Spectroscopy
To obtain the target locations, we centered on nearby bright stars and used an offset corresponding to the difference between the SDSS coordinate of the star and the best-fit radio transition coordinate. The broad profile was allowed to vary in amplitude, width (up to 10,000 km/s) and central wavelength (up to 6Å). We additionally marginalize three nuisance parameters corresponding to i) the total zero-point shift for the spectrum, ii) the total shift in the wavelength solution (corresponding to a redshift uncertainty < 0.002) and iii) the local slope for the continuum emission.
Radio Follow-up Observations
Archival X-ray Source
There was no detection when co-observed with the GSC 10-20 keV band, which has a similar 1𝜎 sensitivity to the other two bands (Sugizaki et al., 2011). This outburst is one of 9 MAXI GRBs classified as a MAXI Unidentified Short Soft Transition (MUSST) due to the lack of specimens in further observations (Sugizaki et al., 2011), (Lipunov et al. 2014, GCN#16688, Vargas et al. 2014, GCN#16689) and no detection in the highest-energy MAXI band.
Consistency of VT 1210+4956’s Position with the GRB 140814A
At the time of the burst, the separation is ~0.34 degrees, less than the 1𝜎 uncertainty of the Gaussian response. We additionally used MXSCANCUR to check whether the effective surface area at position VT 1210+4956 was nonzero at the time of burst detections.
False Alarm Probability
Peak Flux and Frequency
Shock Radius and Magnetic Field
Due to the likely aspherical CSM geometry, we marginalize over 𝑓 by assuming that it is drawn from a uniform distribution between 0.1 and 0.5.
Energy in the Shock
Using measurements of the magnetic field and the shock velocity, we can estimate the density of the gas swept up between the follow-up epochs. Material is swept up by the shock at a speed of 𝑣1 ≈ 𝑣shock and the mass density is related to the number density by 𝜌 = 𝜇𝑚𝑝𝑛, where 𝜇 is the average atomic mass.
Total Shocked Mass in the Circumstellar Shell
Mass Loss Rates and CSM Velocities
Given that our observed mass-loss rates are higher, we take this as an approximate lower bound on the mass-loss rate in the wind scenarios. Given that most interactions occur on the main sequence or during the first expansion to a red giant, we adopt the same lower fiducial limit on the mass-loss rate.
Constraints on the Source of MAXI 140814A
In type II supernovae, the narrow H𝛼 line indicates the rate of mass loss before the explosion. A collimated outflow does not change the lower limit of the Lorentz source factor derived above.
Supplementary Text
Second, if the stellar envelope is quiescent at the time the explosion energy is released (i.e., there is no double explosion), photons produced after the shock are emitted at the shock burst and their typical observed energy is 100 keV (Ehud Nakar and Re'em Sari , 2012), which is higher than observed. If the source radiation is directed towards the observer (i.e. the angle between the source velocity and the line of sight is less than 1/𝛾), the limits are similar to the spherical case, but if the radiation is directed away from the observer, the limits are tighter (Matsumoto, Ehud Nakar and Piran, 2019).
Consistency Checks
There are no cataloged supernovae located within the variable source region that exploded within ∼1 month of the X-ray transition ( Guillochon et al., 2017 ). In a search of the SIMBAD database, we found no known flare stars or X-ray binaries within the MAXI localization region.
Constraints on an Optical Supernova
There are also no cataloged galaxies within the MAXI variable source localization region closer than 10 Mpc (Cook et al., 2019). The PTF and Pan-STARRS limits exclude superluminous supernovae (e.g. De Cia et al., 2018) and luminous type IIn supernovae where large amounts of mass are lost near the explosion (Fransson et al., 2014).
Free-free Absorption and Asymmetric CSM
The lack of free absorption requires an explanation, given the high CSM density. Any dense ionizing gas that has not yet been shaken along the line of sight would only increase free absorption.
Rate of similar radio transients
A high temperature, driven by Compton heating from the central X-ray source, has been invoked to explain a similar phenomenon in the fast blue optical transient AT 2018cow (A. Y. Q. Ho, E. Sterl Phinney, et al., 2019). Hydrodynamic simulations have found that the CSM of binary interaction generally forms a toroidal geometry that can be orders of magnitude denser in the midplane than along the poles (Schrøder et al., 2020; MacLeod, Ostriker, and Stone, 2018).
Supplementary Tables
Scott and Readhead, 1977) to estimate the typical strength of the magnetic field in the emitting region. An upper limit on the age of the nebula can be derived from the fact that VT 1137-0337 was detected as a transient.
VT 1137-0337: An emerging pulsar wind nebula
Abstract
VT 1137-0337 is located in the brightest region of a dwarf galaxy at a luminosity distance of 121.6 Mpc. Jets launched by various classes of accreting black holes also struggle to account for the combination of observational properties of VT 1137-0337.
Introduction
In Section 4.3 we discuss the discovery of VT 1137-0337 and its association with its host galaxy, a starburst dwarf. In section 4.6, we evaluate the compliance of VT 1137-0337 with a wide range of classes of transient, variable and persistent radio sources.
Discovery of VT 1137-0337
If the source diameter 𝑑 is chosen on the scale of the scattering disk (corresponding to a diameter of 𝑟 𝑠 ∼ 2×1017 cm at the distance from the host galaxy), the RMS modulation is further reduced by a factor of (𝑟 𝑠/ ) 7/6. Here we present a rate estimate assuming that VT 1137-0337 is a representative sample of the population from which it was drawn (option 1).
Observations and Model Fitting
As shown in the top panel of Figure 4.2, the calibrated phase calibrator spectrum varies smoothly within each epoch. In the first two epochs, the phase calibrator spectrum is well described by a single power law.
Properties of the transient and its environment
If not, this is an upper limit on the quiescent X-ray luminosity of the galaxy. The [SII] 6717Å /6731Å ratio is sensitive to the density of the radiative region, with a small dependence on metallicity.
Astrophysical Analogues and Possible Models
In the lower half of the mass range ∼ 102−3.5𝑀⊙ many of the same arguments regarding stellar mass BHs apply. The behavior of the radio light curve depends on 𝐸¤𝑑, the adiabatic cooling of the radio-emitting electrons and the development of the magnetic field in the nebula.
Summary and Conclusions
We believe that a nebula driven by spindown or field decay is a reasonable explanation for VT 1137-0337. What VT 1137-0337 can teach us about the young wind nebula pulsar population as a whole.
Appendix: Free-free Emission
If 𝑓𝑡 is ℎ∼1, the corresponding lower bound is extreme for a region emitting radio-free emissions. Given that the emission within a dwarf galaxy appeared on a time scale of ~20 years, we conclude that free-free emission is not a reasonable explanation for VT 1137-0337.
Appendix: Literature Transients and Flat Spectrum Sources
PRS/FRB 190520B: The spectral index and luminosity come from the values reported in Niu et al. GRB 980703: We estimate a spectral index of ∼∼ 0.6 from the late data points plotted in Fig. 1 by Granot and Alexander J.
Expanding the search for slow radio transients to the entire north-
Other samples
Identification of transients and variables in VLASS Epoch 2 vs Epoch 1180
Conclusions and next directions
Summary
Transient detection ideas for the near future
![LeMMINGs III. The e-MERLIN legacy survey of the Palomar sample: exploring the origin of nuclear radio emission in active and inactive galaxies through the [O iii] – radio connection](data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==)