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Register a new userDetermining the viewing angle of neutron star merger jets with VLBI radio images
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Added by
Joseph John Fern\\'andez
Publication date
2021
fields
Physics
and research's language is
English
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Very long base interferometry (VLBI) radio images recently proved to be essential in breaking the degeneracy in the ejecta model for the neutron star merger event GW170817. We discuss the properties of synthetic radio images of merger jet afterglow by using semi-analytic models of laterally spreading or non-spreading jets. The image centroid initially moves away from the explosion point in the sky with an apparent superlumianal velocity. After reaching a maximum displacement its motion is reversed. This behavior is in line with that found in full hydrodynamics simulations. Since the evolution of the centroid shift and jet image size are significantly different in the two jet models, observations of these characteristics for very bright events might be able to confirm or constrain the lateral expansion law of merger jets. We explicitly demonstrate how $theta_{rm obs}$ is obtained by the centroid shift of radio images or its apparent velocity provided the ratio of the jet core size $theta_{c}$ and the viewing angle $theta_{rm obs}$ is determined by afterglow light curves. We show that a simple method based on a point-source approximation provides reasonable angular estimates ($10-20%$ errors at most). By taking a sample of structured Gaussian jet results, we find that the model with $theta_{rm obs} sim 0.32$ rad can explain the main features of the GW170817 afterglow light curves and the radio images.
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Gravitational waves have been detected from a binary neutron star merger event, GW170817. The detection of electromagnetic radiation from the same source has shown that the merger occurred in the outskirts of the galaxy NGC 4993, at a distance of 40 megaparsecs from Earth. We report the detection of a counterpart radio source that appears 16 days after the event, allowing us to diagnose the energetics and environment of the merger. The observed radio emission can be explained by either a collimated ultra-relativistic jet viewed off-axis, or a cocoon of mildly relativistic ejecta. Within 100 days of the merger, the radio light curves will distinguish between these models and very long baseline interferometry will have the capability to directly measure the angular velocity and geometry of the debris.
VLBI and JVLA observations revealed that GW170817 involved a narrow jet ($ theta_j approx 4^circ $) that dominated the afterglow peak at our viewing angle, $ theta_{rm obs} approx 20^circ $. This implies that at the time of the afterglow peak, the observed signal behaved like an afterglow of a top-hat jet seen at $ theta_{rm obs} gg theta_j $, and it can be modeled by analytic expressions that describe such jets. We use a set of numerical simulations to calibrate these analytic relations and obtain generic equations for the peak time and flux of such an afterglow as seen from various observing angles. Using the calibrated equations and the estimated parameters of GW170817, we estimate the detectability of afterglows from future double neutron star mergers during the Advanced LIGO/Virgo observation run O3. GW170817 took place at a relatively low-density environment. Afterglows of similar events will be detectable only at small viewing angles, $ theta_{rm obs} lesssim 20^circ $, and only $sim 20% $ of the GW detections of these events will be accompanied by a detectable afterglow. At higher densities, more typical to sGRB sites, up to $ 70% $ of the GW detections are expected to be followed by a detectable afterglow, typically at $ theta_{rm obs} sim 30^circ $. We also provide the latest time one should expect an afterglow detection. We find that for typical parameters, if the jet emission had not been detected within about a year after the merger, it is unlikely to be ever detected.
Jets can become collimated as they propagate through dense environments and understanding such interactions is crucial for linking physical models of the environments to observations. In this work, we use 3D special-relativistic simulations to study how jets propagate through the environment created around a neutron star merger remnant by neutrino-driven winds. We simulate four jets with two different initial structures, top-hat and Gaussian, and two luminosities. After jet breakout, we study the angular jet structures and the resulting afterglow light curves. We find that the initial angular structures are efficiently washed out during the propagation, despite the small wind mass of only $sim 10^{-3}$ M$_odot$. The final structures depend, however, on the jet luminosity, as less energetic jets are more strongly collimated. Although entrainment of baryons leads to only moderate outflow Lorentz factors ($approx 40$), all simulated jets can well reproduce the afterglow observed in the aftermath of GW170817. The inferred physical parameters (e.g. inclination angle, ambient particle number density), however, vary substantially between the fits and appear to be sensitive to smaller details of the angular jet shape, indicating that observationally inferred parameters may depend sensitively on the employed jet models.
The LIGO/Virgo Consortium (LVC) released a preliminary announcement of a candidate gravitational wave signal, S190426c, that could have arisen from a black hole-neutron star merger. As the first such candidate system, its properties such as masses and spin are of great interest. Although LVC policy prohibits disclosure of these properties in preliminary announcements, LVC does release the estimated probabilities that this system is in specific categories, such as binary neutron star, binary black hole and black hole-neutron star. LVC also releases information concerning relative signal strength, distance, and the probability that ejected mass or a remnant disc survived the merger. In the case of events with a finite probability of being in more than one category, such as is likely to occur with a black hole-neutron star merger, it is shown how to estimate the masses of the components and the spin of the black hole. This technique is applied to the source S190426c.
The first observation of a binary neutron star coalescence by the Advanced LIGO and Advanced Virgo gravitational-wave detectors offers an unprecedented opportunity to study matter under the most extreme conditions. After such a merger, a compact remnant is left over whose nature depends primarily on the masses of the inspiralling objects and on the equation of state of nuclear matter. This could be either a black hole or a neutron star (NS), with the latter being either long-lived or too massive for stability implying delayed collapse to a black hole. Here, we present a search for gravitational waves from the remnant of the binary neutron star merger GW170817 using data from Advanced LIGO and Advanced Virgo. We search for short ($lesssim1$ s) and intermediate-duration ($lesssim 500$ s) signals, which includes gravitational-wave emission from a hypermassive NS or supramassive NS, respectively. We find no signal from the post-merger remnant. Our derived strain upper limits are more than an order of magnitude larger than those predicted by most models. For short signals, our best upper limit on the root-sum-square of the gravitational-wave strain emitted from 1--4 kHz is $h_{rm rss}^{50%}=2.1times 10^{-22}$ Hz$^{-1/2}$ at 50% detection efficiency. For intermediate-duration signals, our best upper limit at 50% detection efficiency is $h_{rm rss}^{50%}=8.4times 10^{-22}$ Hz$^{-1/2}$ for a millisecond magnetar model, and $h_{rm rss}^{50%}=5.9times 10^{-22}$ Hz$^{-1/2}$ for a bar-mode model. These results indicate that post-merger emission from a similar event may be detectable when advanced detectors reach design sensitivity or with next-generation detectors.
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