Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe Optical Afterglow of GW170817: An Off-axis Structured Jet and Deep Constraints on a Globular Cluster Origin
133
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We present a revised and complete optical afterglow light curve of the binary neutron star merger GW170817, enabled by deep Hubble Space Telescope (HST) F606W observations at $approx!584$ days post-merger, which provide a robust optical template. The light curve spans $approx 110-362$ days, and is fully consistent with emission from a relativistic structured jet viewed off-axis, as previously indicated by radio and X-ray data. Combined with contemporaneous radio and X-ray observations, we find no spectral evolution, with a weighted average spectral index of $langle beta rangle = -0.583 pm 0.013$, demonstrating that no synchrotron break frequencies evolve between the radio and X-ray bands over these timescales. We find that an extrapolation of the post-peak temporal slope of GW170817 to the luminosities of cosmological short GRBs matches their observed jet break times, suggesting that their explosion properties are similar, and that the primary difference in GW170817 is viewing angle. Additionally, we place a deep limit on the luminosity and mass of an underlying globular cluster of $L lesssim 6.7 times 10^{3},L_{odot}$, or $M lesssim 1.3 times 10^{4},M_{odot}$, at least 4 standard deviations below the peak of the globular cluster mass function of the host galaxy, NGC4993. This limit provides a direct and strong constraint that GW170817 did not form and merge in a globular cluster. As highlighted here, HST (and soon JWST) enables critical observations of the optical emission from neutron star merger jets and outflows.
rate research
Read More
We present new observations of the binary neutron star merger GW170817 at $Delta tapprox 220-290$ days post-merger, at radio (Karl G. Jansky Very Large Array; VLA), X-ray (Chandra X-ray Observatory) and optical (Hubble Space Telescope; HST) wavelengths. These observations provide the first evidence for a turnover in the X-ray light curve, mirroring a decline in the radio emission at $gtrsim5sigma$ significance. The radio-to-X-ray spectral energy distribution exhibits no evolution into the declining phase. Our full multi-wavelength dataset is consistent with the predicted behavior of our previously published models of a successful structured jet expanding into a low-density circumbinary medium, but pure cocoon models with a choked jet cannot be ruled out. If future observations continue to track our predictions, we expect that the radio and X-ray emission will remain detectable until $sim 1000$ days post-merger.
The jet structure of short gamma-ray bursts (GRBs) has been controversial after the detection of GRB 170817A as the electromagnetic counterparts to the gravitational wave event GW170817. Different authors use different jet structures for calculating the afterglow light curves. We formulated a method to inversely reconstruct a jet structure from a given off-axis GRB afterglow, without assuming any functional form of the structure. By systematically applying our inversion method, we find that more diverse jet structures are consistent with the observed afterglow of GRB 170817A within errors: such as hollow-cone, spindle, Gaussian, and power-law jet structures. In addition, the total energy of the reconstructed jet is arbitrary, proportional to the ambient density $n_0$, with keeping the same jet shape if the parameters satisfy the degeneracy combination $n_0 varepsilon_mathrm{B}^{(p+1)/(p+5)} varepsilon_mathrm{e}^{4(p-1)/(p+5)} = mathrm{const.}$. Observational accuracy less than $sim 6$ per cent is necessary to distinguish the different shapes, while the degeneracy of the energy scaling would be broken by observing the spectral breaks and viewing angle. Future events in denser environment with brighter afterglows and observable spectral breaks are ideal for our inversion method to pin down the jet structure, providing the key to the jet formation and propagation.
In light of the most recent observations of late afterglows produced by the merger of compact objects or by the core-collapse of massive dying stars, we research the evolution of the afterglow produced by an off-axis top-hat jet and its interaction with a surrounding medium. The medium is parametrized by a power law distribution of the form $n(r)propto r^{-k}$ is the stratification parameter and contains the development when the surrounding density is constant ($k=0$) or wind-like ($k=2$). We develop an analytical synchrotron forward-shock model when the outflow is viewed off-axis, and it is decelerated by a stratified medium. Using the X-ray data points collected by a large campaign of orbiting satellites and ground telescopes, we have managed to apply our model and fit the X-ray spectrum of the GRB afterglow associated to SN 2020bvc with conventional parameters. Our model predicts that its circumburst medium is parametrized by a power law with stratification parameter $k=1.5$.
The X-ray emission of gamma-ray bursts (GRBs) is often characterized by an initial steep decay, followed by a nearly constant emission phase (so called plateau) which can extend up to thousands of seconds. While the steep decay is usually interpreted as the tail of the prompt gamma-ray flash, the long-lasting plateau is commonly associated to the emission from the external shock sustained by energy injection from a long lasting central engine. A recent study proposed an alternative interpretation, ascribing both the steep decay and the plateau to high-latitude emission (HLE) from a structured jet whose energy and bulk Lorentz factor depend on the angular distance from the jet symmetry axis. In this work we expand over this idea and explore more realistic conditions: (a) the finite duration of the prompt emission, (b) the angular dependence of the optical depth and (c) the lightcurve dependence on the observer viewing angle. We find that, when viewed highly off-axis, the structured jet HLE lightcurve is smoothly decaying with no clear distinction between the steep and flat phase, as opposed to the on-axis case. For a realistic choice of physical parameters, the effects of a latitude-dependent Thomson opacity and finite duration of the emission have a marginal effect on the overall lightcurve evolution. We discuss the possible HLE of GW170817, showing that the emission would have faded away long before the first Swift-XRT observations. Finally, we discuss the prospects for the detection of HLE from off-axis GRBs by present and future wide-field X-ray telescopes and X-ray surveys, such as eROSITA and the mission concept THESEUS.
Binary neutron-star mergers (BNSMs) are among the most readily detectable gravitational-wave (GW) sources with LIGO. They are also thought to produce short $gamma$-ray bursts (SGRBs), and kilonovae that are powered by r-process nuclei. Detecting these phenomena simultaneously would provide an unprecedented view of the physics during and after the merger of two compact objects. Such a Rosetta Stone event was detected by LIGO/Virgo on 17 August 2017 at a distance of $sim 44$ Mpc. We monitored the position of the BNSM with ALMA at 338.5 GHz and GMRT at 1.4 GHz, from 1.4 to 44 days after the merger. Our observations rule out any afterglow more luminous than $3times 10^{26}~{rm erg,s}^{-1},{rm Hz}^{-1}$ in these bands, probing $>$2--4 dex fainter than previous SGRB limits. We match these limits, in conjunction with public data announcing the appearance of X-ray and radio emission in the weeks after the GW event, to templates of off-axis afterglows. Our broadband modeling suggests that GW170817 was accompanied by a SGRB and that the GRB jet, powered by $E_{rm AG,,iso}sim10^{50}$~erg, had a half-opening angle of $sim20^circ$, and was misaligned by $sim41^circ$ from our line of sight. The data are also consistent with a more collimated jet: $E_{rm AG,,iso}sim10^{51}$~erg, $theta_{1/2,,rm jet}sim5^circ$, $theta_{rm obs}sim17^circ$. This is the most conclusive detection of an off-axis GRB afterglow and the first associated with a BNSM-GW event to date. Assuming a uniform top-hat jet, we use the viewing angle estimates to infer the initial bulk Lorentz factor and true energy release of the burst.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
