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تسجيل مستخدم جديدThe Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. VI. Radio Constraints on a Relativistic Jet and Predictions for Late-Time Emission from the Kilonova Ejecta
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We present Very Large Array (VLA) and Atacama Large Millimeter/sub-millimeter Array ALMA radio observations of GW,170817, the first Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo gravitational wave (GW) event from a binary neutron star merger and the first GW event with an electromagnetic (EM) counterpart. Our data include the first observations following the discovery of the optical transient at both the centimeter ($13.7$ hours post merger) and millimeter ($2.41$ days post merger) bands. We detect faint emission at 6 GHz at 19.47 and 39.23 days after the merger, but not in an earlier observation at 2.46 d. We do not detect cm/mm emission at the position of the optical counterpart at frequencies of 10-97.5 GHz at times ranging from 0.6 to 30 days post merger, ruling out an on-axis short gamma-ray burst (SGRB) for energies $gtrsim 10^{48}$ erg. For fiducial SGRB parameters, our limits require an observer viewer angle of $gtrsim 20^{circ}$. The radio and X-ray data can be jointly explained as the afterglow emission from an SGRB with a jet energy of $sim 10^{49}-10^{50}$ erg that exploded in a uniform density environment with $nsim 10^{-4}-10^{-2}$ cm$^{-3}$, viewed at an angle of $sim 20^{circ}-40^{circ}$ from the jet axis. Using the results of our light curve and spectral modeling, in conjunction with the inference of the circumbinary density, we predict the emergence of late-time radio emission from the deceleration of the kilonova (KN) ejecta on a timescale of $sim 5-10$ years that will remain detectable for decades with next-generation radio facilities, making GW,170817 a compelling target for long-term radio monitoring.
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We present a simple analytic model, that captures the key features of the emission of radiation from material ejected by the merger of neutron stars (NS), and construct the multi-band and bolometric luminosity light curves of the transient associated
with GW170817, AT,2017gfo, using all available data. The UV to IR emission is shown to be consistent with a single $approx0.05$,M$_odot$ component ejecta, with a power-law velocity distribution between $approx 0.1,c$ and $>0.3,c$, a low opacity, {$kappa<1$,cm$^2$,g$^{-1}$}, and a radioactive energy release rate consistent with an initial $Y_{rm e}<0.4$. The late time spectra require an opacity of $kappa_ uapprox0.1$,cm$^2$,g$^{-1}$ at 1 to $2mu$m. If this opacity is provided entirely by Lanthanides, their implied mass fraction is $X_{rm Ln}approx10^{-3}$, approximately 30 times below the value required to account for the solar abundance. The inferred value of $X_{rm Ln}$ is uncertain due to uncertainties in the estimates of IR opacities of heavy elements, which also do not allow the exclusion of a significant contribution to the opacity by other elements (the existence of a slower ejecta rich in Lanthanides, that does not contribute significantly to the luminosity, can also not be ruled out). The existence of a relatively massive, $approx 0.05$,M$_odot$, ejecta with high velocity and low opacity is in tension with the results of numerical simulations of NS mergers.
The binary neutron star (BNS) merger GW170817 was the first astrophysical source detected in gravitational waves and multi-wavelength electromagnetic radiation. The almost simultaneous observation of a pulse of gamma-rays proved that BNS mergers are
associated with at least some short gamma-ray bursts (GRBs). However, the gamma-ray pulse was faint, casting doubts on the association of BNS mergers with the luminous, highly relativistic outflows of canonical short GRBs. Here we show that structured jets with a relativistic, energetic core surrounded by slower and less energetic wings produce afterglow emission that brightens characteristically with time, as recently seen in the afterglow of GW170817. Initially, we only see the relatively slow material moving towards us. As time passes, larger and larger sections of the outflow become visible, increasing the luminosity of the afterglow. The late appearance and increasing brightness of the multi-wavelength afterglow of GW170817 allow us to constrain the geometry of its ejecta and thus reveal the presence of an off-axis jet pointing about 30 degrees away from Earth. Our results confirm a single origin for BNS mergers and short GRBs: GW170817 produced a structured outflow with a highly relativistic core and a canonical short GRB. We did not see the bright burst because it was beamed away from Earth. However, approximately one in 20 mergers detected in gravitational waves will be accompanied by a bright, canonical short GRB.
We present UV, optical, and NIR photometry of the first electromagnetic counterpart to a gravitational wave source from Advanced LIGO/Virgo, the binary neutron star merger GW170817. Our data set extends from the discovery of the optical counterpart a
t $0.47$ days to $18.5$ days post-merger, and includes observations with the Dark Energy Camera (DECam), Gemini-South/FLAMINGOS-2 (GS/F2), and the {it Hubble Space Telescope} ({it HST}). The spectral energy distribution (SED) inferred from this photometry at $0.6$ days is well described by a blackbody model with $Tapprox 8300$ K, a radius of $Rapprox 4.5times 10^{14}$ cm (corresponding to an expansion velocity of $vapprox 0.3c$), and a bolometric luminosity of $L_{rm bol}approx 5times10^{41}$ erg s$^{-1}$. At $1.5$ days we find a multi-component SED across the optical and NIR, and subsequently we observe rapid fading in the UV and blue optical bands and significant reddening of the optical/NIR colors. Modeling the entire data set we find that models with heating from radioactive decay of $^{56}$Ni, or those with only a single component of opacity from $r$-process elements, fail to capture the rapid optical decline and red optical/NIR colors. Instead, models with two components consistent with lanthanide-poor and lanthanide-rich ejecta provide a good fit to the data, the resulting blue component has $M_mathrm{ej}^mathrm{blue}approx 0.01$ M$_odot$ and $v_mathrm{ej}^mathrm{blue}approx 0.3$c, and the red component has $M_mathrm{ej}^mathrm{red}approx 0.04$ M$_odot$ and $v_mathrm{ej}^mathrm{red}approx 0.1$c. These ejecta masses are broadly consistent with the estimated $r$-process production rate required to explain the Milky Way $r$-process abundances, providing the first evidence that BNS mergers can be a dominant site of $r$-process enrichment.
We present Chandra and VLA observations of GW170817 at ~521-743 days post merger, and a homogeneous analysis of the entire Chandra data set. We find that the late-time non-thermal emission follows the expected evolution from an off-axis relativistic
jet, with a steep temporal decay $F_{ u}propto t^{-1.95pm0.15}$ and a simple power-law spectrum $F_{ u}propto u^{-0.575pm0.007}$. We present a new method to constrain the merger environment density based on diffuse X-ray emission from hot plasma in the host galaxy and we find $nle 9.6 times 10^{-3},rm{cm^{-3}}$. This measurement is independent from inferences based on the jet afterglow modeling and allows us to partially solve for model degeneracies. The updated best-fitting model parameters with this density constraint are a fireball kinetic energy $E_0 = 1.5_{-1.1}^{+3.6}times 10^{49},rm{erg}$ ($E_{iso}= 2.1_{-1.5}^{+6.4}times10^{52}, rm{erg}$), jet opening angle $theta_{0}= 5.9^{+1.0}_{-0.7},rm{deg}$ with characteristic Lorentz factor $Gamma_j = 163_{-43}^{+23}$, expanding in a low-density medium with $n_0 = 2.5_{-1.9}^{+4.1} times 10^{-3}, rm{cm^{-3}}$ and viewed $theta_{obs} = 30.4^{+4.0}_{-3.4}, rm{deg}$ off-axis. The synchrotron emission originates from a power-law distribution of electrons with $p=2.15^{+0.01}_{-0.02}$. The shock microphysics parameters are constrained to $epsilon_{rm{e}} = 0.18_{-0.13}^{+0.30}$ and $epsilon_{rm{B}}=2.3_{-2.2}^{+16.0} times 10^{-3}$. We investigate the presence of X-ray flares and find no statistically significant evidence of $ge2.5sigma$ of temporal variability at any time. Finally, we use our observations to constrain the properties of synchrotron emission from the deceleration of the fastest kilonova ejecta with energy $E_k^{KN}propto (Gammabeta)^{-alpha}$ into the environment, finding that shallow stratification indexes $alphale6$ are disfavored.
We present a comprehensive comparison of the properties of the radio through X-ray counterpart of GW170817 and the properties of short-duration gamma-ray bursts (GRBs). For this effort, we utilize a sample of 36 short GRBs spanning a redshift range o
f $z approx 0.12-2.6$ discovered over 2004-2017. We find that the counterpart to GW170817 has an isotropic-equivalent luminosity that is $approx 3000$ times less than the median value of on-axis short GRB X-ray afterglows, and $gtrsim10^{4}$ times less than that for detected short GRB radio afterglows. Moreover, the allowed jet energies and particle densities inferred from the radio and X-ray counterparts to GW170817 and on-axis short GRB afterglows are remarkably similar, suggesting that viewing angle effects are the dominant, and perhaps only, difference in their observed radio and X-ray behavior. From comparison to previous claimed kilonovae following short GRBs, we find that the optical and near-IR counterpart to GW170817 is comparatively under-luminous by a factor of $approx 3-5$, indicating a range of kilonova luminosities and timescales. A comparison of the optical limits following short GRBs on $lesssim 1$ day timescales also rules out a blue kilonova of comparable optical isotropic-equivalent luminosity in one previous short GRB. Finally, we investigate the host galaxy of GW170817, NGC4993, in the context of short GRB host galaxy stellar population properties. We find that NGC4993 is superlative in terms of its large luminosity, old stellar population age, and low star formation rate compared to previous short GRB hosts. Additional events within the Advanced LIGO/VIRGO volume will be crucial in delineating the properties of the host galaxies of NS-NS mergers, and connecting them to their cosmological counterparts.
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