arXiv subjects

R. Chornock

Publications and source records attributed to R. Chornock.

At least 55 records · Page 3Linked to original sources

arXiv2018

Target of Opportunity Observations of Gravitational Wave Events with LSST

The discovery of the electromagnetic counterparts to the binary neutron star merger GW170817 has opened the era of GW+EM multi-messenger astronomy. Exploiting this breakthrough requires increasing samples to explore the diversity of kilonova behaviour and provide more stringent constraints on the Hubble constant, and tests of fundamental physics. LSST can play a key role in this field in the 2020s, when the gravitational wave detector network is expected to detect higher rates of merger events involving neutron stars ($\sim$10s per year) out to distances of several hundred Mpc. Here we propose comprehensive target-of-opportunity (ToOs) strategies for follow-up of gravitational-wave sources that will make LSST the premiere machine for discovery and early characterization for neutron star mergers and other gravitational-wave sources.

astro-ph.HE↗

arXiv2018

An embedded X-ray source shines through the aspherical AT2018cow: revealing the inner workings of the most luminous fast-evolving optical transients

We present the first extensive radio to gamma-ray observations of a fast-rising blue optical transient (FBOT), AT2018cow, over its first ~100 days. AT2018cow rose over a few days to a peak luminosity $L_{pk}\sim4\times 10^{44}$ erg/s exceeding those of superluminous supernovae (SNe), before declining as $\propto t^{-2}$. Initial spectra at $\lesssim 15$ days were mostly featureless and indicated large expansion velocities v~0.1c and temperatures reaching 30000 K. Later spectra revealed a persistent optically-thick photosphere and the emergence of H and He emission features with v~sim 4000 km/s with no evidence for ejecta cooling. Our broad-band monitoring revealed a hard X-ray spectral component at $E\ge 10$ keV, in addition to luminous and highly variable soft X-rays, with properties unprecedented among astronomical transients. An abrupt change in the X-ray decay rate and variability appears to accompany the change in optical spectral properties. AT2018cow showed bright radio emission consistent with the interaction of a blastwave with $v_{sh}$~0.1c with a dense environment ($\dot M\sim10^{-3}-10^{-4}\,M_{\odot}yr^{-1}$ for $v_w=1000$ km\s). While these properties exclude Ni-powered transients, our multi-wavelength analysis instead indicates that AT2018cow harbored a "central engine", either a compact object (magnetar or black hole) or an embedded internal shock produced by interaction with a compact, dense circumstellar medium. The engine released $\sim10^{50}-10^{51.5}$ erg over $\sim10^3-10^5$ s and resides within low-mass fast-moving material with equatorial-polar density asymmetry ($M_{ej,fast}\lesssim0.3\,\rm{M_{\odot}}$). Successful SNe from low-mass H-rich stars (like electron-capture SNe) or failed explosions from blue supergiants satisfy these constraints. Intermediate-mass black-holes are disfavored by the large environmental density probed by the radio observations.

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arXiv2018

A Hydrogen-Poor Superluminous Supernova with Enhanced Iron-Group Absorption: A New Link Between SLSNe and Broad-Lined Type Ic SNe

We present optical observations of the Type I superluminous supernova (SLSN-I) SN2017dwh at $z\!\approx\!0.13$, which reached $M_{i}\!\approx\!-21$ mag at peak. Spectra taken a few days after peak show an unusual and strong absorption line centered near 3200Å that we identify with Co II, suggesting a high fraction of synthesized $^{56}$Ni in the ejecta. By $\sim\!1$ month after peak, SN2017dwh became much redder than other SLSNe-I, instead strongly resembling broad-lined Type Ic supernovae (Ic-BL SNe) with clear suppression of the flux redward of $\sim\!5000$ Å, providing further evidence for a large mass of Fe-group elements. Late-time upper limits indicate a $^{56}$Ni mass of $\lesssim 0.6$ M$_\odot$, leaving open the possibility that SN2017dwh produced a $^{56}$Ni mass comparable to SN1998bw ($\approx\!0.4$ M$_\odot$). Fitting the light curve with a combined magnetar and $^{56}$Ni model using ${\tt MOSFiT}$, we find that the light curve can easily accommodate such masses without affecting the inferred magnetar parameters. We also find that SN2017dwh occurred in the least-luminous detected host galaxy to date for a SLSN-I, with $M_{B} = -13.5$ mag and an implied metallicity of $Z\!\sim\!0.08$ $Z_\odot$. The spectral properties of SN2017dwh provide new evidence linking SLSNe-I with Type Ic-BL SNe, and in particular the high Fe-group abundance may be due to enhanced $^{56}$Ni production or mixing due to asphericity. Finally, we find that SN2017dwh represents the most extreme end of a correlation between continuum shape and Co II absorption strength in the near-peak spectra of SLSNe-I, indicating that Fe-group abundance likely accounts for some of the variation in their spectral shapes.

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arXiv2018

A Decline in the X-ray through Radio Emission from GW170817 Continues to Support an Off-Axis Structured Jet

We present new observations of the binary neutron star merger GW170817 at $Δt\approx 220-290$ 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 $\gtrsim5σ$ 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.

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arXiv2018

Spitzer Space Telescope Infrared Observations of the Binary Neutron Star Merger GW170817

We present Spitzer Space Telescope 3.6 and 4.5 micron observations of the binary neutron star merger GW170817 at 43, 74, and 264 days post-merger. Using the final observation as a template, we uncover a source at the position of GW170817 at 4.5 micron with a brightness of 22.9+/-0.3 AB mag at 43 days and 23.8+/-0.3 AB mag at 74 days (the uncertainty is dominated by systematics from the image subtraction); no obvious source is detected at 3.6 micron to a 3-sigma limit of >23.3 AB mag in both epochs. The measured brightness is dimmer by a factor of about 2-3 times compared to our previously published kilonova model, which is based on UV, optical, and near-IR data at <30 days. However, the observed fading rate and color (m_{3.6}-m_{4.5}> 0 AB mag) are consistent with our model. We suggest that the discrepancy is likely due to a transition to the nebular phase, or a reduced thermalization efficiency at such late time. Using the Spitzer data as a guide, we briefly discuss the prospects of observing future binary neutron star mergers with Spitzer (in LIGO/Virgo Observing Run 3) and the James Webb Space Telescope (in LIGO/Virgo Observing Run 4 and beyond).

astro-ph.HE↗

arXiv2018

The fraction of ionizing radiation from massive stars that escapes to the intergalactic medium

The part played by stars in the ionization of the intergalactic medium remains an open question. A key issue is the proportion of the stellar ionizing radiation that escapes the galaxies in which it is produced. Spectroscopy of gamma-ray burst afterglows can be used to determine the neutral hydrogen column-density in their host galaxies and hence the opacity to extreme ultra-violet radiation along the lines-of-sight to the bursts. Thus, making the reasonable assumption that long-duration GRB locations are representative of the sites of massive stars that dominate EUV production, one can calculate an average escape fraction of ionizing radiation in a way that is independent of galaxy size, luminosity or underlying spectrum. Here we present a sample of NH measures for 138 GRBs in the range 1.6 ~0.005, with a 98% confidence upper limit of ~0.015. This analysis suggests that stars provide a small contribution to the ionizing radiation budget of the IGM at z<5, where the bulk of the bursts lie. At higher redshifts, z>5, firm conclusions are limited by the small size of the GRB sample, but any decline in average HI column-density seems to be modest. We also find no indication of a significant correlation of NH with galaxy UV luminosity or host stellar mass, for the subset of events for which these are available. We discuss in some detail a number of selection effects and potential biases. Drawing on a range of evidence we argue that such effects, while not negligible, are unlikely to produce systematic errors of more than a factor ~2, and so would not affect the primary conclusions. Given that many GRB hosts are low metallicity, high specific star-formation rate, dwarf galaxies, these results present a particular problem for the hypothesis that such galaxies dominated the reionization of the universe.

astro-ph.GA↗

arXiv2018

The Type I Superluminous Supernova PS16aqv: Lightcurve Complexity and Deep Limits on Radioactive Ejecta in a Fast Event

[Abridged] We present UV/optical observations of PS16aqv (SN 2016ard), a Type I superluminous supernova (SLSN-I) classified as part of our search for low-$z$ SLSNe. PS16aqv is a fast evolving SLSNe-I that reached a peak absolute magnitude of $M_{r} \approx -22.1$. The lightcurves exhibit a significant undulation at 30 rest-frame days after peak, with a behavior similar to undulations seen in the slowly fading SLSN-I SN 2015bn. This similarity strengthens the case that fast and slow SLSNe-I form a continuum with a common origin. At $\approx\!80$ days after peak, the lightcurves exhibit a transition to a slow decline, followed by significant subsequent steepening, indicative of a plateau phase or a second significant undulation. Deep limits at $\approx280$ days after peak imply a tight constraint on the nickel mass, $M_{\rm Ni} \lesssim 0.35$ M$_{\odot}$ (lower than for previous SLSNe-I), and indicate that some SLSNe-I do not produce significantly more nickel than normal Type Ic SNe. Using MOSFiT, we model the lightcurve with a magnetar central engine model and find $P_{\rm spin} \approx 0.9$ ms, $B \approx 1.5 \times 10^{14}$ G, and $M_{\rm ej} \approx 16$ M$_{\odot}$. The implied rapid spin-down time and large reservoir of available energy coupled with the high ejecta mass may account for the fast evolving lightcurve and slow spectroscopic evolution. We also study the location of PS16aqv in its host galaxy and find that it occurred at an offset of $2.46 \pm 0.21$ kpc from the central star-forming region. We find the host galaxy exhibits low metallicity and spatially varying extinction and star formation rate, with the explosion site exhibiting lower values than the central region. The complexity seen in the lightcurves of PS16aqv and other events highlights the importance of obtaining well-sampled lightcurves for exploring deviations from a uniform decline.

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arXiv2018

The Complete Light-curve Sample of Spectroscopically Confirmed Type Ia Supernovae from Pan-STARRS1 and Cosmological Constraints from The Combined Pantheon Sample

We present optical light curves, redshifts, and classifications for 365 spectroscopically confirmed Type Ia supernovae (SNe Ia) discovered by the Pan-STARRS1 (PS1) Medium Deep Survey. We detail improvements to the PS1 SN photometry, astrometry and calibration that reduce the systematic uncertainties in the PS1 SN Ia distances. We combine the subset of 279 PS1 SN Ia ($0.03 < z < 0.68$) with useful distance estimates of SN Ia from SDSS, SNLS, various low-z and HST samples to form the largest combined sample of SN Ia consisting of a total of 1048 SN Ia ranging from $0.01 < z < 2.3$, which we call the `Pantheon Sample'. When combining Planck 2015 CMB measurements with the Pantheon SN sample, we find $Ω_m=0.307\pm0.012$ and $w = -1.026\pm0.041$ for the wCDM model. When the SN and CMB constraints are combined with constraints from BAO and local H0 measurements, the analysis yields the most precise measurement of dark energy to date: $w0 = -1.007\pm 0.089$ and $wa = -0.222 \pm0.407$ for the w0waCDM model. Tension with a cosmological constant previously seen in an analysis of PS1 and low-z SNe has diminished after an increase of $2\times$ in the statistics of the PS1 sample, improved calibration and photometry, and stricter light-curve quality cuts. We find the systematic uncertainties in our measurements of dark energy are almost as large as the statistical uncertainties, primarily due to limitations of modeling the low-redshift sample. This must be addressed for future progress in using SN Ia to measure dark energy.

astro-ph.CO↗

arXiv2018

Measuring Dark Energy Properties with Photometrically Classified Pan-STARRS Supernovae. II. Cosmological Parameters

We use 1169 Pan-STARRS supernovae (SNe) and 195 low-$z$ ($z < 0.1$) SNe Ia to measure cosmological parameters. Though most Pan-STARRS SNe lack spectroscopic classifications, in a previous paper (I) we demonstrated that photometrically classified SNe can be used to infer unbiased cosmological parameters by using a Bayesian methodology that marginalizes over core-collapse (CC) SN contamination. Our sample contains nearly twice as many SNe as the largest previous SN Ia compilation. Combining SNe with Cosmic Microwave Background (CMB) constraints from Planck, we measure the dark energy equation of state parameter $w$ to be -0.989$\pm$0.057 (stat$+$sys). If $w$ evolves with redshift as $w(a) = w_0 + w_a(1-a)$, we find $w_0 = -0.912 \pm 0.149$ and $w_a =$ -0.513$\pm$0.826. These results are consistent with cosmological parameters from the Joint Lightcurve Analysis and the Pantheon sample. We try four different photometric classification priors for Pan-STARRS SNe and two alternate ways of modeling CC SN contamination, finding that no variant gives a $w$ differing by more than 2% from the baseline measurement. The systematic uncertainty on $w$ due to marginalizing over CC SN contamination, $σ_w^{\textrm{CC}} = 0.012$, is the third-smallest source of systematic uncertainty in this work. We find limited (1.6$σ$) evidence for evolution of the SN color-luminosity relation with redshift, a possible systematic that could constitute a significant uncertainty in future high-$z$ analyses. Our data provide one of the best current constraints on $w$, demonstrating that samples with $\sim$5% CC SN contamination can give competitive cosmological constraints when the contaminating distribution is marginalized over in a Bayesian framework.

astro-ph.CO↗

arXiv2018

A Precise Distance to the Host Galaxy of the Binary Neutron Star Merger GW170817 Using Surface Brightness Fluctuations

The joint detection of gravitational waves and electromagnetic radiation from the binary neutron star (BNS) merger GW170817 has provided unprecedented insight into a wide range of physical processes: heavy element synthesis via the $r$-process; the production of relativistic ejecta; the equation of state of neutron stars and the nature of the merger remnant; the binary coalescence timescale; and a measurement of the Hubble constant via the "standard siren" technique. In detail, all of these results depend on the distance to the host galaxy of the merger event, NGC4993. In this paper we measure the surface brightness fluctuation (SBF) distance to NGC4993 in the F110W and F160W passbands of the Wide Field Camera 3 Infrared Channel on the Hubble Space Telescope (HST). For the preferred F110W passband we derive a distance modulus of $m{-}M=33.05\pm0.08\pm0.10$ mag, or a linear distance $d=40.7\pm1.4\pm1.9$ Mpc (random and systematic errors, respectively); a virtually identical result is obtained from the F160W data. This is the most precise distance to NGC4993 available to date. Combining our distance measurement with the corrected recession velocity of NGC4993 implies a Hubble constant $H_0=71.9\pm 7.1~km~s^{-1}~Mpc^{-1}$. A comparison of our result to the GW-inferred value of $H_0$ indicates a binary orbital inclination of $i\,{\gtrsim}\,137~°$. The SBF technique can be applied to early-type host galaxies of BNS mergers to ${\sim\,}100$ Mpc with HST and possibly as far as ${\sim\,}300$ Mpc with the James Webb Space Telescope, thereby helping to break the inherent distance-inclination degeneracy of the GW signals at distances where many future BNS mergers are likely to be detected.

astro-ph.GA↗

arXiv2018

Jets in Hydrogen-poor Super-luminous Supernovae: Constraints from a Comprehensive Analysis of Radio Observations

The energy source powering the extreme optical luminosity of hydrogen-stripped Superluminous Supernovae (SLSNe-I) is not known, but recent studies have highlighted the case for a central engine. Radio and/or X-ray observations are best placed to track the fastest ejecta and probe the presence of outflows from a central engine. We compile all the published radio observations of SLSNe-I to date and present three new observations of two new SLSNe-I. None were detected. Through modeling the radio emission, we constrain the sub-parsec environments and possible outflows in SLSNe-I. In this sample we rule out on-axis collimated relativistic jets of the kind detected in Gamma-Ray Bursts (GRBs). We constrain off-axis jets with opening angles of 5\arcdeg\ (30\arcdeg) to energies of $\rm{E_k<4\times10^{50}\,erg}$ ($\rm{E_k<10^{50}\,erg}$) in environments shaped by progenitors with mass-loss rates of $\dot{M}<10^{-4}\,M_{\odot}\,{\rm yr}^{-1}$ ($\dot{M}<10^{-5}\,M_{\odot}\,{\rm yr}^{-1}$) for all off-axis angles, assuming fiducial values $ε_e=0.1$ and $ε_B=0.01$. The deepest limits rule out emission of the kind seen in faint un-collimated GRBs (with the exception of GRB\,060218), and from relativistic supernovae. Finally, for the closest SLSN-I SN 2017egm we constrained the energy of an uncollimated non-relativistic outflow like those observed in normal SNe to $E_{\rm k}\lesssim10^{48}$ erg.

astro-ph.HE↗

arXiv2018

The Binary Neutron Star event LIGO/VIRGO GW170817 a hundred and sixty days after merger: synchrotron emission across the electromagnetic spectrum

We report deep Chandra, HST and VLA observations of the binary neutron star event GW170817 at $t<160$ d after merger. These observations show that GW170817 has been steadily brightening with time and might have now reached its peak, and constrain the emission process as non-thermal synchrotron emission where the cooling frequency $ν_c$ is above the X-ray band and the synchrotron frequency $ν_m$ is below the radio band. The very simple power-law spectrum extending for eight orders of magnitude in frequency enables the most precise measurement of the index $p$ of the distribution of non-thermal relativistic electrons $N(γ)\propto γ^{-p}$ accelerated by a shock launched by a NS-NS merger to date. We find $p=2.17\pm0.01$, which indicates that radiation from ejecta with $Γ\sim3-10$ dominates the observed emission. While constraining the nature of the emission process, these observations do \emph{not} constrain the nature of the relativistic ejecta. We employ simulations of explosive outflows launched in NS ejecta clouds to show that the spectral and temporal evolution of the non-thermal emission from GW170817 is consistent with both emission from radially stratified quasi-spherical ejecta traveling at mildly relativistic speeds, \emph{and} emission from off-axis collimated ejecta characterized by a narrow cone of ultra-relativistic material with slower wings extending to larger angles. In the latter scenario, GW170817 harbored a normal SGRB directed away from our line of sight. Observations at $t\le 200$ days are unlikely to settle the debate as in both scenarios the observed emission is effectively dominated by radiation from mildly relativistic material.

astro-ph.HE↗

arXiv2018

Hydrogen-Poor Superluminous Supernovae from the Pan-STARRS1 Medium Deep Survey

We present light curves and classification spectra of 17 hydrogen-poor superluminous supernovae (SLSNe) from the Pan-STARRS1 Medium Deep Survey (PS1 MDS). Our sample contains all objects from the PS1 MDS sample with spectroscopic classification that are similar to either of the prototypes SN2005ap or SN2007bi, without an explicit limit on luminosity. With a redshift range $0.3 < z < 1.6$, PS1MDS is the first SLSN sample primarily probing the high-redshift population; our multi-filter PS1 light curves probe the rest-frame UV emission, and hence the peak of the spectral energy distribution. We measure the temperature evolution and construct bolometric light curves, and find peak luminosities of $(0.5-5) \times 10^{44}$ erg s$^{-1}$ and lower limits on the total radiated energies of $(0.3-2) \times 10^{51}$ erg. The light curve shapes are diverse, with both rise- and decline times spanning a factor of $\sim 5$, and several examples of double-peaked light curves. When correcting for the flux-limited nature of our survey, we find a median peak luminosity at 4000 Å of $M_{\rm 4000} = -21.1$ mag, and a spread of $σ= 0.7$ mag.

astro-ph.HE↗

arXiv2017

Improved constraints on H0 from a combined analysis of gravitational-wave and electromagnetic emission from GW170817

The luminosity distance measurement of GW170817 derived from GW analysis in Abbott et al. 2017 (here, A17:H0) is highly correlated with the measured inclination of the NS-NS system. To improve the precision of the distance measurement, we attempt to constrain the inclination by modeling the broad-band X-ray-to-radio emission from GW170817, which is dominated by the interaction of the jet with the environment. We update our previous analysis and we consider the radio and X-ray data obtained at $t<40$ days since merger. We find that the afterglow emission from GW170817 is consistent with an off-axis relativistic jet with energy $10^{48}\,\rm{erg} 100$ days of the X-ray and radio emission will lead to tighter constraints.

astro-ph.CO↗

arXiv2017

The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. II. UV, Optical, and Near-IR Light Curves and Comparison to Kilonova Models

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 at $0.47$ 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 $T\approx 8300$ K, a radius of $R\approx 4.5\times 10^{14}$ cm (corresponding to an expansion velocity of $v\approx 0.3c$), and a bolometric luminosity of $L_{\rm bol}\approx 5\times10^{41}$ 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.

astro-ph.HE↗

arXiv2017

The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. VIII. A Comparison to Cosmological Short-duration Gamma-ray Bursts

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 of $z \approx 0.12-2.6$ 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.

astro-ph.HE↗

arXiv2017

The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. III. Optical and UV Spectra of a Blue Kilonova From Fast Polar Ejecta

We present optical and ultraviolet spectra of the first electromagnetic counterpart to a gravitational wave (GW) source, the binary neutron star merger GW170817. Spectra were obtained nightly between 1.5 and 9.5 days post-merger, using the SOAR and Magellan telescopes; the UV spectrum was obtained with the \textit{Hubble Space Telescope} at 5.5 days. Our data reveal a rapidly-fading blue component ($T\approx5500$ K at 1.5 days) that quickly reddens; spectra later than $\gtrsim 4.5$ days peak beyond the optical regime. The spectra are mostly featureless, although we identify a possible weak emission line at $\sim 7900$ Å at $t\lesssim 4.5$ days. The colours, rapid evolution and featureless spectrum are consistent with a "blue" kilonova from polar ejecta comprised mainly of light $r$-process nuclei with atomic mass number $A\lesssim 140$. This indicates a sight-line within $θ_{\rm obs}\lesssim 45^{\circ}$ of the orbital axis. Comparison to models suggests $\sim0.03$ M$_\odot$ of blue ejecta, with a velocity of $\sim 0.3c$. The required lanthanide fraction is $\sim 10^{-4}$, but this drops to $<10^{-5}$ in the outermost ejecta. The large velocities point to a dynamical origin, rather than a disk wind, for this blue component, suggesting that both binary constituents are neutron stars (as opposed to a binary consisting of a neutron star and a black hole). For dynamical ejecta, the high mass favors a small neutron star radius of $\lesssim 12$ km. This mass also supports the idea that neutron star mergers are a major contributor to $r$-process nucleosynthesis.

astro-ph.HE↗

arXiv2017

The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. VII. Properties of the Host Galaxy and Constraints on the Merger Timescale

We present the properties of NGC 4993, the host galaxy of GW170817, the first gravitational wave (GW) event from the merger of a binary neutron star (BNS) system and the first with an electromagnetic (EM) counterpart. We use both archival photometry and new optical/near-IR imaging and spectroscopy, together with stellar population synthesis models to infer the global properties of the host galaxy. We infer a star formation history peaked at $\gtrsim 10$ Gyr ago, with subsequent exponential decline leading to a low current star formation rate of 0.01 M$_{\odot}$ yr$^{-1}$, which we convert into a binary merger timescale probability distribution. We find a median merger timescale of $11.2^{+0.7}_{-1.4}$ Gyr, with a 90% confidence range of $6.8-13.6$ Gyr. This in turn indicates an initial binary separation of $\approx 4.5$ R$_{\odot}$, comparable to the inferred values for Galactic BNS systems. We also use new and archival $Hubble$ $Space$ $Telescope$ images to measure a projected offset of the optical counterpart of $2.1$ kpc (0.64$r_{e}$) from the center of NGC 4993 and to place a limit of $M_{r} \gtrsim -7.2$ mag on any pre-existing emission, which rules out the brighter half of the globular cluster luminosity function. Finally, the age and offset of the system indicates it experienced a modest natal kick with an upper limit of $\sim 200$ km s$^{-1}$. Future GW$-$EM observations of BNS mergers will enable measurement of their population delay time distribution, which will directly inform their viability as the dominant source of $r$-process enrichment in the Universe.

astro-ph.HE↗