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 0.12 - 2.6 discovered over 2004-2017. We find that the counterpart to GW170817 has an isotropic-equivalent luminosity that is 3000 times less than the median value of on-axis short GRB X-ray afterglows, and 104 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-infrared (NIR) counterpart to GW170817 is comparatively under-luminous by a factor of 3 - 5 , indicating a range of kilonova luminosities and timescales. A comparison of the optical limits following short GRBs on 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, NGC 4993, in the context of short GRB host galaxy stellar population properties. We find that NGC 4993 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 Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo volume will be crucial in delineating the properties of the host galaxies of neutron star-neutron star (NS-NS) mergers, and connecting them to their cosmological counterparts.
The localization of the repeating fast radio burst (FRB) 121102 to a low-metallicity dwarf galaxy at z = 0.193, and its association with a luminous quiescent radio source, suggests the possibility ...that FRBs originate from magnetars, formed by the unusual supernovae that occur in such galaxies. We investigate this possibility via a comparison of magnetar birth rates, the FRB volumetric rate, and host galaxy demographics. We calculate average volumetric rates of possible millisecond magnetar production channels, such as superluminous supernovae (SLSNe), long and short gamma-ray bursts (GRBs), and general magnetar production via core-collapse supernovae (CCSNe). For each channel, we also explore the expected host galaxy demographics using their known properties. We determine for the first time the number density of FRB emitters (the product of their volumetric birth rate and lifetime), Gpc−3, assuming that FRBs are predominantly emitted from repetitive sources similar to FRB 121102 and adopting a beaming factor of 0.1. By comparing rates, we find that production via rare channels (SLSNe, GRBs) implies a typical FRB lifetime of ∼30-300 years, in good agreement with other lines of argument. The total energy emitted over this time is consistent with the available energy stored in the magnetic field. On the other hand, any relation to magnetars produced via normal CCSNe leads to a very short lifetime of ∼0.5 years, in conflict with both theory and observation. We demonstrate that due to the diverse host galaxy distributions of the different progenitor channels, many possible sources of FRB birth can be ruled out with host galaxy identifications. Conversely, targeted searches of galaxies that have previously hosted decades-old SLSNe and GRBs may be a fruitful strategy for discovering new FRBs and related quiescent radio sources, and determining the nature of their progenitors.
We present observations of PS16dtm (also known as SN 2016ezh), a luminous transient that occurred at the nucleus of a narrow-line Seyfert 1 galaxy hosting a 106 M black hole. The light curve shows ...that PS16dtm exhibited a plateau phase for ∼100 days, during which it showed no color evolution, maintained a blackbody temperature of K, and radiated at approximately the Eddington luminosity of the supermassive black hole (SMBH). The spectra exhibit multicomponent hydrogen emission lines and strong Fe ii emission, show little time evolution, and closely resemble the spectra of NLS1s while being distinct from those of Type IIn supernovae (SNe IIn). Moreover, PS16dtm is undetected in the X-rays to a limit an order of magnitude below an archival X-ray detection of its host galaxy. These observations strongly link PS16dtm to activity associated with the SMBH and are difficult to reconcile with an SN origin or known forms of active galactic nucleus (AGN) variability. Therefore, we argue that PS16dtm is a tidal disruption event (TDE) in which the accretion of the stellar debris powers the rise in the continuum and excitation of the preexisting broad-line region, while obscuring the X-ray-emitting region of the preexisting AGN disk. We predict that PS16dtm will remain bright for years and that the X-ray emission will reappear on a similar timescale as the accretion rate declines. Placing PS16dtm in the context of other TDEs, we find that TDEs in AGN galaxies are more efficient and reach Eddington luminosities, likely due to interaction of the stellar debris with the preexisting accretion disk.
ABSTRACT
We present and analyse a new tidal disruption event (TDE), AT2017eqx at redshift z = 0.1089, discovered by Pan-STARRS and ATLAS. The position of the transient is consistent with the nucleus ...of its host galaxy; the spectrum shows a persistent blackbody temperature T ≳ 20 000 K with broad H i and He ii emission; and it peaks at a blackbody luminosity of L ≈ 1044 erg s−1. The lines are initially centred at zero velocity, but by 100 d, the H i lines disappear while the He ii develops a blueshift of ≳ 5000 km s−1. Both the early- and late-time morphologies have been seen in other TDEs, but the complete transition between them is unprecedented. The evolution can be explained by combining an extended atmosphere, undergoing slow contraction, with a wind in the polar direction becoming visible at late times. Our observations confirm that a lack of hydrogen a TDE spectrum does not indicate a stripped star, while the proposed model implies that much of the diversity in TDEs may be due to the observer viewing angle. Modelling the light curve suggests AT2017eqx resulted from the complete disruption of a solar-mass star by a black hole of ∼106.3 M⊙. The host is another Balmer-strong absorption galaxy, though fainter and less centrally concentrated than most TDE hosts. Radio limits rule out a relativistic jet, while X-ray limits at 500 d are among the deepest for a TDE at this phase.
ABSTRACT We report the discovery of transient radio emission from the nearby optically discovered tidal disruption event (TDE) ASASSN-14li (distance of 90 Mpc), making it the first typical TDE ...detected in the radio, and unambiguously pointing to the formation of a non-relativistic outflow with a kinetic energy of (4-10) × 1047 erg, a velocity of 12,000-36,000 km s−1, and a mass of 3 × 10−5-7 × 10−4 M . We show that the outflow was ejected on 2014 August 11-25, in agreement with an independent estimate of the timing of super-Eddington accretion based on the optical, ultraviolet, and X-ray observations, and that the ejected mass corresponds to about 1%-10% of the mass accreted in the super-Eddington phase. The temporal evolution of the radio emission also uncovers the circumnuclear density profile, on a scale of about 0.01 pc, a scale that cannot be probed via direct measurements even in the nearest supermassive black holes. Our discovery of radio emission from the nearest well-studied TDE to date, with a radio luminosity lower than all previous limits, indicates that non-relativistic outflows are ubiquitous in TDEs, and that future, more sensitive, radio surveys will uncover similar events.
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 Southern Astrophysical Research and Magellan telescopes; the UV spectrum was obtained with the Hubble Space Telescope at 5.5 days. Our data reveal a rapidly fading blue component ( T 5500 K at 1.5 days) that quickly reddens; spectra later than 4.5 days peak beyond the optical regime. The spectra are mostly featureless, although we identify a possible weak emission line at ∼7900 at t 4.5 days. The colors, 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 140 . This indicates a sightline within θ obs 45 ° of the orbital axis. Comparison to models suggests ∼0.03 M of blue ejecta, with a velocity of ∼ 0.3 c . The required lanthanide fraction is ∼ 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 12 km. This mass also supports the idea that neutron star mergers are a major contributor to r-process nucleosynthesis.
We present a near-infrared spectral sequence of the electromagnetic counterpart to the binary neutron star merger GW170817 detected by Advanced Laser Interferometer Gravitational-wave Observatory ...(LIGO)/Virgo. Our data set comprises seven epochs of J+H spectra taken with FLAMINGOS-2 on Gemini-South between 1.5 and 10.5 days after the merger. In the initial epoch, the spectrum is dominated by a smooth blue continuum due to a high-velocity, lanthanide-poor blue kilonova component. Starting the following night, all of the subsequent spectra instead show features that are similar to those predicted in model spectra of material with a high concentration of lanthanides, including spectral peaks near 1.07 and 1.55 m. Our fiducial model with 0.04 M of ejecta, an ejection velocity of v = 0.1c, and a lanthanide concentration of Xlan = 10−2 provides a good match to the spectra taken in the first five days, although it over-predicts the late-time fluxes. We also explore models with multiple fitting components, in each case finding that a significant abundance of lanthanide elements is necessary to match the broad spectral peaks that we observe starting at 2.5 days after the merger. These data provide direct evidence that binary neutron star mergers are significant production sites of even the heaviest r-process elements.
Atom probe tomography and transmission electron microscopy (TEM) have been used to determine the location and distribution of carbon and alloying elements associated with the complex structural ...changes that occur at the atomic and nanoscale in 4340 steel after quenching to martensite and tempering at 325, 450 or 575°C. Tempering at 325°C resulted in carbide formation without partitioning of chromium, manganese, molybdenum, aluminum, nickel or phosphorus, but with early-stage silicon rejection from the carbide. TEM verified the presence of cementite and the Bagaryatsky orientation relationship with the tempered martensite matrix and detected complex precipitate structures. Tempering at 450 or 575°C developed concentrations of all alloying elements at ferrite–cementite interfaces: chromium, manganese and molybdenum partitioned into the cementite, and silicon, aluminum, nickel and phosphorus were clearly rejected from the cementite. These results provide direct evidence for staged cementite growth, where early-stage growth likely occurs under paraequilibrium conditions, followed by initial silicon redistribution and subsequent alloying element redistribution during late-stage growth. Tempering at 575°C induced spheroidization of the cementite, loss of the Bagaryatsky orientation relationship, and phosphorus concentrations at Cottrell atmospheres within the cementite and at ferrite–cementite interfaces, correlating with early observations of the retardation of spheroidization by phosphorus.
Abstract
We present the largest and deepest late-time radio and millimeter survey to date of superluminous supernovae (SLSNe) and long-duration gamma-ray bursts (LGRBs) to search for associated ...nonthermal synchrotron emission. Using the Karl G. Jansky Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA), we observed 43 sources at 6 and 100 GHz on a timescale of ∼ 1–19 yr post-explosion. We do not detect radio/millimeter emission from any of the sources, with the exception of a 6 GHz detection of PTF10hgi, as well as the detection of 6 GHz emission near the location of the SLSN PTF12dam, which we associate with its host galaxy. We use our data to place constraints on central engine emission due to magnetar wind nebulae and off-axis relativistic jets. We also explore nonrelativistic emission from the SN ejecta, and place constraints on obscured star formation in the host galaxies. In addition, we conduct a search for fast radio bursts (FRBs) from some of the sources using VLA phased-array observations; no FRBs are detected to a limit of 16 mJy (7
σ
; 10 ms duration) in about 40 minutes on source per event. A comparison to theoretical models suggests that continued radio monitoring may lead to detections of persistent radio emission on timescales of ≳ a decade.
Abstract
The joint detection of gravitational waves (GWs) 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, NGC 4993. In this Letter we measure the surface brightness fluctuation (SBF) distance to NGC 4993 in the F110W and F160W passbands of the Wide Field Camera 3 Infrared Channel (WFC3/IR) on the
Hubble Space Telescope
(
HST
). For the preferred F110W passband we derive a distance modulus of
(
m
−
M
)
=
33.05
±
0.08
±
0.10
mag, or a linear distance
d
= 40.7 ± 1.4 ± 1.9 Mpc (random and systematic errors, respectively); a virtually identical result is obtained from the F160W data. This is the most precise distance to NGC 4993 available to date. Combining our distance measurement with the corrected recession velocity of NGC 4993 implies a Hubble constant
H
0
= 71.9 ± 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
≳ 137°. The SBF technique can be applied to early-type host galaxies of BNS mergers to ∼100 Mpc with
HST
and possibly as far as ∼300 Mpc with the
James Webb Space Telescope
, thereby helping to break the inherent distance-inclination degeneracy of the GW data at distances where many future BNS mergers are likely to be detected.
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