| 1. |
- Evans, P. A., et al.
(författare)
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Swift and NuSTAR observations of GW170817 : Detection of a blue kilonova
- 2017
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 358:6370, s. 1565-1569
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Tidskriftsartikel (refereegranskat)abstract
- With the first direct detection of merging black holes in 2015, the era of gravitational wave (GW) astrophysics began. A complete picture of compact object mergers, however, requires the detection of an electromagnetic (EM) counterpart. We report ultraviolet (UV) and x-ray observations by Swift and the Nuclear Spectroscopic Telescope Array of the EM counter part of the binary neutron star merger GW170817. The bright, rapidly fading UV emission indicates a high mass (approximate to 0.03 solar masses) wind-driven outflow with moderate electron fraction (Y-e approximate to 0.27). Combined with the x-ray limits, we favor an observer viewing angle of approximate to 30 degrees away from the orbital rotation axis, which avoids both obscuration from the heaviest elements in the orbital plane and a direct view of any ultrarelativistic, highly collimated ejecta (a gamma-ray burst afterglow).
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| 2. |
- Kasliwal, M. M., et al.
(författare)
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Illuminating gravitational waves : A concordant picture of photons from a neutron star merger
- 2017
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 358:6370, s. 1559-
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Tidskriftsartikel (refereegranskat)abstract
- Merging neutron stars offer an excellent laboratory for simultaneously studying strong-field gravity and matter in extreme environments. We establish the physical association of an electromagnetic counterpart (EM170817) with gravitational waves (GW170817) detected from merging neutron stars. By synthesizing a panchromatic data set, we demonstrate that merging neutron stars are a long-sought production site forging heavy elements by r-process nucleosynthesis. The weak gamma rays seen in EM170817 are dissimilar to classical short gamma-ray bursts with ultrarelativistic jets. Instead, we suggest that breakout of a wide-angle, mildly relativistic cocoon engulfing the jet explains the low-luminosity gamma rays, the high-luminosity ultraviolet-optical-infrared, and the delayed radio and x-ray emission. We posit that all neutron star mergers may lead to a wide-angle cocoon breakout, sometimes accompanied by a successful jet and sometimes by a choked jet.
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| 3. |
- Mooley, K., et al.
(författare)
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A mildly relativistic wide-angle outflow in the neutron-star merger event GW170817
- 2018
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 554:7691, s. 207-210
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Tidskriftsartikel (refereegranskat)abstract
- GW170817 was the first gravitational-wave detection of a binary neutron-star merger. It was accompanied by radiation across the electromagnetic spectrum and localized to the galaxy NGC 4993 at a distance of 40 megaparsecs. It has been proposed that the observed γ-ray, X-ray and radio emission is due to an ultra-relativistic jet being launched during the merger (and successfully breaking out of the surrounding material), directed away from our line of sight (off-axis). The presence of such a jet is predicted from models that posit neutron-star mergers as the drivers of short hard-γ-ray bursts. Here we report that the radio light curve of GW170817 has no direct signature of the afterglow of an off-axis jet. Although we cannot completely rule out the existence of a jet directed away from the line of sight, the observed γ-ray emission could not have originated from such a jet. Instead, the radio data require the existence of a mildly relativistic wide-angle outflow moving towards us. This outflow could be the high-velocity tail of the neutron-rich material that was ejected dynamically during the merger, or a cocoon of material that breaks out when a jet launched during the merger transfers its energy to the dynamical ejecta. Because the cocoon model explains the radio light curve of GW170817, as well as the γ-ray and X-ray emission (and possibly also the ultraviolet and optical emission), it is the model that is most consistent with the observational data. Cocoons may be a ubiquitous phenomenon produced in neutron-star mergers, giving rise to a hitherto unidentified population of radio, ultraviolet, X-ray and γ-ray transients in the local Universe.
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| 4. |
- Broderick, J. W., et al.
(författare)
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LOFAR 144-MHz follow-up observations of GW170817
- 2020
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 494:4, s. 5110-5117
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Tidskriftsartikel (refereegranskat)abstract
- We present low-radio-frequency follow-up observations of AT 2017gfo, the electromagnetic counterpart of GW170817, which was the first binary neutron star merger to be detected by Advanced LIGO-Virgo. These data, with a central frequency of 144 MHz, were obtained with LOFAR, the Low-Frequency Array. The maximum elevation of the target is just 13 degrees.7 when observed with LOFAR, making our observations particularly challenging to calibrate and significantly limiting the achievable sensitivity. On time-scales of 130-138 and 371-374 d after the merger event, we obtain 3s upper limits for the afterglow component of 6.6 and 19.5mJy beam(-1), respectively. Using our best upper limit and previously published, contemporaneous higher frequency radio data, we place a limit on any potential steepening of the radio spectrum between 610 and 144 MHz: the two-point spectral index alpha(610)(144) greater than or similar to -2.5. We also show that LOFAR can detect the afterglows of future binary neutron star merger events occurring at more favourable elevations.
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| 5. |
- Mooley, K., et al.
(författare)
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Superluminal motion of a relativistic jet in the neutron-star merger GW170817
- 2018
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 561:7723, s. 355-359
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Tidskriftsartikel (refereegranskat)abstract
- The binary neutron-star merger GW1708171was accompanied by radiation across the electromagnetic spectrum2and localized2to the galaxy NGC 4993 at a distance3of about 41 megaparsecs from Earth. The radio and X-ray afterglows of GW170817 exhibited delayed onset4–7, a gradual increase8in the emission with time (proportional to t0.8) to a peak about 150 days after the merger event9, followed by a relatively rapid decline9,10. So far, various models have been proposed to explain the afterglow emission, including a choked-jet cocoon4,8,11–13and a successful-jet cocoon4,8,11–18(also called a structured jet). However, the observational data have remained inconclusive10,15,19,20as to whether GW170817 launched a successful relativistic jet. Here we report radio observations using very long-baseline interferometry. We find that the compact radio source associated with GW170817 exhibits superluminal apparent motion between 75 days and 230 days after the merger event. This measurement breaks the degeneracy between the choked- and successful-jet cocoon models and indicates that, although the early-time radio emission was powered by a wide-angle outflow8(a cocoon), the late-time emission was most probably dominated by an energetic and narrowly collimated jet (with an opening angle of less than five degrees) and observed from a viewing angle of about 20 degrees. The imaging of a collimated relativistic outflow emerging from GW170817 adds substantial weight to the evidence linking binary neutron-star mergers and short γ-ray bursts.
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| 6. |
- Andreoni, Igor, et al.
(författare)
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GROWTH on S190814bv : Deep Synoptic Limits on the Optical/Near-infrared Counterpart to a Neutron Star-Black Hole Merger
- 2020
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 890:2
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Tidskriftsartikel (refereegranskat)abstract
- On 2019 August 14, the Advanced LIGO and Virgo interferometers detected the high-significance gravitational wave (GW) signal S190814bv. The GW data indicated that the event resulted from a neutron star-black hole (NSBH) merger, or potentially a low-mass binary BH merger. Due to the low false-alarm rate and the precise localization (23 deg(2) at 90%), S190814bv presented the community with the best opportunity yet to directly observe an optical/near-infrared counterpart to an NSBH merger. To search for potential counterparts, the GROWTH Collaboration performed real-time image subtraction on six nights of public Dark Energy Camera images acquired in the 3 weeks following the merger, covering >98% of the localization probability. Using a worldwide network of follow-up facilities, we systematically undertook spectroscopy and imaging of optical counterpart candidates. Combining these data with a photometric redshift catalog, we ruled out each candidate as the counterpart to S190814bv and placed deep, uniform limits on the optical emission associated with S190814bv. For the nearest consistent GW distance, radiative transfer simulations of NSBH mergers constrain the ejecta mass of S190814bv to be M-ej < 0.04 M-circle dot at polar viewing angles, or M-ej < 0.03 M-circle dot if the opacity is kappa < 2 cm(2)g(-1). Assuming a tidal deformability for the NS at the high end of the range compatible with GW170817 results, our limits would constrain the BH spin component aligned with the orbital momentum to be chi < 0.7 for mass ratios Q < 6, with weaker constraints for more compact NSs.
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| 7. |
- Kasliwal, M. M., et al.
(författare)
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Spitzer mid-infrared detections of neutron star merger GW170817 suggests synthesis of the heaviest elements
- 2022
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Ingår i: Monthly Notices of the Royal Astronomical Society: Letters. - : Oxford University Press (OUP). - 1745-3925 .- 1745-3933. ; 510:1, s. L7-L12
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Tidskriftsartikel (refereegranskat)abstract
- We report our Spitzer Space Telescope observations and detections of the binary neutron star merger GW170817. At 4.5 μm, GW170817 is detected at 21.9 mag AB at +43 days and 23.9 mag AB at +74 days after merger. At 3.6 μm, GW170817 is not detected to a limit of 23.2 mag AB at +43 days and 23.1 mag AB at +74 days. Our detections constitute the latest and reddest constraints on the kilonova/macronova emission and composition of heavy elements. The 4.5 μm luminosity at this late phase cannot be explained by elements exclusively from the first abundance peak of the r-process. Moreover, the steep decline in the Spitzer band, with a power-law index of 3.4 ± 0.2, can be explained by a few of the heaviest isotopes with half-life around 14 d dominating the luminosity (e.g. 140Ba, 143Pr, 147Nd, 156Eu, 191Os, 223Ra, 225Ra, 233Pa, 234Th) or a model with lower deposition efficiency. This data offers evidence that the heaviest elements in the second and third r-process abundance peak were indeed synthesized. Our conclusion is verified by both analytics and network simulations and robust despite intricacies and uncertainties in the nuclear physics. Future observations with Spitzer and James Webb Space Telescope will further illuminate the relative abundance of the synthesized heavy elements.
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