Two gravitational events might be a single lensed one

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Discussion Overview

The discussion centers on the gravitational wave events GW170809 and GW170814, exploring the hypothesis that they may represent a single event magnified by gravitational lensing. Participants examine the implications of this idea for the understanding of gravitational wave sources, their distances, and the potential for future observations to clarify the situation.

Discussion Character

  • Exploratory, Debate/contested, Technical explanation

Main Points Raised

  • Some participants propose that the similarity in parameters and the five-day difference in timing between the two gravitational wave events suggest they could be a single lensed event.
  • Others question whether the acoustic signatures of the events would correlate closely if they originated from the same source, indicating a potential shift in time.
  • It is noted that the signals observed are indeed very similar, with a time shift of four days.
  • Participants express uncertainty about whether a viable lensing candidate has been identified, citing challenges related to localization without an optical component.
  • Some suggest that the angular separation and time difference could allow for estimates of the mass and distance of the lensing object, aiding in future searches.
  • Concerns are raised about the localization providing only a rough direction in the sky, with no significant angular separation between the events.
  • There is a suggestion that a binary neutron star merger with an optical component would enhance future measurements.
  • Clarifications are made regarding the number of detectors used in the observations, with acknowledgment of the large area for potential localization.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether the events are a single lensed occurrence or separate events. Multiple competing views and uncertainties remain regarding the implications of gravitational lensing and the challenges of identifying lensing candidates.

Contextual Notes

Limitations include the dependence on the presence of an optical component for better localization and the unresolved nature of the angular separation between the events.

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It has been proposed that the gravitational wave events GW170809 and GW170814 , named after their date (5 days difference) were actually a single one which appeared twice and magnified due to gravitational lensing. Both events have very similar parameters and come from the same area in the sky.

Twin LIGO/Virgo Detections of a Viable Gravitationally-Lensed Black Hole Merger

Some good discussion on reddit

More generally, the authors of the paper argue that most of the observed events could come from lensing. It would mean the sources are smaller - they would fit to the mass range of stellar black holes we know from other observations. It would also mean they are farther away. Due to the way LIGO/Virgo work we don't expect to see multiple images often, but they should still happen. Future observation runs will easily distinguish between these options. If we see more repetitions then this is probably the case, if not it is probably not.
 
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Wouldn’t their acoustic signatures correlate closely but shifted in time if they were from the same source?
 
That's what they see. Very similar signals, just shifted by 4 days.
 
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mfb said:
That's what they see. Very similar signals, just shifted by 4 days.
Has there been a search done for a viable lensing candidate?
 
I haven't seen anything. Without an optical component the localization is not good, probably too many galaxies in range.
 
mfb said:
I haven't seen anything. Without an optical component the localization is not good, probably too many galaxies in range.
Not to give up just yet, all is not lost - With their approximate angular separation plus that five day time difference it should be possible to come up with an approximate mass and distance to the lensing object and so have an idea of what they're looking for and where!
 
The localization just gives a rough direction in the sky. There is no significant (!) angular separation between the events.

A binary neutron star merger with an optical component would be great in future measurements.
 
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mfb said:
The localization just gives a rough direction in the sky. There is no significant (!) angular separation between the events.

A binary neutron star merger with an optical component would be great in future measurements.
Oops, I forgot they were found using only two detectors...
 
Three detectors (2*LIGO and VIRGO), at least the second one, but the possible area is still 90 square degrees (340 for the other event).
 

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