5 ms·

From an observational astronomy point of view, perhaps the most interesting thing about the report is that by including the Virgo detector with both LIGO detect

From an observational astronomy point of view, perhaps the most interesting thing about the report is that by including the Virgo detector with both LIGO detectors, the uncertainty in the direction the gravitational wave event was reduced by a factor of 10! The "error ellipse" on the sky is now ~60 square degrees as opposed to ~600. This makes it much more feasible to do rapid observational follow-up with other observatories at all wavelengths (radio, optical, X-ray, etc.), to search for the counterparts. Many observatories like the VLA, Chandra, Fermi, Gemini, have a rapid response trigger for the detection events, where within minutes they will stop their current operations. Observations in the EM spectrum are crucial for learning about a huge number of questions in astrophysics, like what kinds of galaxies do the merger events originate from, where are they located in those galaxies, how does the luminosity decay at different wavelengths, etc. It will be seriously exciting when the first counterpart is discovered!

sleavey 9y ago
Here's a skymap [1] that really drives home the point about how much having a third detector helps. [1] http://www.virgo-gw.eu/skymap.html http://www.virgo-gw.eu/skymap.html
- stefco_ 9y ago
  Having extra detectors also helps hugely with uptime. We're currently only active and collecting clean data ~50% of the time for each detector. We need at least two detectors active for a detection, and need three detectors for this kind of beautiful directional isolation. So having extra detectors decreases our skymap areas significantly, which helps in EM followup, but it also significantly reduces GW detector network downtime. And it allows for graceful degradation; with 5 detectors, you can afford to have a couple detectors down and still detect an event with good direction reconstruction. We also have tons of downtime for upgrades. Now that our second observing run is over, we have a ~1.5yr upgrade cycle. There's no point in running VIRGO without LIGO (except for engineering reasons); having more detectors will increase flexibility in this regard, too. When KAGRA and LIGO India come online, we should see much better skymaps, but we will also see much higher uptimes (and hence higher detection rates). It's especially exciting to see improved direction reconstruction given that VIRGO has faced setbacks this year with their mirrors. I wasn't expecting to see such beautiful skymaps until our third observing run! Needless to say it was an exciting summer :) - grad student working on LIGO
  - saganus 9y ago
    1.5 years for an upgrade? Can you expand on why it takes so much time to do an upgrade? I'm guessing it's due to the complexity of the system, kinda like with the LHC? or is it something different? I mean, having equipment that expensive on downtime for more than a year seems like a lot!
    - sleavey 9y ago
      The required time is due to how complicated the instruments are, and how hard it is to tune them such that they achieve the sensitivities they are designed to achieve across such a wide frequency band. In the downtime until O3, new optics are to be installed. As these are 40kg each and suspended from monolithic fused silica fibres (each a couple hundred microns thick), this process is complicated and requires great care to avoid damaging either the fibres or the optics during installation. In addition, both LIGO and Virgo are going to get squeezed light sources installed, which allow for reduction of limiting quantum noise at high frequencies. These are immensely sensitive to light loss in the optical path, so commissioners need to be very careful to minimised scattered light and ensure good alignment. After making all of the planned upgrades, the interferometers then need to be recommissioned: all of the control loops (of which there are thousands) need to be re-tuned from their settings as of now, given the changes in the behaviour of the interferometer due to the upgrades. This takes a lot of person power and a lot of time to get right - for the first observing run this process was basically also 1.5 years.
      - saganus 9y ago
        Wow! Great explanation. Thanks! I just can't start to fathom the complexity of these instruments. Also... how cool that you get to work on such interesting field and with such cool "lab" devices!
      - ssijak 9y ago
        Could be easily put in a SF novel about 3000 something year and I would not blink.
      - stefco_ 9y ago
c3534l 9y ago
A factor of 10 would be impressive enough, but a factor of 3,628,800 is incomprehensible.
- vorotato 9y ago
  you could say it's astronomical
- lpellis 9y ago
  I think the ! after the 10 was for emphasis not factorial :)
viraptor 9y ago
> a rapid response trigger for the detection events, where within minutes they will stop their current operations Is that a large enough margin? What would be typical delay between noticing gravitational wave event and seeing the related EM waves?
- kkylin 9y ago
  Not an astrophysicist, but my guess is both gravitational and EM waves travel at the speed of light and there should not be a big delay. The limiting factor is the duration of the astronomical event in question, which may only be minutes or hours. (Someone with actual knowledge should jump in. :-)
  - stefco_ 9y ago
    We predict that the speed of light and the speed of GWs is the same. One benefit of EM followup is that you can test this hypothesis (you should see roughly simultaneous GW and EM radiation); if it isn't true, then our understanding of general relativity is flawed. Yes, if you have a long event, that smears out the time during which EM radiation might have been emitted. But for just observing the event, you expect a long-lived afterglow. It starts out hot and radiates gamma and x-rays, then cools through UV, visible, IR, etc. all the way to radio waves for weeks after the event. You can learn a lot about the event and the material that was ejected from it from a detailed analysis of this cooldown.
    - SomeStupidPoint 9y ago
      On long flights, don't we expect the light to arrive after the gravitational waves because it interacts more strongly with the particles that both sets of waves traverse? Both kinds of waves are slowed when passing through media, but because EM is much stronger than gravity, the effect from interstellar dust should be much stronger on EM than on gravity. Even a 1-in-a-million slowdown of the EM versus gravity would mean that everything outside of ~100ly would arrive at least an hour later. Every intergalactic bit of light is passing through at least a few thousand lightyears of galactic dust, and potentially much more.

sleavey 9y ago

Here's a skymap [1] that really drives home the point about how much having a third detector helps. [1] http://www.virgo-gw.eu/skymap.html http://www.virgo-gw.eu/skymap.html

stefco_ 9y ago
Having extra detectors also helps hugely with uptime. We're currently only active and collecting clean data ~50% of the time for each detector. We need at least two detectors active for a detection, and need three detectors for this kind of beautiful directional isolation. So having extra detectors decreases our skymap areas significantly, which helps in EM followup, but it also significantly reduces GW detector network downtime. And it allows for graceful degradation; with 5 detectors, you can afford to have a couple detectors down and still detect an event with good direction reconstruction. We also have tons of downtime for upgrades. Now that our second observing run is over, we have a ~1.5yr upgrade cycle. There's no point in running VIRGO without LIGO (except for engineering reasons); having more detectors will increase flexibility in this regard, too. When KAGRA and LIGO India come online, we should see much better skymaps, but we will also see much higher uptimes (and hence higher detection rates). It's especially exciting to see improved direction reconstruction given that VIRGO has faced setbacks this year with their mirrors. I wasn't expecting to see such beautiful skymaps until our third observing run! Needless to say it was an exciting summer :) - grad student working on LIGO
- saganus 9y ago
  1.5 years for an upgrade? Can you expand on why it takes so much time to do an upgrade? I'm guessing it's due to the complexity of the system, kinda like with the LHC? or is it something different? I mean, having equipment that expensive on downtime for more than a year seems like a lot!
  - sleavey 9y ago
    The required time is due to how complicated the instruments are, and how hard it is to tune them such that they achieve the sensitivities they are designed to achieve across such a wide frequency band. In the downtime until O3, new optics are to be installed. As these are 40kg each and suspended from monolithic fused silica fibres (each a couple hundred microns thick), this process is complicated and requires great care to avoid damaging either the fibres or the optics during installation. In addition, both LIGO and Virgo are going to get squeezed light sources installed, which allow for reduction of limiting quantum noise at high frequencies. These are immensely sensitive to light loss in the optical path, so commissioners need to be very careful to minimised scattered light and ensure good alignment. After making all of the planned upgrades, the interferometers then need to be recommissioned: all of the control loops (of which there are thousands) need to be re-tuned from their settings as of now, given the changes in the behaviour of the interferometer due to the upgrades. This takes a lot of person power and a lot of time to get right - for the first observing run this process was basically also 1.5 years.
    - saganus 9y ago
      Wow! Great explanation. Thanks! I just can't start to fathom the complexity of these instruments. Also... how cool that you get to work on such interesting field and with such cool "lab" devices!
    - ssijak 9y ago
      Could be easily put in a SF novel about 3000 something year and I would not blink.
    - stefco_ 9y ago

c3534l 9y ago

A factor of 10 would be impressive enough, but a factor of 3,628,800 is incomprehensible.

vorotato 9y ago
you could say it's astronomical
lpellis 9y ago
I think the ! after the 10 was for emphasis not factorial :)

viraptor 9y ago

> a rapid response trigger for the detection events, where within minutes they will stop their current operations Is that a large enough margin? What would be typical delay between noticing gravitational wave event and seeing the related EM waves?

kkylin 9y ago
Not an astrophysicist, but my guess is both gravitational and EM waves travel at the speed of light and there should not be a big delay. The limiting factor is the duration of the astronomical event in question, which may only be minutes or hours. (Someone with actual knowledge should jump in. :-)
- stefco_ 9y ago
  We predict that the speed of light and the speed of GWs is the same. One benefit of EM followup is that you can test this hypothesis (you should see roughly simultaneous GW and EM radiation); if it isn't true, then our understanding of general relativity is flawed. Yes, if you have a long event, that smears out the time during which EM radiation might have been emitted. But for just observing the event, you expect a long-lived afterglow. It starts out hot and radiates gamma and x-rays, then cools through UV, visible, IR, etc. all the way to radio waves for weeks after the event. You can learn a lot about the event and the material that was ejected from it from a detailed analysis of this cooldown.
  - SomeStupidPoint 9y ago
    On long flights, don't we expect the light to arrive after the gravitational waves because it interacts more strongly with the particles that both sets of waves traverse? Both kinds of waves are slowed when passing through media, but because EM is much stronger than gravity, the effect from interstellar dust should be much stronger on EM than on gravity. Even a 1-in-a-million slowdown of the EM versus gravity would mean that everything outside of ~100ly would arrive at least an hour later. Every intergalactic bit of light is passing through at least a few thousand lightyears of galactic dust, and potentially much more.