HEN News 03/2024 (The one with alphabet soup in it [O4, LIGO, KAGRA, VIRGO,...])

American Association of Variable Star Observers (AAVSO)
Thu, 04/04/2024 - 21:16

There are news from the gravitational wave observatories: The planned pause in the O4 observation run is almost over, and the O3b part of the run is scheduled to resume on April 10th, 0800 Pacific time. Low latency alerts are already active already now. So it's time again to be prepared for the vent everybody is waiting for: an alert about the next Binary Neutron Star (BNS) merger event with sky localization that would allow amateurs and professionals alike to hunt down any optical counterpart!

Details can be found at https://observing.docs.ligo.org/plan/

There is quite some drama hiding behind the Gantt-chart-like diagram on that page. If you compare this chart to earlier versions from just before the start of O4 in early 2023 [1], you will notice that all observatories did not quite live up to the high expectations: some missed by just a bit, and others by *a lot*. Let's look at them individually:

The two LIGO instruments in the US missed their sensitivity targets, but not by much: detector performance is measured in the distance at which you would be able to detect a BNS merger with a certain fixed signal-to-noise-ratio. For LIGO, the initial goal was 160-190+ Mpc, the current plan gives a target of 150-160+ Mpc. Fair enough.

For VIRGO in Italy, the goal in early 2023 was 80-115Mpc, but the state of the detector was such that it was decided shortly before the start of O4a that it needed more work and it didn't join O4a at all. VIRGO will now join O4b, but expectations were lowered to a range of 40-80Mpc and current performance around 55 Mpc, which is not so much better than the  range during O3 (VIRGO: 40-50Mpc, for comparison: LIGO: 100-140Mpc in O3). Can you imagine how painful it must be to face your funding agency, telling them that all the money they poured into your instrument between O3 and O4 not only failed to improve the performance as expected, but that your detectors isn't even able to operate at all in O4a? I guess this must have been like the moment when scientists realized that Hubble had a serious problem after it was switched on, only that VIRGO scientists had more time to see it coming I guess.

The newest addition to the GW detector zoo, KAGRA in Japan, is also facing challenges. KAGRA has many ambitious design features: all of these GW detectors consist of very long vacuum tubes that together house a laser interferometer, but KAGRA's tubes are installed in cavities inside a mountain, and not on the surface as with LIGO and VIRGO. Also parts of KAGRA are cryogenic optics. According to the latest plan, KAGRA will re-join O4 only near the end of the O4 run and the BNS merger range will be around 10 Mpc (for comparison, the single only (!) BNS merger observed so far in both GW and electro-magnetic wavelengths was at a distance of ca 40Mpc). Recent earthquakes near Japan didn't help either.

Gravitational Wave astronomy is still very much based on experimental, cutting edge technology because off-the-shelf stuff just isn't good enough to advance the field. So setbacks are, perhaps, to be expected. No plan survives contact with physics.

For us amateurs, all of this actually isn't too relevant (upps, maybe I should have mentioned this before going on a rant...). The events that we can hope to detect the optical counterparts of are probably within (say) 100 Mpc, and closer is better of course. VIRGO joining the O4 run will help to narrow down the search region of close events that are of particular interest for amateurs, even at its current sensitivity.

Let's keep our fingers crossed and let us keep watching the phone apps like Astro-Colibri that will inform us instantly when an alert for a BNS event is issued!

Good Luck!


[1] https://dcc.ligo.org/public/0172/G2002127/018/ObsScen_timeline.jpg

American Association of Variable Star Observers (AAVSO)
There are about 50 galaxies…

There are about 50 galaxies within the local group, or within 2 Mpc.  So detections at 10 to 150 Mpc is  amazing. But then the technology that goes into those interferometers is amazing. Are VIRGO and KAGRA squeezing yet?



American Association of Variable Star Observers (AAVSO)
Unfortunately, most of those…

Unfortunately, most of those galaxies are just dwarf galaxies with relatively few stars in them. I think the optimistic rate estimation per volume and time (assuming a spherical-cow-in-space-like uniform distribution of mergers I guess, not the actual densities in our neighborhood) is about 1000 or 2000 mergers per Gigaparsec cubed per year...and if you do the math, it doesn't look so good wrt the Local Group. Or in simpler terms: We might get an event like GW170817 which was at 40 Mpc every few months to years if we are lucky. The volume at distance 2Mpc is 20^3 times smaller so ... 8000 times ... so I would not put my money on a BNS merger in the local group within anybody's lifetime here.

And yes, VIRGO is already using "squeezed light" [1] and specifically "frequency dependent squeezing", the latter KAGRA plans to add in future observing runs.

I see you have some deeper interest and understanding for those detectors, so reading the internal and external outreach "LIGO magazine" might be interesting. E.g. this is a recent article on the challenges facing VIRGO [2]. Some of the stuff in that article is very technical but you also get an idea about the human aspects of working, and sometimes failing, in these high-gain-high-risk experiments.



[1] https://en.wikipedia.org/wiki/Squeezed_states_of_light

[2] https://www.ligo.org/magazine/LIGO-magazine-issue24.pdf#page=12


American Association of Variable Star Observers (AAVSO)

Thanks for the references. I've been out of touch for about 5 years. No contact anymore with the people actually doing the technical work to squeeze. I worked with some of them on interferometers in the past.

Good hunting on the next run.