I’d like to look back on April and highlight some things that might be of interest for HEN members. So let’s start with April 1st, Fools Day. There were some hilarious papers posted on arxiv, here are just a few examples:
1) Multi-Messenger Astrology : https://arxiv.org/abs/2403.19749
Yeah, why be limited to observations in the visual spectrum for astrology if we have so many more wavelengths to observe today. Makes perfect nonsense :-)
2) Circular reasoning: Solving the Hubble tension with a non-π value of π : https://arxiv.org/abs/2403.20219
People have tried to explain the different values that you get for the Hubble constant from different observational methods by suggesting that some (astro)-physical constants may not actually be constant after all. This paper explains away the tension with an alternative value for Pi. Hmmmmm….
On the Gravitational Wave front, the second half of the 4th observation run, “O4b”, started on April 10th. And this time, the Italian VIRGO observatory joined the two US-American LIGO instruments.
Since then, we are all still waiting for that Binary-Neutron-Star (BNS) merger event that might have an observable counterpart in the visual optical spectrum. But we got the next best thing, a Black-Hole-Neutron-Star (BHNS) merger event alert on April 22, https://gracedb.ligo.org/superevents/S240422ed/view/
The sky localization, with the help from Virgo, was quite good (50% confidence area was “just” 72 square degrees, 90% ==> 259 deg^2), but at a distance of 188+/-43Mpc, it was unlikely to be observable in EM by amateurs (or even professionals).
Still, this event created a bit of excitement when on April 26th, the brand new “Einstein Probe” X-Ray telescope reported a new transient source that was consistent with this narrow sky localization of S240422ed. Was this the X-ray afterglow of the merger? The “Kilonova Catcher” project alerted its volunteer amateur observers, but nothing was found. The almost full moon didn’t help with observing this area, but a GCN was published nevertheless to document the effort. It was good target practice. Better luck next time!! https://gcn.nasa.gov/circulars/36326
The new “Einstein Probe“ is an interesting new space asset which might generate more PRO-AM collaboration projects in the future. This project is mainly managed by the Chinese Academy of Sciences but has contributions from the European Space Agency and European science institutes like the Max-Planck-Institute for Extraterrestrial Physics
https://www.mpe.mpg.de/7867826/EinsteinProbe
https://en.wikipedia.org/wiki/Einstein_Probe
It has some similarities with the Chandra mission, which made news in March and April as well, tho in a less happy way: The announcement of suggested budget cuts (which are seen to imply a ramping down of the mission to the point where the telescope is expected to be shut down in perhaps three years from now or sooner) caused quite some backlash in the X-ray community. Even popular YouTube science channels chimed in like “Veritasium” https://youtube.com/shorts/sE-RUu9ClsU?si=Vti4biNGesA8Otcj
Perhaps Einstein Probe can fill the void once Chandra will haved closed its X-ray eyes forever, but many researchers especially in the US would have hoped that the US had launched its own replacement before killing Chandra, as was the case when Hubble will pass the baton to JWST (sort of). It’s a bit of an Arecibo-moment for the US X-ray astronomy community I guess. If you browse the AAVSO alerts and campaigns, you fill find a few calls-for-observations that were coordinated with CHANDRA. It will be missed.
Oh, and then there was the LIGO/VIRGO alert S240429an on (you guessed it) April 29th, which was about a possible Binary Neutron Star merger in just around 14 Mpc distance, so basically just next door in astrophysical terms. Exactly what we are waiting for … but it soon was retracted as caused by instrument artifacts. Too bad. But if you want to see what an alert would look like that we are waiting for so desperately, here is the link: https://gracedb.ligo.org/superevents/S240429an/view/
Clear Skies & keep listening for GCN alerts (and watch T CrB , of course)
HBE