Archive for September, 2021

Waiting for U Sco: nearby gamma ray transient detected

September 29, 2021
The Fermi Gamma-ray Space Telescope (source: NASA)

Two days ago I wrote Waiting for U Sco. Today, here are a few excerpts from ATel #14941 (The Astronomer’s Telegram):

The Large Area Telescope (LAT), one of two instruments on the Fermi Gamma-ray Space Telescope, has detected a new transient gamma-ray source, Fermi J1623-1752.

A possible counterpart to the LAT transient is the recurrent nova U Scorpii with position offset by 0.18 deg from the LAT position, thus just outside the 95% confidence error circle, but within the 99% confidence error circle. U Sco has a known recurrence interval of 10.3 years (Schaefer 2010, ApJS, 187, 275) prior to its outburst in January 2010, thus a new outburst from U Sco could be anticipated. The last observation of U Sco in the AAVSO lightcurve database was on 2021 Sep 19.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. In consideration of the ongoing activity of this source, we encourage multi-wavelength observations, particularly optical follow-up of U Sco.

ATel #14941

I read about this in a post by Alexandre Amorim (Brazil) in the AAVSO novae forum. Robert Fidrich (Hungary) soon followed up with an iTelescope observation to say that there was no eruption visible.

Given that U Sco almost certainly had an outburst in its “seasonal gap” (unobservable due to position on the sky), probably in 2017, what this current gamma ray event means seems pretty uncertain.

I’ll continue imaging the U Sco region until it’s too low tonight (in between the clouds) and, consistent with my last post, whenever I can thereafter.

Waiting for U Sco

September 27, 2021

When the recurrent nova U Sco goes into outburst again, the image below shows where it will appear (see cross-hair style position marker), with the bright star Antares, and those bracketing it, shown for reference:

source: Stellarium

Zooming in shows two stars to the right of the nova position marker. Looking at previous outbursts, U Sco’s peak brightness should be somewhere between these two (around magnitude 7.5 to 8, so a binocular or small telescope object).

source: Stellarium

I’m taking images from my backyard of the U Sco region when I can. The image below (click it to enlarge) was taken on Sep 10 2021. I’ve taken a few since but want to step up the regularity. The red arrow shows where U Sco will appear and the green arrow shows the two stars pointed to above.

In any case, I won’t have the same cause to complain about missing the next outburst of U Sco that Leslie Peltier did about missing the T CrB outburst in 1946 (as related near the end of a recent talk I gave about RS Oph), given that my observing frequency is likely to be less and over a much shorter timeframe.

Addendum: Having said all this, given that there was almost certainly an unobserved eruption in the 2017 seasonal gap, as announced in a March 2019 AAVSO forum post, what does this imply for the prediction made by Brad Schaefer of 2020+/-0.7 in a paper published in June 2019? And yet, see the next post…

The Crab Nebula

September 25, 2021

We live only a few conscious decades, and we fret ourselves enough for several lifetimes.

(Christopher Hitchens)

I recently gave a 5 minute talk to a non-technical group about the Crab Nebula (M1, the first object in Charles Messier’s catalog) that related to a sense of wonder in Science. I thought the brief content was worth sharing here.

Chinese astronomers in 1054 witnessed a “guest star”, an explosion that gave rise to a remnant several light years wide called the Crab Nebula.

The object is several thousand light-years distant from earth, yet visible in a small telescope, although with nothing like the detail shown in the Hubble Space Telescope image of Figure 1!

Figure 1: The Crab Nebula, a Hubble Space Telescope image mosaic (source: NASA)

At the heart of the nebula is an object composed only of neutrons, a spoonful of which would weigh a billion tonnes, spinning 30 times per second, energising the nebula, wave-like rings expanding outward from the centre like ripples in a pond (Figure 2).

The original star ran out of the elements that had sustained its nuclear fusion for millions of years, its core collapsing then rebounding in a fraction of a second, ending as a supernova explosion.

Figure 2: The central region of the Crab Nebula (source: NASA)

The event synthesised elements heavier than iron that spread out into the space between the stars, to be incorporated into new stars and their planetary systems, some perhaps going on to develop life.

The iron in our blood, along with trace elements like zinc and iodine, come from such stars.

So, not only are we star-stuff, as Carl Sagan liked to say, but in fact we owe our existence to massive stars that burned bright and lived short lives (millions vs the billions of years our sun will last) then died, so that we could live for just a few decades.

RS Ophiuchi talk for The Astronomical Society of South Australia

September 4, 2021

A few days ago, on September 1 2021, I gave a short talk (15 minutes) to the Astronomical Society of South Australia about RS Ophiuchi and the 10 known recurrent novae more generally.

The talk starts at the 1 hour 7 minute 39 second mark (cued up here) in the YouTube video after the main speaker, Associate Professor Gary Hill, who gave a talk about observing astronomical phenomena with high energy neutrinos using a neutrino observatory he helped to construct in Antarctica. He was a hard act to follow.

On the subject of RS Ophiuchi, here is an updated visual light curve for September 4 2021 with my binocular observations in purple.

One thing I talked about was the most recent and the next predicted outbursts of recurrent novae. I neglected to add U Scorpii to the list which is predicted to go into outburst in the next year or two. Unlike T Corona Borealis which is likely to reach magnitude 2.5 or 3 sometime before 2026, U Sco will only be magnitude 7.5 or 8 at maximum (starting from around magnitude 18), fading by several magnitudes within a week.