Archive for the ‘Astronomy’ Category

U Sco update

June 8, 2022

Within a day, the nova reached a peak magnitude of 7.8, then dropped two magnitudes, becoming around 5 times dimmer in actual brightness (magnitude is logarithmic).

The AAVSO Alert Notice details the decline of the last outburst over a few months and flares with an amplitude of around half a magnitude that began several days after the peak, for which the mechanism is unknown.

Outburst of the Recurrent Nova U Sco

June 8, 2022

In late September 2021, I wrote about the recurrent nova U Scorpii: Waiting for U Sco and Waiting for U Sco: nearby gamma ray transient detected.

The last image I took of the region around U Sco in September showed no sign of it, but within the last day it has undergone an outburst, and appears to have quickly peaked, as per previous events (the last being in January 2010) before declining again. The nova reached a visual magnitude of 7.8 on June 7 2022 up from its usual magnitude 18 or thereabouts, a difference in brightness of around 25 times (each magnitude being a factor of 2.512 brighter than the whole number magnitude on either side of it). As I write, U Sco has declined to around magnitude 8.4.

The light curves show the current and 2010 outburst, and a close up of the current outburst. It’s interesting note that there appears to have been some evidence of an increase in magnitude in the few months leading up to the outburst.

I took a DSLR image of the region around the nova tonight. U Sco is marked with a red arrow.

While I don’t have images that are suitable for DSLR photometry from tonight (the sky was less than ideal), nor did I have the chance to make a visual (binocular or telescope) observation, from the image I estimate the nova’s magnitude to be around 8.4 (magnitude 8.3 and 8.6 comparison stars are marked with green arrows), which is consistent with the current photometry in the AAVSO International Database.

The following images show the U Sco region tonight (June 2022) and late last September for comparison, in different sky conditions and positions on the sky. The second image has been rotated to match the current event’s sky rotation. Flicking up and down between the images will help reveal the nova, aided by the arrowed image above if necessary.

June 2022 outburst (5 seconds, ISO 800, f2.0, Canon 1100D)
U Sco region in late September 2021 (3 seconds, ISO 800, f2.0, Canon 1100D)

For more information about recurrent novae, I gave a talk in early September 2021 to ASSA about another recurrent nova RS Oph. See also this AAVSO forum page regarding the current outburst.

EDIT: Here is the AAVSO alert notice: https://www.aavso.org/aavso-alert-notice-779

Making sense of variable star observations

February 23, 2022

I recently gave a talk via ASSA@Home about making the connection between variable star observations (from the AAVSO International Database and Kepler mission) with the processes in the star systems themselves.

I gave examples of pulsating variables (Chi Cyg, RS Pup, RR Lyr, T Umi), an Algol type eclipsing binary (ASAS J035812+1629.7), the enigmatic luminous blue variable eta Car, and the recurrent nova RS Oph, showed videos linking light curves with stellar processes, inspected time series observations, created phase plots and carried out analyses such as period search and time-frequency analysis using VStar.

While not as polished and more ad hoc and exploratory than last year’s RS Oph recurrent nova eruption talk for ASSA, it was fun and seemed to be appreciated, despite less than best audio and video at times. It’s also not easy watching yourself give a talk. 🙂

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 Decline Continues

August 16, 2021

The expected decline of the recurrent nova RS Oph continues, almost 3 magnitudes in 7 days. I’ve managed only 5 observations with a couple of cloudy nights between the last two. Luna at 60% illumination in nearby Scorpius tonight made estimating its magnitude (7.2) in 7×50 binoculars a little trickier tonight.

Recurrent Nova RS Ophiuchi Update

August 12, 2021

Since my Aug 9 observation of RS Oph, the AAVSO alert notice was published by Sara Beck: Alert Notice 752: Rare Outburst of Recurrent Nova RS Ophiuchi. My 4.5 visual estimate was listed there along with others from Australia, Brazil, the UK, the US, Belgium and Russia.

The next night was cloudy and wet, but there were enough clear periods tonight for another estimate, 5.7 visual, as shown below:

The last 2 years of RS Oph visual data (including photometry from CCDs, DSLRs)

The next two plots show more detail:

The last week of RS Oph visual data
The last few days of RS Oph visual data

I suggested in my last post that based upon previous outbursts, RS Oph will have reduced by 1 magnitude in 2 days, which it has done. A little more actually, from 4.4 on Aug 9 to 5.7 on Aug 11.

These objects are a reminder that we live in a violent, ever changing universe.

Recurrent Nova RS Ophiuchi in outburst!

August 10, 2021

The recurrent nova RS Ophiuchi is in outburst for the first time since 2006!

Other known outbursts were observed in 1985, 1967, 1958, 1933, and 1898.

RS Oph outbursts since 1933

The current outburst is shown at upper right of the plot that includes outbursts from 1933 onward.

Around 15 visual observations of the current outburst had been recorded in the AAVSO International Database and the nova had reached around magnitude 4.6 at the time I started writing.

The nova will be visible to the unaided eye from some locations and easily visible in binoculars from suburban locations (at least here in South Australia).

Only a handful of recurrent novae are known and the time between outbursts tends to be measured on a timescale of decades, averaging 20 years for RS Oph but varying +/- 10 or so years.

These systems involve a giant star (in this case, a slow irregular variable with a period of a few hundred days) and a white dwarf star pair in which mass is transferred from the giant to the white dwarf, forming an accretion disk which eventually undergoes a runaway thermonuclear reaction.

Example of a Cataclysmic Variable (RS Oph) from http://apod.nasa.gov/apod/image/0607/rsoph_pparc_big.jpg
Artist’s impression of RS Oph (from APOD)

RS Oph is around 5000 light years distant and so of course, the event we now see happened around 3000 BC.

The magnitude of the RS Oph binary star system increases rapidly from magnitude 10 or 11 to around 4.5 in the space of day, taking around 100 days to return to the baseline in a characteristic decay curve.

Within 2 days it will diminish by around 1 magnitude and by around 2 magnitudes within 5 days.

So, clear skies would be nice! The local forecast does not look great for Adelaide over the next few days.

It was cloudy by the time I read the outburst notification tonight and first started writing this post, but the sky cleared at around 1 am!

I observed the nova low in the west through 7×50 binoculars and estimated its magnitude to be 4.5 (Julian Date 2459436.16237 which is almost 15:54 UT or 01:24 Australian Central Standard Time):

The last month or so of observations. My magnitude 4.5 observation is shown in the cross-hairs.
The last few days of observations. My magnitude 4.5 observation is shown in the cross-hairs.

The 43 and 46 comparison stars (at about 5 and 11 o’clock with respect to the nova) on the following AAVSO finder chart were used for the estimate:

The finder chart needs to be rotated clockwise somewhat more than 90 degrees but less than 180 degrees to match the orientation of the sky shown in the Stellarium image below:

Note the position of the three roughly magnitude 3.5 stars in a straight line above the nova which is below and to the right of the middle of these stars (in the Stellarium image), with Scorpius at far left. These 3 stars occupy much of the finder chart near the nova (which is above and to the right of the middle star on the chart).

While every variable star type has its own interesting qualities, I find novae to be the most fascinating sub-type of cataclysmic variables and probably of all variable types because of the power and unpredictability involved. They are a reminder that we live in a violent, hostile yet endlessly fascinating universe, that we are, as Bernard Lovell said: In the Centre of Immensities.

It will be interesting to see whether the brightness has now peaked. If previous outbursts are anything to go by, it may have. But we’ll see! Novae and their much rarer cousins, recurrent novae, can really keep me up late at night (it’s now 2:55am here)!

Possible Nova in Sagittarius

April 6, 2021

Western Australian amateur astronomer Andrew Pearce has discovered a possible nova (PNV J17581670-2914490) in Sagittarius on April 4. At that time its visual magnitude was around 8.8.

Andrew reported in a message to Variable Stars South today that the object has been classified via spectroscopy as a classical nova.

The nova’s visual magnitude is currently around 8 or a little brighter. As always with these objects, it will be interesting to see how bright it gets.

Here is a finder chart suitable for binoculars (something of a challenge at the current brightness in 7x50s):

Rotating this finder chart a bit more than 90 degrees anti-clockwise will roughly match this image from Stellarium at around midnight AEST.

Zooming out gives more positional context:

Further out still shows even more familiar landmarks in Sagittarius and Scorpius.

I have yet to observe or image the nova but hope to do so over the next few days.

EDIT: After writing this I went out and had an initial look at the area. It was after midnight and the nova was in a good position from my backyard. I was just able to glimpse it in 7×50 binoculars but not well enough to make an estimate. So I took a few quick untracked shots of the region from which it appears that it’s probably around magnitude 7.8 (visual) which is consistent with what I glimpsed through binoculars.

I’ll keep an eye on it over the next few nights and hopefully take some better images.

EDIT: the weather did not improve early enough while the nova was still bright enough for me to observe.