An apparent optical transient near IC 2539

[ngcles]

Hi All,

I'm hoping for your help on this one.

Last night I was observing the small faint nearly edge-on galaxy IC 2539 in Antlia at about 9.30pm local time (10.30 UT) on 28032009 at Bargo NSW. The conditions were very good. I'd rate the seeing at 8/10 and somewhere around 0.7" or possibly slightly better. The transparency I'd rate as a 7 verging on 8/10 with a SQM-L showing a reading 10mins previous of 21.12 equating to a ZLM about 6.2. I was observing at x185 with a 12mm TII with an 18" f/4.9 with Argonavis DSC and Servocat drive and goto.

A POSS II red 5 arc-min square image of the galaxy which shows the position of the possible transient is here:

http://stdatu.stsci.edu/cgi-bin/dss_search?v=poss2ukstu_red&r=10+04+17.00&d=-31+21+40.0&e=J2000&h=5.0&w=5.0&f=gif&c=none&fov=NONE&v3=

There were three stars near the galaxy that I could see while observing -- TYC 7170-1368-1 is on-axis with the galaxy to the SW -- about 1.4 arc-mins from a rough centre of the galaxy in PA 208 has a magnitude of 12.06. I could also see two very faint stars just off the W flank of the SW end of the galaxy. These were USNO 0525-12640582 at mag 15(??) and USNO 0525-1264033 at mag 15.75.These two are respectively -- 39" distant from centre in PA 238.9 and 56 arc-seconds in PA 225.Both of these were very faint in the eyepiece and averted vision. On the image, there is a further faint star between TYC 7170-1368-1 and USNO 0525-1264033. This is USNO 0525-12640453 (mag 16.95) and was not seen.

As I was observing the galaxy, I noticed the star USNO 0525-12640582, or at least at a position coincident with that star brightened quite a bit to become almost as bright as TYC 7170-1368-1. The apparent brightening ramped up evenly over about 3 seconds and remained apparently constant for about another 3 or 4 seconds and then faded somewhat more quickly back to "normal" in perhaps about a second. I'd estimate it reached mag 12.7 or thereabouts -- I didn't get long to make the estimate. Nothing else in the field or near the galaxy showed anything similar at the same time (or any other time) -- this was the only object to do this. No movement in the apparent transient was detected/observed while the event occurred.

I am a reasonably experienced observer. No alcohol consumed in the previous 14 days.Illicit drugs not involved. No prescription or non-prescription drugs for at least 14 days. The equivalent of 1 cup of normal strength tea consumed about 10 hrs before -- caffeine apparently not an issue. 8 hrs sleep the previous night. Had been awake just 11 hrs before making this observation. Not feeling tired at all. Had only just started "serious" observing 10 mins before.

I believe this apparent observation is of a non-spurious nature -- of course I have no back-up and there is no objective corroboration. I feel pretty confident in the observation. The event lasted at most 10 seconds, probably more like 6 or 7 and there was no time to gain confirmation from other observers. I am unable to say whether it was the star that brightened or something in the background or foreground at a coincident position caused the apparent observed brightening.

I've never seen anything like this before. Should this be reported somewhere? Is there an apparent and logical explanation?

Feel free to comment.



Best,


Les D

[gary]

Hi Les,

Interesting observation.

Such a short duration.

In the literature to date, what is the shortest duration microlensing event ever observed?
Microlensing events usually occur over a period of days. For example, in this paper
of 10 years back, the shortest duration event recorded was 5 days.
http://adsabs.harvard.edu/full/1999A&A...344L..49A

Best Regards

Gary
Mt. Kuring-Gai

[ngcles]

Hi All,

I have received a reply from the highly respected Amateur now professional astronomer Brian Skiff that was posted on AMASTRO and it seems I'm not the first person to see this type of event (not with this star though!)

"Les' star is a 4".5 pair of similar brightness on DSS and 2MASS images. The southeastern component is slightly brighter, and the2MASS colors are those of a garden-variety K0 giant or mid-K dwarf;statistically in this part of the sky it is somewhat more likely to be a dwarf rather than a giant---both very ordinary sorts of stars,
in either case, so nothing obvious about the star color.

I have seen just this sort of phenomenon once, just as Les
has described: that is, a perfectly ordinary field star flaring up
by several magnitudes, then fading back to the previous brightness.
My event was shorter, lasting only a couple seconds total.
Looking at my observing records, the star was GSC 5706-10179 =
UCAC2 28786920 (18 54 34.75 -08 49 55.6 [J2000, UCAC2]),
which is the mag 13 star on the WSW edge of the large planetary
nebula IC 1295. My notes (1981 March 15) say the star flared by
2.5 magnitudes, then dropped back, followed by irregular flickering
for a few seconds. None of the other stars in the field (obviously
a lot of them!) did anything. I was using a 25cm Newtonian at
the time. The star has the color of a slightly reddened K-giant,
which are ubiquitous in the Milky Way.
Back in 1981 I knew about ordinary flare stars, but knew they
didn't erupt that fast (the timescale violates light-travel-time
and simultaneity principles). But we know now about funny stuff
happening on very short timescales, but getting them to happen
around plain-vanilla stars is problematic.

Since we are only beginning to explore this time-domain in
astronomy, it is not surprising there's no ready explanation for
these sorts of observations. I dunno what's going on, but aside
from probably being more fatigued (Scutum in mid-March from the
northern hemisphere....that's 4am!), I am just as convinced as Les
of the reality of what I saw. If it was something new in the field,
you could readily invoke physio/psychological effects, but the event
occurred on an identifiable star.

Watch this space!

\Brian"

So I might not have been dreaming afterall ...

It seems too short to be a flare from a flare-type star (many K-stars and most M-type stars are flare stars). They normally last many minutes and it is waaaaaaay to slow to be a micro-lense event. No explaination yet. Maybe it is a rare and as yet unknown type of event -- who knows :shrug:

If you've got a suggestion, give a hoy!

Best,

Les D

[Terry B]

What you need to do is track down an image taken of the field across that time. Maybe the ASAS was aiming the correct way.:shrug:
If an image is available it should show an increase in the mag- even if it was of short duration. Good luck.

[astroron]

Les, I to have seen the same phenomenon a few years ago, I informed Dr Ian Wilson an Astronomer who used to work as a professional and he like Brian was unable to give an explanation:shrug:
I am sure there a few experienced amateur astronomers out there who have the same observations:thumbsup:

[OneOfOne]

I can only think of one comment myself....wooo hoooo! Lucky guy :(