Mysterious Flare-Like Object in M31

The following images document a "mysterious flare-like object" that appeared in only one of several 10-second exposures of the Andromeda Galaxy (M31) central region.  I am baffled by the one image showing a star-like object where none are found in any other images I've taken.

Location Overview

First, here's an overview of M31, with an arrow to show the region of interest.


Figure 1.
  Mosaic of M31, consisting of several individual CCD exposures (taken October 22, 2000).  The arrow points to the location where the "mysterious object" will briefly appear August 23, 2001.

Here's another overview image.

Figure 2.  Larger scale view of the central region, showing location of mystery event. FOV = 27 x 18 'arc. 

Mystery Event Observations
 

Figure 3.
  This 2x zoom is the sum of 4 images, one of which contains the mysterious object, and showing a rectangular region to be used in creating a time-sequence.  The rectangle has a vertical dimension of 7.1 minutes of arc.

The rectangular region is used to illustrate the appearance of the flare-like object, in the following sequence.


Figure 4.
  Here's the sequence showing the appearance of a flare-like object in the second frame.  Exposure times are all 10 seconds, taken August 23, 2001, at 7:37:30, 7:41:26, 7:44:19 and 7:46:48 UT.

Here's a detail of the flare, showing noise and resolution at the pixel level.


Figure 5.
  Blow-up of a portion of the second frame in the previous sequence, showing the "flare" with pixel resolution.  Image area has dimensions 15.5 "arc (wide) x 15.3 "arc (high).  Each pixel has a dimension of 1.101 "arc.  (There is NO Gamma adjustment of this image.)

An analysis of the "flare" feature shows that it is Gaussian in shape, with FWHM (full-width-half-maximum) dimensions of 2.2 and 1.8 "arc (seconds of arc).  The brightest star inthis image, which appears over-exposed in this presentation, is in fact not over-exposed in the original image, and it also has a Gaussian shape but has FWHM dimensions of 3.1 and 3.4 "arc.  The larger width for the nearby star might be accounted for if the "flare" lasted much less than 10 seconds, so that the "atmospheric seeing" moved the image around by a much smaller amount than for the star.

The nearby star is at RA/DE = 00:43:16.365 +41 16 30.10 (UCAC4), with V ~ 12.235. The "mystery event" was at RA/DE = 00:43:17.205 +41:16:41.90, with V ~ 14.2.

What Could It Be?

If it's an object that was nearby and moving, then the fact that it's image size is only 1.9x2.2 "arc places severe restrictions on the angular velocity.  Moreover, if it was moving slowly enough to register a small image it should have been visible in the before and after images.  Only if it blinked could these two constraints be satisfied.  Some hypotheses I'll address are:

    1) satellite sun glint
    2) airplane strobe light
    3) cosmic ray hitting CCD
    4) optical SETI

Hypothesis 1 is impossible, since the observations were at midnight.  (Satellite pass information has nothing after 6:45 UT for that night, whereas the observed feature occurs at 07:41 UT.)  I "blink checked" the other images to test for presence of other anomalous features, and for the same object at another location, and found none.

Hypothesis 2 can be addressed by asking how small a strobe light would have to be to register an image as small as 2.1 "arc?  And  how short a time is required for a very small strobe light to be "on" to register a 2.1 "arc image?  The view was essentially overhead, so assume the hypothetical airplane was at a range of about 35,000 feet, travelling at Mach 0.75 (250 m/s).  Let's ask the question "How large could a light source be and still not smear the image of it on my CCD by an amount that I could detect?"  I shall assume that the smearing is less than 1.5 "arc in order that an "atmospheric seeing" image size of 1.5 "arc would not exceed the measured 2.1 "arc.  At 35,000 feet the physical source size would have to be less than 3 inches!  Travelling at 250 [m/s] means a strobe light would not only have to be smaller than 3 inches but it would have to be "on" for less than 0.3 milliseconds.  If the strobe were 1 inch in diameter, it could be "on" about 200 microseconds, but no longer.  Until I find out about airplane strobe lights I cannot rule out this theory.

Hypothesis 3 would require that a cosmic ray affect neighboring pixels in a way that somewhat mimics a Gaussian response.  The following two figures show my readouts of the RA and Dec intensity verus pixel location "shape" of the flare-like image.

Figure 6East/west shape of the "flare-like" image (red), with a Gaussian fit (green) having FWHM of 2.0 pixels (2.2 "arc).

Figure 7.  North/south shape of the "flare-like image" (red) and a Gaussian fit (green) having a Gaussian width of 1.7 pixels.

The green fits are Gaussian hand-fitted shapes, with degeres of freedom for intensity, location and width.  The goodness of fit is typical for stars.  Nevertheless, unless I obtain information on exactly how cosmic rays affect a field of CCD pixels, I cannot rule out this hypothesis.

Hypothesis 4 is the most intriguing: "Optical SETI."  Could an optical wave front stay well shaped to cross 2 million light years and still produce a flash shorter than 10 seconds?  I assume that since radio waves can (pulsars have no trouble staying coherent for milllisecond lengths travelling over a few hundred light years within our galaxy), optical ones also can.

Subsequent Observations

The following night I spent several hours observing M31, and found nothing unusual - no "flares."  Here's an average of most of those images, using a 2.4x "focal reducer" lens to increase the sky area coverage (by a factor 5.8).


Figure 8.
  The arrows point to no flare feature where it had appeared the night before, in this average of 8 images totaling almost 7 minutes of exposure time.  was taken on the evening of Aug 23/24.  Image size is 31.1 x 21.0 'arc.

In addition to inspecting all images for a flare, I "blinked" many pairs to search for a flare feature somewhere else.  My thought was that whatever unusual effect caused the first flare feature, whether it was an airplane strobe light or a cosmic ray hit of a CCD pixel, the same event might produce another "flare" at some other random location - but none appeared.  Whatever the original event was, it is relatively rare.


Figure 9.  The cross-hairs indicate where NO flare feature was seen on the night of August 24/25. Image size is 34 x 21 'arc.

11.5 years later I again observed this region of interest for several hours, and found nothing.

Conclusion 

I conclude that the most likely explanation for the one image on 2001.08.23 showing the "mystery event" is Hypothesis 3, above: a cosmic ray strike that arrived with a close to normal incidence to the CCD surface.  Whereas most cosmic ray defects are streaks, occasionally one will occur that resembles the PSF of nearby stars, and when this happens it will be tempting to attribute the defect to something with greater significance!

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This site opened:  August 23, 2001 Last Update:  August 25, 2001

Updated
2013.01.12 with a most-likely interpretation.