On August 25th, the SOHO and STEREO spacecraft observed a previously undiscovered sungrazing comet. Although thousands of “sungrazers” have been observed by the spacecraft’s coronagraphs, this one was significantly brighter than most. In fact, it reached magnitude +3 on August 27th, before fully disintegrating. The object was a Kreutz-group member, part of a vast family that includes many great comets, most notably C/1843 D1, C/1965 S1 and C/2011 W3.
Figure 1: SOHO/LASCO C2 image extract showing the comet only hours before it vanished. The apparent “tail” is actually more likely a trail of debris, a result of the comet’s disintegration. Image credit: ESA/NASA SOHO/LASCO C2.
In the early hours of August 26th (UT), amateur astronomers Junda Liu and Peiyuan Sun reported the presence of an unknown comet entering the lower left edge of the SOHO/LASCO C3 FOV (Figure 2). Based on its trajectory, it was obvious that it was a Kreutz-group comet on its way to perihelion. Both the discoverers are members of the Sungrazer Project, a citizen science project dedicated to discovering near-sun comets in SOHO/LASCO and STEREO/SECCHI data. In fact, this was the first comet discovered by Junda Liu.Figure 2: A SOHO/LASCO C3 image extract showing the comet as it appeared only hours after the discovery images. Although apparently small, it is much brighter than most Kreutz-group comets would appear at that distance from the Sun. Image credit: ESA/NASA SOHO/LASCO C3.
It is rare that a Kreutz-group comet is bright enough to be observed at the very edge of the SOHO/LASCO C3 FOV. Most sungrazers of this family are only visible much closer to the Sun, when they reach their very brightest (10-15 solar radii. Knight and Battams, 2014). These tiny comets tend to disappear after only a few hours, as they rapidly disintegrate and fade below the instrument’s threshold (Figure 3). According to Karl Battams (NRL): “It’s maybe once every 3 or 4 years we discover a Sungrazer at the edge of the C3 field of view like this!” Consequently, as the comet was already easily detectable when entering the FOV, it was evidently going to become a very bright object. Figure 3: A “typical” Kreutz-group comet as seen in SOHO/LASCO C3. The comet is designated SOHO-2572 and the images are from August 18th, 2013. Although having been present in the FOV for more than a day, this object was only detectable (for only a few hours) when at its brightest, before it rapidly faded below the instrument’s limiting magnitude. The low image resolution, along with the comet’s faint nature, made it impossible to detect any tail. Image credit: ESA/NASA SOHO/LASCO C3.
As expected, the comet brightened significantly over the course of the next couple of days (Figure 4). By August 27th, it had reached magnitude +3. Its brightness was such that it slightly saturated the SOHO/LASCO detectors. However, by 08:00 UT that same day, the object started fading dramatically, clearly indicating that the nucleus was disintegrating. This was sadly expected, as this tends to be the case of all Kreutz-group comets discovered by SOHO. Indeed, although relatively bright, the sungrazers detected by SOHO (and STEREO) are generally intrinsically faint (small). Based on the values provided by Knight et al. (2010), one can assume that the comet probably had an initial diameter of a only few tens of meters. The object was never recovered after it passed behind SOHO/LASCO’s occulting disk.Figure 4: SOHO/LASCO animation showing the comet’s inbound journey towards perihelion. Note how the comet rapidly brightened, before suddenly fading. The apparent “tail” is actually more likely to be a trail of debris, a result of the comet’s disintegration. Image credit: ESA/NASA SOHO/LASCO C2.
In addition to having been observed by SOHO, the comet was sufficiently bright to clearly appear in STEREO/SECCHI’s real time (beacon) COR2-A images (Figure 5). Unlike SOHO/LASCO, real time SECCHI data are of very low resolution. These are, however, exchanged by higher quality data a few days later. Figure 6 shows the comet as seen in a fully processed COR2-A image taken on August 26th. Note the difference in the comet’s trajectory as seen from STEREO, relative to SOHO. This is due to their different in locations in space, hence perspective.Figure 5: Animation of COR2-A real time beacon images showing the demise of the new Kreutz-group comet. Note the difference in its trajectory relative to that observed by SOHO. This is due to STEREO-A’s location ~60° behind the Earth and SOHO. The images were taken on August 27th. Image credit: NASA/SSC STEREO/SECCHI COR2-A.
Perhaps the comet’s most obvious feature was its prominent tail, which grew progressively longer as it neared the Sun. Rather than being an “ordinary” tail, it was probably a debris trail – remnants from the comet’s vanishing nucleus. The “tail” was also clearly visible in COR2-A, however its morphology differed significantly from the “streak-like” appearance observed by SOHO (Figure 1 and 4). This was [again] due to the spacecraft’s differing perspectives.Figure 6: A full-resolution STEREO/SECCHI COR2-A image extract showing the comet. Notice the tail’s (debris trail’s) curved nature, compared to the “streak-like” morphology observed by SOHO/LASCO. Image credit: NASA/SSC STEREO/SECCHI COR2-A
Although now-gone, the comet is still being studied. As Karl Battams posted on Twitter: “I might actually resurrect this subject next week, pending some additional analyses. There might have been something slightly different about this particular Kreutz compared to most other Kreutz, but I need to look at some extra data first”. In addition, I believe that it is important to study the full resolution STEREO/SECCHI COR1-A images from August 27th, once those are made available. It is possible that the comet may have been faintly detectable in those.
Information from various Twitter posts, formal articles (references included in the article), and via the Sungrazer Project website.
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