OIII arc in Gemini

An arc-shaped emission nebula (denoted as the arc in the rest of the text) in the constellation Gemini, with dominant [OIII] lines and an apparent length of about 17°, was detected and is presented on this page. This object has already been recorded by a far-UV survey. What is new, however, is that this object can be seen in visible light and that it is a strong [OIII] emitter.

Click on the image for a full scale version.

Intensity ratios

In the following images, the three wavelengths are shown separately to facilitate comparison.

[OIII]	Hα
[SII]	Luminance measurement regions

The following table contains the results of intensity measurements for the regions marked in the upper image. The average intensity in Rayleighs is given in each case. The region labeled bg was used as a background reference.

Region	Hα	[OIII]	[SII]
K1a	0.02	2.04	0.06
K1a'	1.68	1.53	1.14
K1b	2.20	0.63	2.27
K1c	0.24	0.72	1.54

For the determination of the intensity, some parameters had to be estimated. The transmission ratio of the optics and filters was assumed to be 75% for [OIII] and Hα and 70% for the [SII] doublet. The atmospheric extinction under best conditions was assumed to be 20%. The errors resulting from these estimations only distort the absolute measured values by a factor, but have no influence on the intensity ratios between the wavelengths. More relevant are the errors caused by the background calibration, as they distort the intensity ratios between the individual emission lines. The accuracy of the background reference (the two regions marked bg in the image above) is limited by the fact that the entire region is full of faint background nebulae.

Classification and discussion of the superstructure

The arc seems to be a part of a larger structure. To better assess the situation, a wide field view has been created from data of DR0.2 of the Northern Sky Narrowband Survey.

Visual light

A closer look at the faint filaments east of the arc (left in the images) reveals that they seem to cross the arc, as sketched in panel (a) of the next figure. Furthermore, unlike the arc, these filaments have a structure similar to the other SNR (red and blue) near the galactic plane. Connecting them seems natural. The yellow ellipse in panel (b) of the following figure shows the outline of the projection of a hypothetical bubble containing the SNR filaments that appear to belong together.

(a)

(b)

X-rays

In X-rays, a SNR denoted as the Monogem Ring can be observed. Its progenitor is most likely the same as that of the Pulsar PSR B0656+1. This ring is plotted in white in panel (b) of the figure above. Size and position are taken from Knies et al. (2024), who analyzed the region using the latest X-ray data from the eROSITA telescope array. The same authors also propose the existence of a second SNR in a similar distance and with a similar size. This two SNR scenario is plotted in magenta in the image above, where the eastern (left) SNR is designated G205.6+12.4 while the western one is still denoted as Monogem Ring.

Both scenarios are a good match to the visual structures, including the faint filaments east of the arc. However, the arc does not appear to be part of the Monogem Ring or G205.6+12.4.

Far UV

The arc can also be seen in Galex far-UV (134 nm to 179 nm) images as a thin filament. However, since the data is incomplete and the region is full of similar structures, the arc cannot be recognized as a separate object in Galex images.

Far UV spectroscopy was performed by Kim et al. in the range of 90 nm to 175 nm. In the low resolution CIV (154.8 nm and 155.0 nm) emission lines image (Fig. 1), only the region corresponding to the southern part of the arc (at a galactic longitude from 201° to 208°) becomes visible. In the paper, this region is denoted as R1, and it approximately coincides with the region where the arc apparently overlaps with the Monogem Ring or G205.6+12.4, respectively. Due to the low resolution, it is uncertain whether these emissions originate from the SNR or are the result of some interaction. It is very unlikely that the emissions come directly from the arc, because the northern part (at a galactic longitude from 193° to 201°) is not visible. Furthermore, the authors detected only CIII and CIV emissions, but no OIII. In visible light, however, [OIII] is dominant. The authors interpret this structure as “the blast wave [of the Monogem SNR, see below] with an isolated cloud”.

Interaction between Monogem Ring and arc ?

There is a noticeable similarity between the arc and the position of the Monogem Ring or G205.6+12.4, respectively:

• In the southern part, where the arc and the SNR apparently overlap, the arc can only be seen in [OIII].
• Approximately at the apparent boundary of the SNR, there is a region where only Hα and [SII] emissions can be detected.
• In the northern part, which lies outside the SNR, all three observed emission lines are visible

However, the lack of Hα emissions in the southern part can only be explained by an absence of hydrogen in this region. Since oxygen is still present, this cannot be the result of an interaction between an expanding shell and the arc. Thus, the variation in emission line intensity ratios across the arc is propably caused by a variation in elemental abundance. (The varying [OIII]/[SII] ratio could be explained by differing ionization energy at a constant elemental abundance.)

3D structure of the arc

The OIII arc is probably (a patch of) a surface on a three-dimensional structure referred to as the bubble in this section. What we see is the two-dimensional projection of this surface. What we see is the two-dimensional projection of this surface. Even if the bubble is assumed to be spherical, it is difficult to reconstruct it from just a small fragment. However, due to the high aspect ratio, the projected arc must lie close to the perimeter of the projected bubble.

A rough estimation of the region in which center of a spherical bubble can be expected is depicted by the yellow ellipse in the following figure. The yellow square would be the center if the arc lies on the apparent boundary of a spherical bubble.

Geminga SNR ?

Interestingly, the birthplace of the Geminga pulsar lies within the region of possible centers. The position taken from Pellizza et al. (2019) and Faherty et al. (2007) (both works independently achieve approximately the same position) is marked by the magenta square in the next figure. The magenta circle depicts a hypothetical spherical Geminga SNR with an apparent diameter of 72°. With that assumption, the de-rotated arc would have an aspect ratio of about 3, i.e. it would be quite distorted. However, it is unlikely that the SNR is exactly spherical. Thus, the region of likely centers would be larger as well.

Another filament that might belong to the Geminga SNR is marked by a white line in the figure below. It seems to be connected to the Orion-Eridanus superbubble. However, this structure is assumed to be located below the galactic plane (Ochsendorf, 2015) and therefore can't be the orIgin of the filament.

The estimated age of the Geminga pulsar is 346 kyr (Pellizza et al., 2019). Assuming an ISM density of 0.1 to 0.3 atoms per cm³ (Local bubble: 0.05, Milky way average: 0.5) and a characteristic explosion energy of of 10⁵¹ ergs, the Geminga SNR would be in the snowplough phase and have a radius of 50 to 80 pc (Padmanabhan, 2001). (The calculation mainly depends on density and only weakly on explosion energy.) To correspond to an apparent radius of 36°, the birthplace of Geminga (and thus the SNR center) would have to be at the lower end of the estimated distance range of 90 to 240 pc (Pellizza et al., 2019). In this case, the SNR would expand at least partially into the Local Hot Bubble (LHB).

However, within the LHB, the speed of sound is about 120 km/s (atomic hydrogen at a temperature of 1.1×10⁶ K, see Liu et al., 2017), which means that the shock region of the Geminga SNR would dissolve quickly. (Interestingly, the speed of sound of pure atomic oxygen at the same temperature is about 30 km/s.) Furthermore, the SNR could not sweep up much ISM. That would explain why we don't see more filaments. In particular, the SNR would only become visible where it hits denser (and cooler) ISM — for instance, at the rim of the LHB. In these regions Padmanabhan (2001) predicts visibility in the optical range. Despite of the question of whether the OIII arc belongs to the Geminga SNR, the fact that most of the SNR appears to be invisible supports the assumption that the Geminga pulsar was born in the LHB.

In summary, the hypothesis of the arc being a visual filament of the Geminga SNR remains highly speculative.

Further observations

Weinberger et al. discovered a [SII]+Hα filament. which is also visible in the wide field image presented above. A few similar objects are located in the same region. Some of them are bright in [SII], while others are bright in [OIII]. They appear to be extensions of the elongated southern filament of the Monogem Ring or G205.6+12.4, respectively. The mentioned objects can be found in this Javascript presentation.

Image data

Images where captured with a camera array which is described on the instruments page.

Image data are:

	View #1	View #2
Center position:	RA: 7:44h, DEC: 18:30°	RA: 6:12h, DEC: 14°
FOV:	8.75°×14.31° (RA×DEC, through center)	60°×60° (RA×DEC, through center)
Orientation:	North is up	North is up
Scale:	10 arcsec/pixel (in center at full resolution)	10 arcsec/pixel (in center at full resolution)
Projection type:	Stereographic	Stereographic
Exposure times (Sum of exposure times of all single exposures used to calculate the image):	Hα: 80 h OIII: 55 h SII: 52 h	Data from DR0.2 of the Northern Sky Narrowband Survey

Image processing

Both images were processed differently. The second image (60°×60° FOV) was calculated from DR0.2 of the Northern Sky Narrowband Survey. (Follow the link for a detailed description.) The first image (8.75°×14.31° FOV) is older and is not continuum-subtracted (in contrast to the newer one). Its processing is described as follows.

All image processing steps are deterministic and none of the algorithms use machine learning (often referred to as “AI”), which tends to generate plausible looking fake details. The software used can be downloaded here.

The image processing steps were:

1. Bias correction, dark current subtraction, flatfield correction, noise estimation
2. Alignment and brightness calibration using stars from reference image
3. Stacking with outlier rejection, background estimation and optimal weighting based on noise estimation
4. Star subtraction where star positions and intensities are extracted from continuum images
5. Denoising and deconvolution of both components (stars and residual)
6. Dynamic range compression using non-linear high-pass filter
7. Color composition and tonal curve correction

References

 

Jacqueline Faherty, Frederick M. Walter, and Jay Anderson. The trigonometric parallax of the neutron star Geminga. Astrophysics and Space Science, 308(1-4):225–230, April 2007. [ DOI | http ]

 

I. J. Kim, K. W. Min, K. I. Seon, J. W. Park, W. Han, J. H. Park, U. W. Nam, J. Edelstein, R. Sankrit, and E. J. Korpela. Far-Ultraviolet Observations of the Monogem Ring. The Astrophysical Journal, 665(2):L139–L142, August 2007. [ DOI | http ]

 

Jonathan R. Knies, Manami Sasaki, Werner Becker, Teng Liu, Gabriele Ponti, and Paul P. Plucinsky. A new understanding of the gemini-monoceros x-ray enhancement from discoveries with erosita, 2024. [ arXiv | http ]

 

Bram B. Ochsendorf, Anthony G. A. Brown, John Bally, and Alexander G. G. M. Tielens. Nested Shells Reveal the Rejuvenation of the Orion-Eridanus Superbubble. ApJ, 808(2):111, August 2015. [ DOI | arXiv | http ]

 

T. Padmanabhan. Theoretical Astrophysics, Vol 2. 2001. [ DOI ]

 

L. J. Pellizza, R. P. Mignani, I. A. Grenier, and I. F. Mirabel. On the local birth place of Geminga. Astronomy and Astrophysics, 435(2):625–630, May 2005. [ DOI | arXiv | http ]

 

R. Weinberger, S. Temporin, and B. Stecklum. Detection of an optical filament in the Monogem Ring. Astronomy and Astrophysics, 448(3):1095–1100, March 2006. [ DOI | arXiv | http ]

 

Bing Zhao, Yiqing Guo, and Xunxiu Zhou. Geminga SNR: Possible Candidate of Local Cosmic-Ray Factory (II). Universe, 9(2):93, February 2023. [ DOI | http ]

 

W. Liu, M. Chiao, M. R. Collier, T. Cravens, M. Galeazzi, D. Koutroumpa, K. D. Kuntz, R. Lallement, S. T. Lepri, D. McCammon, K. Morgan, F. S. Porter, S. L. Snowden, N. E. Thomas, Y. Uprety, E. Ursino, and B. M. Walsh. The Structure of the Local Hot Bubble. The Astrophysical Journal, 834(1):33, January 2017. [ DOI | arXiv ]