Possible new water maser flare in W49

[Eduard Mol]

Hi all, 

For a few years I have been tracking the water masers in the star forming region Westerhout 49. Its spectrum has many different features, representing different maser clouds in the region.

Today I made a new observation of the region at 22.2GHz. Already during the recording I noticed something unusual. One feature was very obviously visible in the IFaverage window with only 1 minute of integration time. One could have easily mistaken it for the hydrogen line :-) (The spike on the left is a reference signal used to correct frequency drifting of the LNB LO). 

I just did a quick processing of the data. The integration time is about 42 minutes. I have attached a plot showing the spectrum from previous observations over the past few months, plus the new one from today.

In recent months the peak at 6.9 km/s appears to have steadily increased in brightness, and is now by far the brightest feature. Of course my measurements are not calibrated to flux density so some change in the levels due to different atmospheric absorption or pointing errors would be expected. However, the cluster of features near -5 to -10 km/s has remained relatively stable over the past few months. So the rise of the 6.9 km/s feature (at least relative to the other features) is probably real. Apparently, whatever event triggered the possible new flare at 6.9 km/s  has not had an effect on the other group of masers at -5 to -10 km/s. 

Anyway, it will be interesting to continue observing this source over the coming weeks and months. 

[fasleitung3]

Interesting, I will try to have a look at W49 on Sunday.

Wolfgang

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[fasleitung3]

I can confirm that the 6.9 km/s feature of the water maser spectrum of W49 is brightening. Please find attached two spectra in comparison.

The black spectrum was recorded today, Nov. 30th. The red spectrum was recorded on Nov. 4th.

Wolfgang

[Eduard Mol]

Thanks, interesting stuff going on there! 

Op zo 30 nov 2025 om 20:23 schreef 'fasleitung3' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>

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[Eduard Mol]

Hi all, 

In response to Wolfgang's most recent observations which he shared during the RTOP meeting on Sunday I decided to do a new observation of W49 today. Wolfgang reported that by december 3rd the brightened features at ~5- 10 km/s range had decreased in brightness while the other features have not significantly changed. I thus expected to see a decrease as well. Instead, today's spectrum shows the 6.9 km/s feature at the same level as it was on November 26. The other features, such as the group at -4 to -10 km/s seem a bit dimmer, but this could also be attributed to pointing error or unfavourable observing conditions (cloudy weather). In any case, when compared to the other features in the spectrum the brightest peak does not appear to have dimmed at all.

So either there was somehow a very short dimming of the maser between November 26th and December 10th (unlikely?) or there is some sort of observational/instrumental effect at play here (polarization? maybe the 10 metre dish is partially resolving the source??) 

Anyway, this sort of unexpected stuff is what keeps me excited about radio astronomy :-)

Eduard

[fasleitung3]

According to high resolution plot from VLA (https://arxiv.org/pdf/astro-ph/0407410) the water maser region in W49 seems to be so confined that it will not be resolved with our 10-m dish. So this seems not to be a possible explanation.

Indeed there is pointing error which also makes absolute intensity measurements questionable. However, the relative intensities of various spectral components should be not affected by pointing inaccuracies.

Something to follow up on....

Wolfgang

[Eduard Mol]

Thanks Wolfgang, so we can exclude a partially resolved source from our list of explanations. 

Today I checked the pointing of my dish, because yesterday evening I noticed that the polar alignment of the mount was off. I carefully adjusted the RA and DEC until I was receiving the maximum intensity from W49 (which was easy to do given how bright the source is right now). Compared to my observation from yesterday this makes quite a significant difference in the total intensity, but not the relative intensity of the spectral components compared to one another as you mentioned. 

Next I checked if polarization angle has any effect. The feed of the LNB I’m using is linearly polarized, in the usual configuration on my dish the polarization is roughly east- west oriented, although I don’t know the exact sky polarization angle. I first did a 30 minute recording with this normal “horizontal” polarization, then I turned the LNB by 90 degrees and did another 30 minute recording. There is practically no difference between the two recordings, so the degree of linear polarization seems to be insignificant.

Op do 11 dec 2025 om 09:43 schreef 'fasleitung3' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>

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[Stephen Arbogast]

Well... I had to look this up  and  in case others  are wondering here is what I found...    https://en.wikipedia.org/wiki/Westerhout_49

Thanks  for  the posts.

[fasleitung3]

This is the latest recording from yesterday, Dec. 14th. It now seems that the 6.9 km/s has brightened again compared to my observation from Dec. 3rd. So we are seeing high variability on time scales of a couple of days.

I also noted that the features of this water masers at the velocity range -90 to -45 km/s (not shown here so far) is also changing quite a bit. I will look into that a bit more.

Wolfgang

[Eduard Mol]

Very interesting!

Usually I only observe the bright features at -20- 30 km/s because my SDR has only about 5 MHz bandwidth, so I don’t have any recent data on the features at -45 to -90 km/s. 

Yesterday I did a quick observation of the blueshifted part of W49. I made four spectra with 10 minutes of integration time spaced 4 MHz apart so that there is some overlap between them. There are indeed some relatively bright features at -60 to -75 km/s. Is this what you are referring to?

Op ma 15 dec 2025 om 09:45 schreef 'fasleitung3' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>

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[fasleitung3]

Yes, these are the ones I was referring to. I will have a closer look at the development of this feature once I am back from travelling.

Wolfgang