22.2GHz water maser observations with a small dish

[robin_astro]

4 hours ago, Ed astro said:

Another way to display the changes is by plotting a heatmap of all the spectra

Impressive work again !   I used a similar technique to generate  the image in my avatar (but from data at a much higher frequency )

https://britastro.org/observations/observation.php?id=20100825_230000_95356e4359198628

Cheers

Robin

[Ed astro]

Hi all,

Besides the regular observations of W49 and W51 I have also observed some other maser sources in the last few months. 

In August I managed to detect W75N in Cygnus. The detected peak has an LSR velocity of 11 km/s, which matches quite well with the LSR velocities of this object reported in the maser database MaserDB. 

The star forming region G25.649+1.050 was observed on three evenings, and a weak peak was detected around the expected LSR velocity of +43 km/s. However, the signal-to-noise ratio was too low to call this a positive detection. The peak was also very close to the midpoint of the spectrum where artifacts of the SDR receiver often occur. This source has to be observed another time.

Finally, the Westerhout 3 star forming complex has been observed again for about 1.5 hours yesterday evening. The W3 region contains two water maser sources: W3(OH) and W3(2). W3(OH) is usually the strongest of the two. To my surprise, there was no peak in the spectrum at -48- -50 km/s where I would have expected a strong signal of W3(OH). Instead, three weak peaks are detected at -37, -39 and -41 km/s, which is more consistent with W3(2). Maybe W3(OH) is just unusually weak now, or my frequency or velocity correction is off. However, W51 was also observed the same evening (as part of the monthly observation program on W49 and W51) and the LSR velocity of the peak seemed to be right, so a mistake in the frequency or velocity correction seems less likely. Anyway, the unexpected results are often also the most interesting and fun. 

 

[Victor Boesen]

Brilliant Ed! I thoroughly enjoy these updates

Victor

[Ed astro]

Hi all,

It has been a while since I posted an update on this project. In the past few months I have detected two more water maser sources: Cepheus A and G37.403+0.232

Cepheus A was observed on October 9 and 10. Interestingly, this star forming region has a characteristic double peaked spectrum. The two peaks could be originating from two different maser spots within the star forming region, moving at slightly different velocities. 

 

In the period November 2022- January 2023 five observations have been done pointing the dish at the star forming region W44. The water maser emission from this region is an average velocity of 60 km/s relative to the local standard of rest (LSR) according to the data published on the MaserDB database. Shown below is the averaged spectrum from all five observation sessions (approximately 8 hours of total observing time). A vey faint signal was indeed detected, but it was at -56 km/s, so a bit too low for W44. In fact, the velocity is more consistent with another source, G34.403+0.232, which is located just 10 arcmin north of W44.

 

[Ed astro]

Hi all, 

Here a little update on the water maser project:

In February I opened up the LNB to adjust the frequency of the local oscillator (LO) by carefully tuning the frequency adjust screw.  Now the LO frequency is changed to 20.814 GHz and the 22.235 GHz water line is converted down to 1421 MHz. This is well within the frequency range of my airspy mini SDR, which has a larger bandwidth compared to the nooelec SDR I was using before.

I have also continued doing observations of the strong source W49. The variability is very obvious when looking through all the spectra collected over the past year. The features at -4 - -14 km/s seem to be particularly active.

 

W51 decreased in brightness by two orders of magnitude since February 2022, and is no longer an easily detectable source. In February I therefore decided to pause monthy observations of this source in favour of other masers that are easier to detect now.

[Victor Boesen]

Wow Ed! Once again very impressive results Since you've adjusted the LO frequency of the LNB is there any way you could perhaps benefit from an LNA like the Nooelec H1+ SAWbird?

I can't remember if you are already using some sort of filtered amplifier.

Victor

[Ed astro]

Hi Victor, 

the amplifiers inside the LNB already have plenty of gain, so adding an LNA would not be that useful.

The 1420 MHz output frequency is intentional though, so that I could use an existing 1420 MHz filter if necessary. So far RFI at 22 GHz has been very limited, but with the growth of satellite megaconstellations and other services operating at cm- and mm- wavelengths the RFI situation could soon change….

[Pete Clearsky]

Ed,

This is fascinating!  It made me curious, and perhaps I could try this with similar hardware I have, although I don't have a dish at hand.

However, I have a question though: water vapor in the earth's atmosphere has a strong absorption near 22 GHz, so how does that affect your observation so far away?  I am thinking that the signal from W49 is smaller than the water vapor variability here.  Any comments?

 

[Ed astro]

Hi Pete,

Yes, water vapor in the atmosphere definitely has a noticeable impact on my observations but it is not a huge problem. The 22.2GHz line emitted by water vapor in the atmosphere is spread out over several GHz due to pressure broadening. With the 6 MHz bandwidth of the receiver you see an increased background noise instead of a line, and extraterrestrial signals appear weaker due to absorption losses. The atmosphere's background noise at 22.2 GHz is typically around 30- 40 kelvin near zenith, and path loss is around 10-20%. Atmospheric noise and losses get worse as you point the telescope lower due to the increased airmass, so it is best to observe a source when it is near its highest point above the horizon. 

With regards to variability: because the atmospheric water signal is so much wider than the bandwidth of the receiver and the width of the astrophysical maser lines, it should affect all peaks in the maser’s spectrum nearly equally. In W49 we see the peaks vary independently of each other, which can not be explained by variations in atmospheric conditions. 

[Weatherguy]

Hi, Ed,

That's a great explanation about water vapor effects.  On the other hand, I was wondering whether you could detect or quantify water vapor with the setup you have. As you pointed out, the pressure broadening made water vapor absorption look like background noise within the frequency range.   Also, not sure where you got the 6MHz bandwidth for the receiver.  You LNB picture shows the frequency range is from 21.2 to 22.2 or 1 GHz, right?  Anyway, great work.  You should publish it.

 

[Ed astro]

Hi,

The 6 MHz bandwidth is of the Airspy SDR receiver I’m using.

it is definitely possible to use the 22.2 GHz band for quantifying water vapor in the atmosphere; see for example this article: https://hal.science/hal-00224825/document

An interesting aspect is that due to the effects of pressure broadening the water spectral line becomes narrower with increasing altitude; it is therefore possible to probe water vapor at different heights in the atmosphere by observing at different frequencies away from the rest frequency.

not easy to do though with my dish, since I would need accurate calibration loads.