Cosmic Maser Projects Overview

Introduction

The Maser project pages describe an attempt at the implementation of some form of automated astronomical maser observation system using a small dish – where a ‘small dish’ is in the range of 1 – 3 m in diameter.  Additional personal motivation for these projects is to provide some reassurance that when the time comes to move to a smaller block (hopefully many years from now) I will still be able to carry out radio astronomy activities.

The Software Defined Radio (SDR) used in these projects is the RTL-SDR - which has several variants in terms of quality and frequency range - but all with a maximum reliable bandwidth of 2.4 MHz.

The information contained in these projects has been largely compiled from public sources. Some reference is made to information contained in non-public sources – but wholesale reference is not done – not as a sign of lack of importance or relevance – but rather respecting the non-public nature of those resources.

 

Astrophysical Maser Characteristics

In the 1960s, astrophysical MASER or microwave laser emission was first detected. Initially, the emission mechanism was not well understood, and there was speculation that a new element called "mysterium" could be responsible. However, the spectral line was quickly identified as a transition of the hydroxyl radical (OH), and it was soon recognized that the signal properties could be explained by the process of stimulated emission of radiation amplification.

 

Unlike most cosmic radio sources – which are mostly broadband in nature - astrophysical masers emit signals in a relatively narrow range. The radiation is produced by certain molecules falling back from a higher energy state to a lower energy state – the difference in energy (transition) being emitted as electromagnetic radiation in a relatively narrow range – although some appear as a group of closely-spaced transition spectral lines. Maser sources suitable for observation by the amateur are a product of strong flux and at frequencies for which equipment is reasonably easy to obtain. These masers in order of radiation rest frequency are…

 


In addition to the 12.2 GHz line for Methanol masers, there is a strong line at 6.7 GHz. However, at this time, there is no readily available equipment for this frequency – and so it is not included in this project.


NOTE: The 2.4 MHz bandwidth of the RTL-SDR translates to a range of velocities of about 445 km/s, 60 km/s and 30 km/s at 1612 MHz, 12.2 GHz and 22.2 GHz respectively.  Masers tend to come spatially-grouped but somewhat spread in VLSR velocities. Each target's characteristic spectrum needs to examined to ensure the particular target spectrum line lies within the 2.4 MHz bandwidth. In general, OH and Methanol masers fit suitably within the bandwidth - where the range of velocities afforded by the 2.4 MHz bandwidth at 1612 MHz is wide, whilst the spread of velocities for individual methanol masers is relatively small.  For water masers the narrow range of velocities (~30 km/s) afforded by the 2.4 MHz bandwidth coupled with the wide spread in velocities mean that they are more likely to need careful attention.

Analysis of Requirements

It is useful to work backwards from the accuracy required for the measurement of the observed velocity w.r.t. the Local Standard of Rest (LSR) observed on the maser spectrum (VLSR).

What is VLSR ?

To calculate the VLSR of a maser, astronomers use a technique called Doppler spectroscopy. This involves measuring the shift in frequency of the maser emission compared to a reference frequency. This shift is caused by the relative motion of the maser and the observer along the line of sight. The Doppler shift is given by the formula: Δf/f0 = V/C, where Δf is the change in frequency, f0 is the reference frequency, V is the velocity of the maser along the line of sight, and C is the speed of light. By measuring the Doppler shift of the maser emission, astronomers can determine the velocity of the maser relative to the observer, which is typically given in units of km/s. This velocity is then corrected for the motion of the observer with respect to the LSR to obtain the VLSR of the maser.

 Sign Convention: +ve means the observer is moving away from the position. To correct an observed radial velocity to one of the given rest standards, subtract the appropriate value.

Local Standards of Rest: Two forms of LSR exist.  LSR(K) is a "kinematical" LSR and refers to the average motion of nearby stars. LSR(D) is the "dynamical" LSR, the point in the solar neighbourhood which is in circular orbit around the galactic centre. The kinematical LSR is the one which is generally used for radio astronomy. The adopted kinematical LSR is 20 km/s towards RA 18 00 00.0, DEC +30 00 00 B1900 (= 18 03 50.3 +30 00 17 J2000).

Required Accuracy for VLSR

The calculation of VLSR for a maser spectrum necessarily requires removing the Doppler effect of the observer's own velocity through space as a result of residing on planet Earth which travels in an orbit around the Sun.  The accuracy of that calculation of velocity determines the level of accuracy required in the measurement of VLSR. The observer's radial velocity towards the LSR can be calculated dynamically via a software algorithm or read from an online source. A typical online source is one which has taken the code from a previous HawkRAO webpage (without permission - but that's fine), made some cosmetic changes and put up on another website (http://f4klo.ampr.org/vlsrKLO.php).

Another online source is the ATNF calculator (https://www.narrabri.atnf.csiro.au/observing/obstools/velo.html). These two calculators generally agree to within 0.3 km/s. Given the width of the maser emission in km/s is usually > 1 km/s, a rule of thumb for the required velocity accuracy of +/-0.1 km/s would seem reasonable. Using the speed of light of 299,792 km/s, +/-0.1 km/s is +/-0.33 ppm. Having the accuracy expressed in 'ppm' gives a base figure for calculating various equivalents for the maser line rest frequencies.

Required Accuracy of Measurements

The required accuracy and stability for maser observations can be expressed in several ways. The first relates to the required specification of the test equipment used in measuring the behaviour of the receivers, the second is the behaviour of the receivers themselves.

The test equipment specification is the most readily determined as measurements are done in the relatively stable temperature environment of the observatory room. The behaviour of the receivers is more difficult to characterise as it, by nature, must include the large swing in temperatures encountered in the external environment.

In the table on the right, the adopted velocity accuracy (excluding the accuracy in the calculation of radial velocity) is +/-0.1 km/s. From this a velocity accuracy of +/-0.33 ppm is derived. In place of that value, a conservative value of +/-0.2 ppm is adopted. This is a starting value, which may be adjusted (tightened/relaxed) at a later time if needed. For the three maser species hydroxyl, methanol and water, the required 'Sky' frequency accuracy is calculated. Using these respective values and the nominal IF/RF values the required backend accuracy (the RTL-SDR dongle) is calculated in ppm. It can be seen that for Methanol and Water maser observations, the quoted 0.5 ppm stability of the NESDR Smart dongle is entirely adequate without further analysis, while for Hydroxyl masers the behaviour of the dongle needs to be further examined.

Also, the required RBW setting for the FFT analysis application is calculated as nominally 10 % of the required 'Sky' frequency accuracy..

Usage of Analysis Results

These results will be employed in the design of the various maser observation systems including measurement of the behaviour of LNB/F receivers (where used) and systems designed for automated observations.

The first maser project that is to be attempted has Methanol masers as the target. This is because - taking into consideration all aspects at the time - it was thought to require the least development.

Suitable Maser Targets

The selection of targets for this project is based on visibility at HawkRAO - largely governed by a limited window on the sky due to extensive tree cover - and the nominal signal strength expressed as peak flux density. The following targets are drawn from the maserDB database and details initial targets for each of the spectral lines.

Examining the Maser Database

The maser database (maserDB) was queried by an 'All-sky search', search mode = 'Maser variability' setting.

The example here is for the 'H2O' maser species.

It has been found that the 'Maser variability' setting for search mode returns a list with the most suitable information. Of particular usefulness is the values for maximum and minimum observed fluxes. Combined with the total number of observations from which those values were derived, these fluxes give at least a general guide to masers which may be able to be detected.

After clicking the "Start Search" button a small delay ensues (understandable as there are over 9,000 H2O masers listed in the database for example) before a new page is shown with the list.  There is a link designated 'Export to csv' just above the list header, and when this is hovered over a information pop-up advises the list will be saved in the file '_h2o_var.csv' (other names are given for other species searched for).  The default directory where the file is slated to be saved will be the last-used working directory - which can be changed to something more convenient by navigation in the save dialogue.

Once the file has been saved it can be read into a spreadsheet of choice to be processed.

The following information was drawn from the relevant saved csv file. For the purposes of HawkRAO observations the csv files were curated to leave only a selection of the strongest masers (highest maximum flux) with declinations which are accessible through the tree-restricted sky view at HawkRAO. This was arbitrarily chosen to be declinations < 0°.

MaserDB Input Settings

Selecting Target Candidates

Sensitivity calculations for the various dish antennas are done in the next sub-page to determine whether a number of candidates visible from the HawkRAO location are likely to be detected.