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e-MERLIN Cycle-7 capabilities

Summary

Selected e-MERLIN capabilities are available in cycle-7.

• L-band, C-band and shared-risk K-band (1.3-1.75GHz, 4.5-7.5GHz, 21-24GHz).
• Resolution about 12 mas (K-band) 40 mas (C-band), 150 mas (L-band).
• Up to 0.512 GHz instantaneous bandwidth
• Continuum observations at both C and L-band will use, by default, the most sensitive and interference free parts of the band with expected 1σ rms noise 12 hr on-source of 6-7 μJy/beam (at L and C-band) and with Lovell Telescope under good conditions. This corresponds to a 3σ sensitivity to brightness temperatures around 300 K.
• Full polarization for all observations
• Maximum spectral resolution 10⁷ i.e. 0.03 km/s (C-band), 0.1 km/s (L-band).

Note that when complete, e-MERLIN sensitivity and imaging fidelity at C-band will be improved by more than a factor of two by using the full 2-GHz bandwidth. K-band (around 1.2 cm wavelength) and a wider range of correlator configurations will be offered, see The technical capabilities of e-MERLIN

Sensitivity and imaging capability

During cycle-7 the optimum continuum sensitivity will be achieved in 12 hr on-source, corresponding to about an 18 hr track including calibration, using the Lovell telescope, for targets with declination greater than 20deg.

At C-band, continuum rms values of 7 μJy/beam will be reached by placing the 0.512 GHz bandwidth in the most sensitive part of the band, with the current system (about 5.8-6.3 GHz), assuming minimal losses due to interference giving a useful bandwidth of 0.5 GHz. We will observe using four, adjacent 128-MHz spectral windows.

At L-band, standard continuum observations will be made using eight 64-MHz spectral windows which will be placed contiguously over 0.512 GHz to optimise continuum sensitivity and avoid interference. Typical RFI levels across L-band result in around 10-15% of the total 512MHz band being unusable. Observational tests to date have typically actived sensitivities of 6-10 μJy/beam.

We suggest that continuum observers with no precise frequency requirements just specify L-, C- or K-band, and the optimum range will be chosen at observing time.

Up to four additional spectral windows can be used in spectral line modes within the same 0.512 GHz band, see below.

The nominal cycle-7 ranges for L- and C-bands are given in Table 1, but observing towards the edges of C-band at this stage will be subject to up to double the noise. The Lovell Telescopes is a University of Manchester instrument and its inclusion approximately doubles the sensitivity of e-MERLIN at L- and C-band. Observing time with e-MERLIN, including the Lovell Telesecope is typically oversubscribed by a factor 2 compared to e-MERLIN without the Lovell Telescope (oversubscription rates of 4-6 [with LT] compared to 2-3 [without LT]). As such the e-MERLIN TAC will scientifically assess and allocated Lovell Telescope time to highest ranking projects that provide a suitable justification for its inclusion. Please include in your proposal justifications why the Lovell Telescope is needed for your project. On occasions, and subject to scheduling restrictions, the e-MERLIN observing team reserves the right to substitute allocated e-MERLIN+Lovell telescope observing time for extended (in time) e-MERLIN non-Lovell observing time to match the user's target sensitivities. The noise levels without Lovell are approximately twice those given for continuum without it.

We give 1σ rms noise levels. We suggest that observers aim for at least 5σ sensitivity for detection experiments although 3σ details are reliable around brighter sources. Dynamic range limitations may affect bright sources (several Jy), especially in spectral line mode.

Table 1: cycle-7 continuum observing capabilities of e-MERLIN

¹Observations shorter than about 6 hr sources should be split up into a few shorter cuts to give good hour angle coverage. This will generally be a better use of instrumental time if several targets nearby on the sky can be interleaved. However, the imaging fidelity for any but the most compact sources will not be as good as for a full track.

²Exact field of view depends on frequency and also on integration time and channel width. For cycle-7, for less than about 10% smearing, the integration time of 1 sec limits the radius to about 6 arcmin at C-band, 20 arcmin at L-band (greater than the primary beam FWHMs). The default channel widths of 0.25 MHz or 0.0625 MHz, at C- and L-bands, give radii of about 10 and 30 arcmin, respectively.

Spectral capabilities in cycle-7

cycle-7 will observe in C-band initially for several months and then change to L-band. It will not be possible to change between bands at short notice until later cycles. Up to 512 MHz can be observed instantaneously within one band. This can be located flexibly within C-band but there is a calibration overhead. We recommend that, if you need to observe in more than one 512-MHz range (e.g. for spectral lines around 4.8 and 6 GHz), you do this in separate tracks, or if the entire observation is to take place in a single track, frequency changes are made no more often than every couple of hours.

The total bandwidth potentially available is 512 MHz (in future cycles, wider bandwidth will be available at C- and K-bands). The bandwidth is divided by factors of 2ⁿ into sub-bands. The data are recorded in spectral windows (spw) which can be the same width as the sub-band, or smaller by a factor of 2ⁿ. Spw must not overlap sub-band edges.

• C-band 512 MHz divided into four 128-MHz sub-bands, each used for one spw. Up to another four, narrower spw can be used at higher spectral resolution, placed within the 512 MHz.
• L-band Nominally, 512 MHz is divided into 8 (64-MHz wide) sub-bands for continuum. Up to four additional spws can be used at higher spectral resolution. Note that not all sub-bands may be usable due to RFI. The total available band remains under continued evaluation.
• K-band 512 MHz divided into four 128-MHz sub-bands, each used for one spw. Up to another four, narrower spw can be used at higher spectral resolution, placed within the 512 MHz.

The 512-MHz observing band can be placed anywhere in the available band, but once placed, the sub-band locations are fixed. More than one high-resolution spectral window can be placed in the same sub-band if wanted. For maximum sensitivity, the spw should fit within factor-2 divisions of a sub-band. This is illustrated by some possible configurations shown in Figs. 1 and 2.

In cycle-7, all spw will be divided into 512 channels, each in full polarization.

Spectral resolution

The actual spectral resolution may be broader if Hanning or other smoothing is applied. The sensitivities are given for the optimum part of the band and will be worse towards the band edges, in adverse weather, at low elevation or if interference is present. Bright emission in narrow channels will be more dynamic-range limited. About 10% of edge channels within each band may have lower sensitivity if the spw are not optimally placed.

Table 2: C-band cycle-7 spectral line configurations.

Table 3: L-band cycle-7 spectral line configurations.

Line rest frequencies

e-MERLIN is suited for imaging thermal lines in absorption against a bright radio source, or maser lines. These are the best-known transitions in the available bands. Those in brackets are probably faint.

• L-band
• HI 1420.406 MHz
• OH 1612.231 1665.359 1667.402 1720.530 MHz
• C-band
• OH 4650.656 (4660.242) 4765.562 MHz
• OH (6016.746) 6030.747 6035.092 6049.084 MHz
• H₂CO 4829.641 - 4829.671 MHz
• CH₃OH 6668.619 MHz

Frequencies affected by interference include 1450-1520, 1310-1340 and around 1390 MHz. These frequencies are not always unusable (or others may be affected) but please consult e-MERLIN staff if you want to observe red-shifted lines in these ranges.

Proposal Technical Case

e-MERLIN is queue-scheduled and observing routines are prepared by staff. This means that sources will be observed at the positions and frequencies which you provide now. Please make sure these are accurate as there may not be an opportunity to provide corrections. In exceptional cases (e.g. follow-up of a transient), if you have to give a position which is not better than an arcmin, or hope to provide a better position after the deadline, please indicate this.

e-MERLIN staff will normally choose suitable calibration sources but if you have any preferences (e.g. the same phase reference as other observations) please indicate this.

Observing frequencies

Select the observing band. We recommend that you do not specify the precise frequency for pure continuum observations. The e-MERLIN observing team will pick the most sensitive range at C-band or avoid interference at L-band. If you do need a particular frequency for continuum, please explain why (e.g. for spectral index observations or to combine with other data).

• Frequency Flexibility:During cycle-7, both L-band, C-band and shared-risk K-band proposals will be accepted. However, due to commissioning commitments rapid cycling between L- and C-band will not be permitted. Moderately rapid frequency changes (no faster than one change every 2hrs) will be allowed within C-band (4.5-7.5GHz). However, proposers should note that such changes (e.g. between the ranges 4.5-5.0GHz and 6.5-7.0GHz) within a single observing run, whilst permitted will result in significant additional calibration overheads and are discouraged unless scientifically vital.

If you want to include up to 4 spw in narrower spectral configurations, please provide the following information for each spw:

• Line rest frequency (see list below for some common lines).
• Source velocity. This should be given in the LSR (Local Standard of Rest) reference frame, especially for higher resolution observations where the total spw is less than 100 km/s. Heliocentric velocities can vary by tens km/s depending on the measurement date. For extra-Galactic sources, please specify whether this is the radio convention (preferred) or the optical convention.
• Total velocity width needed for the line in this spw.
• Whether continuum subtraction is required. This is needed if the continuum can be detected in individual spectral channels.
Please indicate if you want us to set up the spectral configuration. This is recommended, but if you want to provide a specification, please discuss this.

Sensitivity

Please give the following information. In the case of spectral lines, please give the flux densities for each line and for any (total bandwidth) continuum.

• Target peak (Jy per e-MERLIN beam)
• Faintest emission of interest (Jy per e-MERLIN beam)
• Desired resolution (this can be varied by a factor of about two be weighting during imaging, at the expense of sensitivity).
• Largest angular size of individual objects to be imaged
• Total angular extent of target

Estimate the approximate time needed to reach your sensitivity limit, stating whether this is including the Lovell telescope. Projects not requiring the highest sensitivity (e.g. those which can be observed without the Lovell in less than a day) are unlikely to get the Lovell except in special cases. The Lovell telescope will be available for up to 8 days in total during cycle-7. This allocation is at the discretion of the University of Manchester and will be available to those projects making the strongest scientific justification for the inclusion of the Lovell Telescope.

Targets will normally be observed for one or more complete runs, i.e. the time when they are above a few degrees elevation. If you request a substantially shorter time, please explain why this is adequate for synthesis imaging (i.e. compactness of target). If your source is at low Dec. (below about 20 deg.), additional time is needed and the N-S resolution will be poor.

• Lovell Telescope:During cycle-7 proposers are invited to request the inclusion of the 76-m Lovell telescope. The inclusion of the Lovell Telescope approximately doubles the sensitivity of the array. The Lovell Telescopes is a University of Manchester instrument and its inclusion is limited within e-MERLIN. Please include in your proposal justifications why the Lovell Telescope is needed for your project.

cycle-7 example spectral configurations

These examples show how to use the available spectral configurations to observe lines and continuum. Up to 4 spectral windows are available for high spectral resolution in cycle-7. The remaining spectral windows will normally be used for continuum. The sensitivity of e-MERLIN means that in many cases, target continuum will be detected but even if not, the same configuration will be used for all calibration sources to ensure phase stability. The spectral windows are placed within factor 2 divisions of the sub-bands and in some cases overlap continuum spw. Note that this does not give additional sensitivity to the same emission.

Imaging OH masers around 6 GHz

Fig 1: Example of placement of 4 tunable spectral windows (spw) aligned with specific spectral lines. Two spws of 2MHz width are centred on 6030.7MHz and 6049.1MHz, with the two remaining spw (1MHz wide) aligned on 6035.1MHz.

The target lines are known to have a total velocity span of 40 km/s (above 20 mJy) per transition. They should be observed at at the best velocity resolution available in order to measure Zeeman splitting. The 6035-MHz line is known to be brightest, with a peak of about 1 Jy, the other lines are 100 mJy or fainter. The continuum is likely to reach a few hundred μJy per MERLIN beam.

The set-up adopted assumes that the observing frequency is close to the rest frequency. In practice the whole set-up can be moved to take account of the LSR motion and source V_LSR. Since e-MERLIN observes at fixed frequency, a few channels (equivalent to a few km/s) at each end of each spectral line spw will be discarded due to the diurnal and other shifts during observations.

Spw of 2 MHz provide a useful velocity width of up to 90 km/s. In order to place each spw so that it falls within a division of a sub-band into a factor of 2, the scheme shown in Fig. 1 is used. The lines fall close to the centre of each spw, with at least +/- 30 km/s available. 2 MHz in 512 channels provides a channel separation of 3.9 kHz or 0.2 km/s. For the brightest line at 6035 MHz, two 1-GHz spw are used side-by-side, to allow a higher spectral resolution of 0.1 km/s.

The four continuum spw of 128-MHz are placed so that their edges are consistent with the higher-resolution spw.

Imaging HI and OH at z = 0.09

Fig 1: Example of placement of 2 tunable spectral windows (spw) aligned with specific spectral lines at L-band. One spw of width 16MHz is centred on the redshifted HI line at 1303.1MHz, with a 32MHz wide continuum spw. The second spw is configured to have a width of 8MHz and is centred on the OH line (1665 and 1667MHz rest) at the same redshift.

The target has HI absorption of up to 10 mJy per e-MERLIN beam at redshift ~0.09, span about 500 km/s, against continuum of up to 100 mJy/beam. A spectral resolution better than 10 km/s is needed. The observing frequency for the 21-cm HI line is about 1303.1 MHz. A 16-MHz spw in 512 channels will give about 7 km/s channel separation and a 3sigma sensitivity of 1.4 mJy/beam per channel in 24 hr on target with the Lovell. The total velocity span in this spw is ~3400 km/s. We centre this, slightly asymmetrically, on 1301 MHz, which still gives over 1300 km/s on the closer side of the HI line.

This is to allow the OH 1665- and 1667-MHz lines to be fitted in optimally. At this redshift, they have observing frequencies of about 1527.9 and 1529.7 MHz, respectively. A spw of 8 MHz centred at observing frequency 1529 MHz will give a velocity span of more than 1500 km/s with a channel separation of 3 km/s, sensitivity 2 mJy/beam under the same conditions as the HI. The velocity width for each line is at least 500 km/s both above and below the central velocity.

The remaining ten spw are used at the full 32-MHz width to provide continuum, including in the sub-bands also containing line spw, in order to provide a good image for absorption and continuum subtraction. The location of the sub-bands used might be modified to avoid interference.