Early Science from the LWA Phase II: a Target List
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Early Science from the LWA Phase II: A Target List Aaron Cohen1, Tracy Clarke1;2, Joseph Lazio1 February 12, 2007 1. Introduction As described in Taylor et al. (2006), the LWA Phase I will consist of a single LWA station near the center of the VLA and another station at a distance similar to that of the Pie Town station. Phase II (the Long Wavelength Intermediate Array or LWIA) will expand this system by adding another seven stations even farther out to create an array with a maximum baseline of between 150 to 200 km. These nine antennas can be used alone or, when observing at 74 MHz, they can be used in combination with the 27 VLA antennas. The LWIA will be a sparse array that likely will not be capable of ionospheric calibration across the full field of view. Therefore it will operate mainly to observe sources that dominate the flux density in their field of view, for which simple self-calibration is sufficient. In this memo, we have examined the 74 MHz VLSS catalog to determine a list of sources that are bright enough and isolated enough that they can be imaged with self-calibration alone. We describe our methodology and present a final source list along with a description of the types of objects in this list. We find that 362 sources meet this criteria. When the effects of bandpass smearing are included, which suppress the signals of outlying sources, this list increases dramatically to 4,824. 2. Determining the Criteria for Self-Calibration Only sources that dominate the flux density within their field of view can be imaged successfully with self-calibration. That is because if other sources have significant relative brightness, they must also be included in the calibration model. At the resolution and frequency of the LWA, these other sources will certainly lie outside the isoplanatic patch of the central source, and therefore full-field ionospheric calibration will be necessary. Therefore, in order for self-calibration to work, we must be able to obtain reliable gain solutions by comparing the measured visibilities to a source model that contains only the central source and not any outlying sources. Our experience with VLA data indicates that self-calibration is successful when a model can fit the data with a signal-to-noise ratio of at least 3:1. If we treat the signals from the outlying sources as \noise", this means that the outlier contibution to the average visibility signal must be no more than 1/3 that of the signal from the central source. 1Naval Research Laboratory, Code 7213, Washington, DC 20375 2Interferometrics, Inc. { 2 { Im{a} a o a total Re{a} Fig. 1.| The total visibility, atotal, is a complex sum of the visibilities of the brightest source, ao, and that of all other sources in field of view. The difference between atotal and ao is a \random walk" sum, which has average magnitude equal to the quadratic sum of the magnitudes of all other source contributions. To investigate what this means, consider the visibility signal, a, from a given baseline at a given time. Because a visibility has both magnitude and phase, a is a complex number. If there are N outlier sources, we can break down a into a sum of contributions from the central source, ao, and the contributions from the outlier sources: a1, a2, ... aN . The difference between atotal and ao is the sum of all other source contributions: N atotal − ao = ai: (1) =1 Xi Because this is a sum of complex numbers with uncorrelated phases (Figure 1), the average mag- nitude of the sum will be the quadratic sum of the magnitudes of each of the ai, just as in the case of a random walk: 1 2 N = 2 hjatotal − aojimean = jaij : (2) =1 ! Xi The original criteria that the average visibility magnitude from all other sources be no more than { 3 { 1/3 that from the central source requires that: 1 2 N = 2 ao > 3 × jaij (3) =1 ! Xi This equation can be related to source flux densities if we assume that the magnitude of a source visibility contribution, ai, is proportional to the flux density of that source, Si, multiplied the primary beam attenuation ρ(θi) which is a function of the angular distance θi of source i from the pointing center. This assumption is true for the case of sources being unresolved or, on average, resolved to the same degree on the average baseline. It would fail for the case of a central source that was significantly more extended than the average outlier source. However, this could be avoided somewhat by only calibrating to the shorter baselines. Proceeding with this assumption results in the following criteria for sources we expect to image with self-calibration alone: 1 2 N = 2 So > 3 × (ρ(θi) Si) (4) =1 ! Xi 3. Determining the Source List To determine how many such sources there are at 74 MHz, we can use the nearly completed VLSS survey (http://lwa.nrl.navy.mil/VLSS). Using the catalog from that survey, we consider each source in comparison to all its surrounding sources. Treating the station as a uniformly illuminated aperature, the field of view FWHM is 1:02 λ/D. Assuming a station diameter of 100m and a frequency of 73.8 MHz (λ = 4:06m) this results in a FWHM of 2.37◦, which determines the value of ρ(θi) in Equation 4 as a Gaussian with this half power diameter. For each source, we consider all outlier sources within a 5 degree radius, and sources that meet the requirement of Equation 4 are included in the final source list. The resulting list includes 362 target sources. These sources are listed in Table 1 along with their identifications which are discussed in Section 4. We note that these targets were selected as candidates solely on the basis of the VLSS survey which has a resolution of 80 arcseconds. It is possible that some of the more diffuse candidates may not have a sufficiently high peak flux density at a resolution of a few arcseconds to be used for self-calibration. Higher resolution observations of the targets from the VLA archive could be used further select from the target list in Table 1. 4. Target Identifications The source list obtained from the VLSS catalog search was passed through batch jobs to both the NASA Extragalactic Database (NED) and the CDS SIMBAD astronomical database for source { 4 { identification. The source searches were carried out within a search radius of 3 arcminutes for NED and 5 arcminutes for SIMBAD around the VLSS source positions. Source matches within 18 arcseconds of the VLSS search position were generally selected as the candidate identification of the target. All candidate sources that showed no identification within 18 arcseconds and/or candidate sources that had very little available information in the literature were followed up with visual identifications by overlaying VLSS radio contours on NVSS images. All sources with large positional offsets (up to 2 arcminutes) have been identified through this process as extended sources. Table 1 lists the 362 sources in our list with the VLSS source Name (Column 1) and an alternative name from the NED/SIMBAD source identification (Column 2). The source type (Column 3) is given as one of the following: QSO - quasar, G - galaxy, GCl - galaxy cluster, RS - radio source (no other type found), SNR - supernova remnant, Sy2 - Syfert 2 galaxy, Grp - galaxy group, relic - galaxy cluster radio relic, halo - galaxy cluster radio halo, MH - radio mini- halo in cluster core, GTrpl - triple galaxy system, GPair - double galaxy system, HCG - Hickson Compact Group, GRG - Giant Radio Galaxy. The other columns give: distance between target and candidate ID in arcminutes (Column 4), Right Ascension from the VLSS catalog in J2000 (Column 5), Declination from the VLSS catalog in J2000 (Column 6), integrated VLSS 74 MHz flux of the source in Jy (Column 7), and the ratio of the target flux density to the quadratic sum of the other sources in the LWA FOV (Column 8). The majority of the targets are extragalactic (98.6%) with only 5 source identified as Galactic SNRs (3C58, Crab, Kepler, W49B, and Cas A). All sources within 3 arcminutes of a galaxy pair, galaxy triple galaxy group or cluster of galaxies were followed up with a literature search to de- termine if the candidate source is associated with AGN emission or if it is related to a halo/relic source. Of the 26 sources associated with a galaxy cluster, 1 is a radio halo (Abell 1914), 2 are radio relics (Abell 85, Abell 566), 2 are mini halos in cooling core clusters (Perseus A, Abell 2199), and 8 are central radio galaxies in cooling core systems (Perseus A, Hydra A, Virgo A, Abell 2052, Abell 2199, Cygnus A, Abell 2597, & Abell 4059). Thus the source list of Table 1 contains sources which are interesting in a large variety of scientific ways, indicating the great scientific potential of even the earliest stages of the LWA. 5. Effects of Bandwidth Smearing Averaging across a nonzero bandwidth will cause radial smearing in sources. This bandwidth smearing increases with distance from the phase center. Normally this is considered a problem, however it actually can aid the efforts to self-calibrate as it suppresses the effects of outlying sources on visibilities relative to the central source of interest. Bandwidth smearing becomes significant when the distance from the image center in units of the synthesized beam-width times the fractional bandwidth approaches unity.