A SMALL AUXILIARY ANTENNA AT ARECIBO
C.J. Salter and Tapasi Ghosh
1. Introduction
The 305-m Arecibo radio telescope is the world’s largest, most sensitive, single-dish radio telescope. It is equipped with receivers between 47 MHz and 10 GHz. In addition to its single-dish capabilities, the telescope also participates in Very Long Baseline Interferometry (VLBI) observations with the VLBA, HSA, EVN and Global VLBI networks.
In the quest for higher sensitivity, VLBI observations have been evolving over the years. For a given VLBI array, this can be achieved by either increasing the bandwidth or the integration time. While bandwidth limitations come either from equipment (in the case of continuum immaging) or from the natural phenomena under study (spectral lines), phase fluctuations due to the propagation of the signal through the Earth’s troposphere and ionosphere limit the basic coherence time of VLBI observations. The technique of “phase-referencing” in VLBI allows the possibility of correcting for these effects and thereby increasing the coherence time by large factors.
1.1 VLBI Phase Referencing:
Phase referencing in VLBI observations has made it possible to study very weak radio sources by increasing the effective coherence time from, at maximum, a few minutes to hours. Currently, some 50% of VLBI observations are carried out using this technique.
Phase-referenced observations can be performed in two modes, nodding style, and in-beam. In nodding-style phase referencing, the antennas switch between the target source and a nearby calibrator, called the phase reference, every few minutes. The duration of one cycle of observing the target and phase reference is called the cycle time, and is typically about 5 minutes. This procedure can be successfully carried out for observations at 1 GHz and above. However, at frequencies below 1 GHz the raw coherence times become very short due to ionospheric effects, requiring the phase calibrator to lie within the (voltage) primary beam of all antennas in the array.
However, phase-referenced VLBI observations with the Arecibo 305-m telescope encounter limitations since the Gregorian dome, located on a suspended platform, has slow slew rates (24o/min in azimuth, 2o.4 /min in zenith angle.) Hence, in a typical phase-referenced observation where the calibrator could be located 3o or more from the target, a significant amount of observing time, often ≥ 50%, is wasted slewing between the two sources, leading lower signal-to-noise ratios. However, phase-referenced VLBI could be performed using a smaller “Auxiliary” Telescope (AT) to track the phase calibrator, while the 305-m antenna observes the target most of the time, only occasionally moving to the calibrator. The effects due to ionospheric/tropospheric phase fluctuations can then be derived from the small-telescope data and applied to the target data from the 305-m dish. While this technique has been successfully applied for observations with the 220-km baseline MERLIN array in the UK, it would be a new approach for VLBI. In fact, implementing its VLBI application will be original research in its own right.
2. Areas of Astronomical Research Benefiting from the Auxiliary Telescope (AT) in Phase Referenced VLBI
2.1 Stellar (radio) Astrometry: In a white paper submitted to the NSF ExoPlanet Task Force, Bower et al. (arXiv:astro-ph/0704.0238v1) explored the possibility of “Radio Astrometric Detection and Characterization of Extra-Solar Planets”. Utilizing the better than 100-microarcsec positional accuracy routinely achieved with the VLBA, they propose carrying out a Radio Interferometric PLanet search (RIPL) that will survey 29 low-mass, active (radio-loud) M-dwarf stars over 3 years. This would have sub-Jovian planet mass sensitivity at distances of ~1 AU from the star. They note that, “Radio astrometric planet searches occupy a unique volume in planet discovery and characterization parameter space, which gives greater sensitivity to planets at large radii than do radial velocity searches. For the VLBA and the expanded VLBA, the targets of radio astrometric surveys are by necessity nearby, low-mass, active stars, which cannot be studied efficiently through the radial velocity method, coronography, or optical interferometry.”
The addition of Arecibo in such surveys would increase the detection sensitivity by a factor of four, making it possible to study objects with one third the mass of Jupiter as companions of stars of similar types as in RIPL. As Arecibo's primary beam is much smaller than that of other telescopes, and the slew rate lower, the availability of a small antenna for phase referencing would be highly beneficial for undertaking such studies.
2.2 A Broad-impact VLBI Measurement of Trigonometric Parallax of Star Clusters: In an impressive work using the VLBA at 8 GHz, Menten et al. (2007, A&A, 474, 515) have determined the trigonometric parallax of several stars in the Orion BN/KL region, allowing them to derive the most accurate value to date (414± 7pc) for the distance to this region. This is about an order of magnitude better than the previous value of 361+168-87 pc determined by Hipparcos from the optical parallax measurement of a single star in this complex. Luminosity-based distance estimates of star-forming regions may be adversely affected by poorly known extinction. The new radio technique is an important way to improve the estimation of distances, and hence luminosities, with subsequent impact on star-formation theories. Once again, the inclusion of Arecibo would permit the extension of these studies to fainter, more distant, star-forming regions.
2.3 Pulsar Astrometry: High-precision astrometry of pulsars over multiple epochs can provide their basic astrometric parameters: positions, proper motions, and annual trigonometric parallaxes. Due to the weakness of most pulsars, with duty cycles of typically <10%, the participation of Arecibo with phase referencing is vital to the success of this exercise. In respect of positional measurements, we note that VLBI estimations are tied to the reference frame of the distant quasars, rather than the Solar-system frame employed by pulsar timing positional estimates. This allows fundamental reference frame ties between the Solar-system and extragalactic (ICRF) frames via measurements of recycled pulsars, which are highly stable rotators.
Proper motion estimates allow pulsars to be traced back to their birth sites and, for very young pulsars, associations with progenitor supernova remnants (SNRs) can be established, providing independent age estimates for the SNRs. Combined with pulsar distance estimates, proper motion measurements lead to estimates of space velocities, allowing a study of the natal kicks imparted to pulsars at the time of their birth. When a parallax measurement is possible, this yields a model-independent estimate for the distance (and hence velocity) of the neutron star. Such measurements, (i) calibrate models of the Galactic electron distribution, (ii) constrain SN core collapse using the velocity estimates, and (c) provide photospheric sizes for hot neutron stars with optically observed thermal surface radiation, which in turn constrains the equation of state of matter at extreme pressures and densities.
2.4 Detection Experiments: Present-day VLBI offers the highest sensitivity radio astronomical observations yet achieved, with noise levels presently approaching 1 mJy/beam for arrays using the world's most sensitive telescopes. Hence, the 305-m Arecibo telescope is being increasingly used in experiments to detect radio emission from very weak, very compact, astronomical targets such as X-ray stars, distant supernovae and their remnants, Gamma-Ray Bursts, and red-dwarf and other stars. For these sensitivity levels to be reached for targets of very low intensity, it is essential that phase-referencing be used.
2.5 VLBI Imaging of Molecular Gas in ULIRGs: Arecibo and the GBT are currently searching for cm-wavelength lines of prebiotic and other molecules in Ultra-Luminous InfraRed Galaxies (ULIRGs). The project has been inspired by the recent Arecibo detection of the prebiotic molecule, methanimine (CH2NH), in the protypical ULIRG/megamaser galaxy, Arp 220 (Salter et al. 2008, AJ, 136, 389). These galaxies are considered to be “extreme mergers” and are heavily obscured at optical wavelengths. Molecular lines from them often show wide velocity widths, caused by line blending due to spatial and velocity overlaps. Detailed studies of maser emission and molecular absorption lines from these objects require phase-referenced VLBI observations, and the presence of Arecibo’s sensitivity in the VLBI array.
3. Astronomy Using the AT as an Independent Single Dish
3.1 Full-Stokes Galactic Plane Continuum Surveys: The small telescope, together with existing Arecibo backends, will enable full-Stokes, continuum surveys using the dual-channel receivers that will be used with the dish. Full-Stokes continuum surveys of the wider Galactic plane at high frequencies with the AT can provide unique databases in a number of ways. Firstly, they can yield full spatial frequency, full-Stokes mapping at previously unmapped wavelengths, with competitive resolution for such extended features as the Galactic background emission, HII complexes, and middle-aged and old SNRs. Comparison with existing lower-frequency surveys would allow accurate estimation of spectral index distributions over these features, providing the ability to perform accurate thermal-nonthermal separation on angular scales between ~1o and 10 arcmin allowing the study of energy injection to the ISM, energy losses for relativistic particles associated with SNRs, and the mechanisms of vertical transport and diffusion of energy from the disk of the Galaxy into the halo and intergalactic space.
Linear polarization measurements are especially important. The appearance of the polarized sky at l > 21 cm is complex. Westerbork at 327 MHz (for high Galactic latitudes) and the 1.4-GHz Canadian Galactic Plane Survey have shown that there is little relationship between total intensity and polarization structures; for the diffuse Galactic synchrotron emission, the bulk of the Galactic Plane imaged at L-band reveals highly structured polarization features with no Stokes I counterparts. The accepted interpretation of this is that, although the Galactic synchrotron emission is intrinsically quite smooth, differential Faraday rotation in the intervening magneto-ionic medium, (the Faraday Screen), imposes fine structure on the polarized emission, i.e. the low-frequency polarized sky is dominated by propagation effects rather than intrinsic emission structure. The signals produced via the Faraday Screen are rather weak, and the limited surface brightness sensitivity of interferometers samples only the strongest. Also, the derived rotation measures (RMs) are noisy due to low signal-to-noise per channel, while missing zero-spacings in interferometric observations lead to complications in interpretation.
The high brightness sensitivity of the AT, coupled with its few arcminute beam size at high frequencies, promises major advances in the study of the magneto-ionic medium. At these frequencies (³ 5 GHz), the effects of Faraday rotation become tiny (Dq µ n-2) and the AT polarization position angles will essentially be those intrinsic to the emission, providing both directions of magnetic fields and a database against which lower frequency polarization distributions can be definitively interpreted. In existing studies of the Faraday Screen, the spectral signatures of the polarized intensity have been examined to seek only a single RM value per image pixel. Such a value corresponds to the RM of the dominant polarized emission component along any given sight-line. However, the Faraday Screen is spread out in depth along each line of sight, with regions of polarized emission at different distances along the sight-line contributing to the observed spectrum with their corresponding foreground Faraday rotation signature. With an appropriate combination of observing frequency, bandwidth and spectral resolution, it should be possible to perform Faraday tomography, wherein the spectral polarized intensity modulations along a given sight-line can be transformed to a set of polarized intensities as a function of Faraday depth (i.e. RM), i.e. a polarized-intensity data cube (quite like a spectral-line data cube) with two dimensions being the sky coordinates and the third being RM. High-frequency images from the AT would be invaluable in pursuing this endeavor.
Away from the Galactic plane, the high latitude regions contain several well-known non-thermal emission structures, notably the North Polar Spur (Loop 1), an object that contains rich small-scale structure, both on its main arc and in internal ridging. Above b = 45 °, low resolution measurements of this nearby (~100 pc distant), old SNR show >70% linear polarization at 1.4 GHz. Higher frequency, higher resolution AT images will directly reveal the detailed magnetic field structure in this object.
We specifically mention the L-band Arecibo GALFA Continuum Transit Survey (GALFACTS), which is being made by an international consortium led by Prof. Russ Taylor (U. Calgary). This full-Stokes survey of the whole sky observable with the 305-m telescope covers 1225 – 1525 MHz, with 8192 frequency channels. At L-band, the Faraday rotation effects on the linearly polarized radiation are considerable, and a continuum survey at much higher frequency, but similar resolution, (HPBW ~ 4 arcmin for GALFACTS), would allow thermal-nonthermal separation, and aid Faraday tomography when combined with GALFACTS. The same situation exists for a large part of the Southern Galactic Plane L-band continuum survey being made with the Parkes radio telescope (HPBW ~ 15 arcmin; Haverkorn et al. 2006, ApJS, 167, 230). AT surveys would also provide vital low-spatial frequency data for future interferometric full-Stokes surveys.
Synergy with GLAST: g-ray emission from our Galaxy is believed to be produced by, a) brehmsstrahlung from the interaction of cosmic-ray electrons and the interstellar gas, and b) the decay of neutral pions produced in interactions between the gas and cosmic-ray protons and heavier nuclei. The former is thought to dominate at <1 MeV, the latter at higher energies. Similar distributions of g-rays are found at low latitudes in both energy ranges, suggesting that the cosmic-ray heavy particle-to-electron ratio is constant over the Galaxy. If so, the g-ray emissivity, hg, is proportional to the product of the cosmic-ray intensity and the total (i.e. neutral and ionized atomic, plus molecular) gas density, r ; hg µ N0 r, where the cosmic-ray energy distribution is given by, N(E) dE = N0 E-G dE. Now, for the synchrotron component of the Galactic radio emission, the emissivity is, h µ N0 B^ (G+1)/2, where B^ is the magnetic field strength perpendicular to the line of sight.
The Galactic distributions of the three quantities, N0, r, B, are all of great astrophysical interest. Arecibo will contribute significantly to a knowledge of r over the accessible sky, with the GALFA consortia providing, a) the 2-dimensional distribution of HI, while b) the thermal-nonthermal separation of the continuum emission mapped by the AT and GALFACTS surveys will provide the 2-D distributions of the thermal emission from HII and the non-thermal synchrotron emission. The 2-D distribution of the molecular gas is already available from CO surveys of similar resolution. Hence, combining Arecibo AT and ALFA results with other radio data and the high-fidelity GLAST g-ray background images will provide the information needed to “unfold” the 2-D distributions and derive the Galactic distributions of N0, r, & B. This would represent a major contribution to our understanding of the detailed distribution of the magnetic field and cosmic rays in the Galactic disk.