10.1 Scientific Plans
The scientific plans for using the Arecibo telescope in PY2009 follow from peer reviewed proposals made by our user communities including the NAIC scientific staff. Scientific proposals in the area of astronomy made for telescope time in PY2009 are concerned with the study of pulsars, the distribution of gas in distant galaxies, studies of our own Milky Way Galaxy and of the stars & star-forming regions within it, and of detailed studies of objects within the Solar System.
The new MOCK (FPGA-based) spectrometers became available for use with the seven-pixel ALFA receiver at the beginning of PY2009. This enables the major primary ALFA surveys pALFA, GALFACTS & AUDS to begin, together with their commensal partners. So PY2009 is going to be a busy year as the time granted to the various ALFA consortia is tailored to the available astronomical hours, as well as being adjusted to enable GASS and the OH-megamaser surveys to proceed.
10.1.1 The ALFA Surveys
AUDS -- The Alfa Ultra-Deep Survey:
In a precursor experiment, the AUDS sub-consortium demonstrated that it could achieve noise levels of less than 50 µJy from integration times of about 40 hours per telescope pointing. This translates into an HI mass sensitivity of a few 108 M⊙ at a redshift z ~ 0.16. AUDS has been waiting for the MOCK spectrometer, with its 300 MHz bandwidth capability for 21 cm observations, to begin their blind HI survey of just 0.36 square degrees. They plan to deploy their unprecedented 50 µJy sensitivity over the redshift range 0 < z < 0.16, so the resulting “ALFA Ultra Deep Survey” (AUDS) survey will be an order of magnitude more sensitive than other HI surveys being carried out at Arecibo. Their main scientific goals are to investigate the evolution of HI gas over cosmological time in the Universe, to explore low-density gas around the edges of galaxies, and, perhaps, to directly detect Lyman-limit systems. This survey will be the deepest “blind” HI survey ever conducted. It will provide for the first time a direct link between HI absorption-line measurements at high and intermediate redshifts and 21-cm emission line measurements at low redshifts. The expected number of HI detections at z > 0.1 will be larger than that of all previous targeted and blind surveys combined.
AUDS begins its observations in November 2008. To achieve their aims, they need to almost monopolize the telescope for about three years in dark time, while their two fields transit. While these fields have been chosen to minimize competition with other users it is, none-the-less, an issue for scheduling other surveys especially ALFALFA . The AUDS program requests a total of 980 hours.
As part of the gALFA consortium, the gALFA Continuum Transit Survey (GALFACTS) proposes to use the Arecibo telescope and its ALFA receiver to carry out a sensitive, high-resolution, spectro-polarimetric survey of the entire Arecibo sky. GALFACTS will be a major observational advance in imaging of the polarized radiation from the Milky Way and other galaxies, and promises a transformational advance in our understanding of the magnetic field of the Milky Way, the properties of the magneto-ionic medium, and the role of magnetic fields in Galactic processes. It will in addition explore the polarization properties of a vast number of extragalactic radio sources with L-band intensities down to sub-mJy levels. The GALFACTS survey is a scientific pathfinder to the Square Kilometre Array in the area of cosmic magnetism, one of the five SKA key science cases. As a single-dish experiment providing zero-spacing data it cannot be surpassed, even when the SKA comes on line. The GALFACTS survey promises to be one of the most important legacy surveys enabled by ALFA.
The radio continuum sky has been surveyed at many frequencies and angular resolutions. However, GALFACTS will provide critical new data for a wide range of cutting-edge continuum science. Areas of investigation include: (1) polarimetric constraints on the large-scale Galactic magnetic field, including the disk-halo interface; (2) magnetic field studies of supernova remnants (SNRs), molecular clouds, and H II regions; (3) 3-D Faraday tomography of the ambient magneto-ionic medium, including turbulent cascades; (4) high-resolution, total-intensity imaging of Galactic loops and spurs, low-surface-brightness SNRs, H II regions, and the general interstellar medium, with separation into thermal and nonthermal components by multiwavelength analysis; (5) an extremely sensitive point-source catalog, including polarimetry and variability constraints; and (6) characterization of the “true” Galactic synchrotron foreground, which is essential for CMB studies at higher frequencies.
The GALFACTS sub-consortium has been waiting for the MOCK spectrometers, as their 300 MHz bandwidth is crucial to the survey’s sensitivity, and particularly important in providing access to a sufficient range of frequencies to unambiguously determine the Faraday rotation of continuum sources. GALFACTS has demonstrated its ability to remove the telescope from the data, after they have made their own careful calibration scans at epochs in proximity to their observations. This survey challenges the technical capacity of the ALFA receiver and its IF/LO system, as it has to operate in the presence of strong radar signals. GALFACTS requires 1512 hours, began in October 2008, and should be completed in about three years. ZoA (Zone of Avoidance) is a commensal partner.
The pulsar community have had some observing time using the 100 MHz bandwidth WAPP spectrometers thus far, as the Mock spectrometers were not available in PY2008. This has enabled them to ramp their activities up for handling the enormous data flow they archive, and to test their detection rate against earlier predictions.
After making a detailed review of the factors affecting their detection rate, pALFA concluded that (1) survey observations should use the new MOCK spectrometers to increase the product of bandwidth and integration time (BT) by a factor ~6, particularly for Galactic longitudes 32° ≤ l ≤ 60° where a significant amount of volume is filled with pulsars inside the solar circle; (2) the pipelines should search to larger DMs than currently; (3) pALFA should expand its intentions to observe commensally with the extragalactic drift-scan survey, ALFALFA, in order to find MSPs & relativistic binaries in directions out of the Galactic plane; (4) pALFA are checking the feasibility of an out-of-plane survey using deeper integrations than those provided by the ALFALFA drift scans.
Exploratory discussions have taken place for processing pALFA data with Einstein@Home clients in areas of parameter space that cannot be searched by Consortium computational facilities owing to throughput issues. In particular, the scheme now being implemented is to de-disperse raw data on a central server and to then send a single time series to an E@H client. There are tens of thousands of E@H clients around the world. The client will then search for circular binary pulsars with orbital periods less than one hour, a task that typically will take about 24 hr. This can be compared with a processing time ~0.5 hr per time series in the acceleration search done in the PRESTO-based pipelines. Given the merger rate of double neutron star binaries and the orbital lifetime at a given orbital period, there should be one binary in the Galaxy with an orbital period between 5 and 10 minutes. There would be larger numbers of incipient mergers currently having longer orbital periods.
The consortium asks for about 450 hours of access to the Galactic Plane window each year: in PY2009 they are primarily in competition with I-gALFA for night time hours. They have commensal partners in ZoA & RRL which both require night time for their operation. Follow up time for pulsar candidates from the survey is primarily granted in twilight and daytime, as pALFA has little problem operating in time that is in much less demand by the other consortia.
- ALFALFA’s progress is discussed in §3. It has had about 59% of its 4400 hour request, and will be allocated about 700 hours in PY2009.
- AGES’s progress is discussed in §3. It has had about 25% of its 2000 hour request, and will be allocated about 350 hours in PY2009.
- I-gALFA began in PY2008, and is discussed in §3. It was allocated 200 hours of Galactic dark time in PY2008. It has complete three of its five fields: the remaining two should be finished in PY2009.
- Zone of Avoidance (ZoA) is a commensal partner of pALFA. It is in practice a blind HI survey for galaxies that seeks to detect the HI emission signal from galaxies that are optically obscured by extinction associated with our Milky Way Galaxy, and/or whose NIR signal is confused by Galactic NIR emission. Its pointing and integration time is under the control of pALFA. ZoA will use a copy of the IF signal obtained by pALFA, and, via the second set of MOCK spectrometers, will process it as an extragalactic redshift search survey across 200 MHz of spectrum. ZOA survey is aimed at uncovering the large scale structure in the distribution of galaxies behind the Milky Way, in the part of the sky where galaxies cannot be observed in the optical due to dust in our Galaxy.
- Radio Recombination Line (RRL) Survey is likewise commensal with pALFA (and ZoA). RRL works with the same IF signal as ZoA, but will process it to survey the hydrogen and helium recombination line spectra from localized sources in our Galaxy. RRL expects to map numerous galactic continuum thermal source from large H II regions to compact galactic objects, including planetary nebulae, in one or more recombination lines. RRLs provide critical probes of physical conditions in the warm diffuse medium. The combination of gALFA-RRL and other surveys (e.g. MSX, ISO, 2MASS, NVSS, GALFACTS, gALFA-H I) offers a powerful multi-wavelength probe of the Galactic ISM.
- TOGS & TOGS2 (Turn On gALFA Spectrometer) are commensal HI surveys of the Arecibo sky. Since gALFA has its own dedicated spectrometer, these surveys run commensally whenever ALFALFA, AGES or GALFACTS are using the telescope. TOGS is discussed in §3.
- GASS is discussed in §3. It only received a meager time allocation in PY2008, but is expected to be allocated about 350 hours in PY2009.
- OH Megamasers in ULIRGS is discussed very briefly in §3. It began in PY2008 and is content to operate in twilight and daytime. Its time allocation ramped up rapidly in the last quarter of PY2008, when it got 205 of its requested 455 hours. It should be completed in PY2009.
10.1.2 Pulsar Timing
Canonical pulsars and MSPs account for ~90% and ~10% of all pulsars, respectively, with relativistic binaries and high-field pulsars comprising < 1%. It would not be surprising to find additional types of pulsars in a high-yield survey. That expectation is a primary driver for the PALFA survey. Pulsars, as extremely precise “clocks” fully embedded in a variety of astrophysical environments, are used to probe those environments and the physics that applies in that environment. This is done by repeatedly timing the pulsars—comparing the measured arrival time of a pulsar’s signal to the time expected on the basis of prior measurements. Any departure from these two times can be attributed to motion of the pulsar resulting from local forces; the nature of those forces can be investigated. In PY2009 the following pulsar timing programs are planned:
1) High precision timing of the double neutron star system J1829+2456 over the next year will enable a direct measurement of the proper motion of the system to be made and from that one can establish how much of the dP/dt results from secular acceleration. A value of dP/dt corrected for proper motion will permit the age of the pulsar to be calculated. Already the age lower limit is 12.9 Gyr. Such a large age already implies that J1829+2456 had a birth spin period close to its current value, 40.1 msec. Knowledge of the transverse velocity can also be used to model the kick produced by the birth of the neutron star and so constrain the initial conditions of the system.
2) Timing observations will begin of PSR J1903+03, the first millisecond pulsar to be discovered in the Arecibo PALFA survey. With a period of 2.15 ms and a dispersion measure of 297 pc cm-3, this is a perfect example of a short period, high DM pulsar to which the PALFA survey is uniquely sensitive. PSR J1903+03 is a uniquely distant and precise pulsar which will allow changes in DM to be tracked accurately. Variations in DM provide a direct measurement of the integrated spectrum of electron density fluctuations in the ISM along the line-of- sight. Conventional wisdom holds that the amplitude of DM variations is related to the square root of the distance to a pulsar; timing of PSR J1903+03 will allow this to be tested over a much greater distance than could be done with any previous ms pulsar.
3) Over the past two years, several mysterious neutron stars have been found with very unusual properties. Among these is the new population of Rotating Radio Transients (RRATs). These objects are characterized by radio bursts with durations between 2 and 30 ms and average intervals between bursts ranging from 4 minutes to 3 hours. PSR J0628+09 is a RRAT discovered in the Arecibo PALFA survey with a period of 1.2 s; timing observations will help greatly in establishing its nature. PSR B1931+24, a 813-ms pulsar with a typical age and magnetic field, is another mysterious transient pulsar. This “sometimes pulsar” mysteriously turns off and on in a quasi-periodic fashion, with intervals between “on” and “off” periods ranging from 25 to 35 days and on intervals lasting 5 to 10 days. This is the first time that something like this has been seen for any pulsar and it points to a massive increase in magnetospheric outflow when the pulsar is on. This object and the RRATs together have challenged our understanding of pulsar emission mechanisms and highlighted how little we know about pulsar properties and populations. Timing observations in PY2009 will address this deficiency.
4) Timing of the double neutron star system PSR B1534+12 will (a) improve the measurement of profile variation on orbital timescales, and hence refine the unique measurement of spin-orbit coupling in a strongly self-gravitating system; (b) improve the measurements of the post-Keplerian timing parameters s and r to improve the purely quasi-static test of general relativity—an important complement to the mixed quasi-static/ radiative test in PSR B1913+16 (the Hulse-Taylor pulsar); (c) better determine dP/dt and hence the GR-derived distance to the pulsar, an important input to the expected event rates for LIGO; (d) monitor dispersion measure changes and provide ephemerides for VLBI observations; and (e) produce a calibrated 2-dimensional map of the pulsar beam shape.
5) Continued timing of the young relativistic binary pulsar PSR J1906+0746 discovered in the PALFA survey. This young pulsar is in an eccentric 4-hour orbit. Timing observations over the last year have facilitated a measurement of the time dilation as well as the shift of periastron passage, resulting in mass estimates for the pulsar and its companion. These indicate that the companion is most likely a second neutron star. On the timescale of years, we expect to measure the orbital period decay, which will over constrain the system and provide a test of strong-field gravity. The pulsar shows strong profile evolution with time, which is being used to investigate the pulsar’s 2-dimensional beam shape and the phenomenon of geodetic precession. Finally, an attempt will be made to identify radio pulsations from the companion star.
6) Simultaneous GBT and Arecibo observations will be made of the bright millisecond pulsars J1713+0747 and B1937+21. These data will be used to explore systematic effects present in high-precision timing and to help answer the question of what ultimately determines the best possible timing accuracy. Resolution of this issue will impact current pulsar timing array experiments that aim to detect nHz gravitation wave background. It will also help determine what kinds of pulsar timing experiments will be possible with future large collecting area radio telescopes such as the SKA.
7) Four recently discovered millisecond pulsars will be timed to determine if they have the rotational stability required for them to be included in pulsar timing arrays.
8) Timing observations of the young, relativistic, binary pulsar J1906+0746 will continue in PY2009. This pulsar is in an eccentric 4-hour orbit. Timing observations over the past few years have been used to measure the time dilation and gravitational redshift as well as the shift of periastron passage, resulting in mass estimates for the pulsar and its companion, which indicate that the companion is a second neutron star. There is currently a marginal detection of the orbital period decay that will be confirmed, or refuted, with the observations planned in PY2008. This measurement will overconstrain the system and provide a test of strong gravity. The pulsar also shows strong profile evolution with time, which is being used to investigate the pulsar’s 2-dimensional beam shape and the phenomenon of geodetic precession. Searches will also be made for radio pulsations from the companion star in all of the data acquired.
9) High precision timing of nine millisecond pulsars at monthly intervals over two years will be used to establish improved determinations of neutron star masses, measure pulsar parallaxes and proper motions, set limits on the presence of a gravitational wave background, and test and measure terrestrial clocks, ephemeredes, and reference frames. Different applications require different observing strategies. For some applications, such as measurement of relativistic secular changes in orbital elements, intense campaigns at several widely spaced epochs are optimal. For other applications, most notably astrometry and gravitational wave background measurements, it is best that the observations be made continuously and uniformly over many years. Pulsars are striking sources of coherent radio emission but the nature of their radio emission is far from understood. The physics of the process by which radio frequency emission is generated in the magnetospheres of neutron stars will be explored in PY2009 through several observational programs at Arecibo, two of which are the following. 1) PSR B1951+32 is a young 39.5-ms pulsar in the core of the supernova remnant CTB 80. The pulsar’s spin-down age of 107 kyr is comparable to the age determined from its proper motion and to the dynamical age of CTB 80. Located near the edge of the core of CTB 80 and moving rapidly away from the center of the remnant, the system represents the interesting stage of pulsar evolution when the neutron star penetrates and interacts with the interstellar medium swept up by the remnant. The pulsar’s relativistic wind then interacts with the supernova shell, re-energizing and distorting it and causing the emission of electromagnetic radiation over a broad range of wavelengths. One would expect that PSR B1951+32 would exhibit giant pulses because it shares many similarities with the prototypical giant pulse pulsar in the Crab nebula. The observations to be conducted at Arecibo in PY2009 aim to improve out knowledge of giant pulses by searching for such emission from PSR B1951+32. Besides being very energetic, giant pulses are characterized by narrow pulse widths, high degrees of polarization, and power law energy distributions. It will be very interesting to see if PSR B1951+32 has all of these characteristics.