Specific scientific objectives of COBRA

# Specific scientific objectives of COBRA

## Millisecond and Binary Pulsars

Coherently de-dispersed pulsar signals permit a wide range of technical and scientific applications. Foremost amongst these is high-precision millisecond pulsar timing. Although pulsars typically exhibit great pulse-to-pulse variability in both pulse shape and strength, profiles integrated over several minutes are generally stable, and may be cross-correlated with a standard profile'' to yield times-of-arrival (TOAs). The highest accuracy TOAs will be produced when both the standard profile and the observed profile are produced from coherently de-dispersed data streams. Timing observations of the fastest millisecond pulsars can be used to study the properties of the neutron stars themselves, to investigate the properties of the interstellar medium, to set limits on the strength of the primordial gravitational wave background and variation of Newton's constant, and in the case of certain double-neutron-star binary pulsars, to test the predictions of Einstein's theory of general relativity. The measurement of the orbital decay of such systems due to gravitational radiation losses allows estimates of their coalescence rate and hence the expected event rate for gravitational-wave detectors such as GEO600, LIGO and LISA (Taylor & Weisberg 1989; Stairs et al. 1998). Timing observations of neutron-star--white-dwarf pulsar systems can also lead to greater understanding of the process of binary evolution, by measuring or setting limits on pulsar and companion masses (Thorset & Chakrabarty 1999). Further information is available from the study of eclipsing pulsars, whose winds are thought to be slowly destroying their companions (Stappers et al. 1996).

## Neutron Star Interiors

The study of the rotational irregularities in young pulsars by regular long-term observation permits investigation of the internal structure of neutron stars (Lyne 1996). For instance, the observation of glitches, sudden increases in rotation rate, allows a kind of rotational seismology which reveals the presence of neutron superfluid interacting with a solid crust. These interactions can also be studied by the observation of the stochastic variations known as timing noise. Both these studies will be rendered much more accurate and valuable through the combined use of wide-bandwidth coherent dedispersion and interference mitigation.

## Searches for Pulsars in Globular Clusters

A further exciting possibility lies in high-frequency, wide-bandwidth searches of globular clusters. Where there are multiple pulsars in one globular cluster, their dispersion measures are very similar, so it is possible to perform a high-time-resolution coherently de-dispersed search of any globular cluster where at least one pulsar is already known. Globular clusters tend to be rich in millisecond pulsars (Camilo et al. 2000), and, with the high frequency of stellar gravitational interactions, are prime grounds for the formation of double-neutron-star binaries and exotic binaries such as pulsar--black-hole systems. Furthermore, millisecond pulsars act as highly sensitive accelerometers and allow us to map the gravitational field of a cluster and hence its mass distribution.

## Searches for Fast Binary Pulsars

Because of the general nature of this powerful computing resource, the proposed equipment would not only serve its main purpose as a data acquisition system, but would also be used during periods when pulsar observations are not being performed. Such projects are, for instance, the analysis of pulsar search data obtained at Jodrell Bank or other radio telescopes. Discovering the most astrophysically exciting tight binary systems requires searching in pulsar acceleration as well as the usual period and dispersion measure. In close binary systems, the observed pulse period will change according to a Doppler shift due to the pulsar's fast orbital motion. So-called acceleration searches'' tackle this problem by resampling the observed time series according to an assumed acceleration value. A full scale search over period, dispersion measure and acceleration parameter spaces is obviously computationally very expensive. Even the unaccelerated analysis of our present on-going survey in collaboration with our international partners requires of order 25 workstation-years of processing. However, the acceleration search can be parallelised in a straightforward manner, so that it can be ideally addressed with the proposed hardware.

In addition to the timing and searching applications, we will also use the instrument to study the polarisation of pulsar radiation which yields valuable clues to the poorly understood radio emission process (Lyne & Manchester 1988). A tremendous advantage of coherent dedispersion instruments is that all four Stokes parameters can be simply and accurately calculated from the de-dispersed time series, thereby eliminating the need for additional polarisation instrumentation. Precision polarimetry with wide-bandwidth coherent dedispersion opens up the possibility of studying in detail the emission properties of millisecond pulsars, with information from both the integrated profiles and single pulses. Single pulses of millisecond pulsars are not well studied since no existing data acquisition system is capable of observing them with a large bandwidth and hence great sensitivity.

## Interference Mitigation

Besides these many applications for pulsar science, i.e. high precision timing, searching, single pulse and polarisation studies, the development of such a flexible receiver also helps to develop strategies for dealing with radio frequency interference. As this is a very important subject for the whole field of radio astronomy and communications generally, we refer to \S\ref{sec:benefit} where we describe the benefits for basic science.

## Contribution to Infrastructure

The Observatory operates several radio telescopes including the 76-m Lovell Telescope and the MERLIN imaging array on behalf of PPARC. Associated with these telescopes are many cryogenic receivers as well as much signal conditioning and data acquisition equipment. There are also over 70 networked workstations used for off-line data reduction, telescope control and engineering purposes.

The proposed equipment will form the foundation of all future pulsar observations, and will greatly improve the quality of the observations. The new equipment (sampler plus computing power) in fact represents the radio-astronomical receiver'' of the future, with which we can manipulate the data at the most fundamental of levels in both frequency and time. In particular, it can be used for the development of interference mitigation techniques which will allow more efficient use of the spectrum, both for the Lovell telescope and for future instruments like the Square Kilometer Array (SKA). The system can also be used as a high-resolution spectrometer for the study of hydroxyl, water and methanol masers, as well as for neutral hydrogen and other molecular species. It will also have the capacity and spectral resolution for a planned multibeam census of star-forming regions in the Galaxy using the upgraded Lovell telescope to study methanol at 4.8 and 6.7 GHz.

Since the equipment provides an enormous computing power, when pulsar observations are not being performed or off-line searches conducted, it can also be used for solving other problems requiring a massive parallel computer, such as the hydro-dynamical codes used by other research groups at the observatory.

All of these applications will provide opportunities to train students in research at the forefront of technology and astrophysics.

## Benefits for Research

In many areas of basic research, such as gravitational, nuclear and plasma physics, pulsars are the only available laboratory. We have indicated a few of these applications in \S\ref{sec:science}, which we will summarize here.

In order to study the strong-field limit of general relativity, binary pulsars are the ideal and only test case. Using timing observations of binary pulsars, the existence of gravitational radiation has been demonstrated, and other predictions of general relativity tested. The variety of binary pulsars observed yields valuable information about the late stages in the evolution of stars. The study of rotational instabilities of young pulsars gives insight in the interior of neutron stars and hence offers the only possibility to investigate the equation-of-state under extreme pressures and densities up to 10^{15} g cm^{-3}. Monitoring the old millisecond pulsars and their behaviour as very stable clocks may lead to a new definition of time standards. Finally, pulsar surface magnetic fields range from 10^4 to 10^9 Tesla and exceed those fields which can be created in terrestrial laboratories by at least three orders of magnitude. The behaviour of electron-positron plasma can therefore be studied under unique circumstances.

An example of broadband interference excision. The left panel shows a grayscale plot of a contaminated 29-minute 610\,MHz observation of PSR B1534+12 taken with the 76\,m Lovell Telescope. Each horizontal line represents ten seconds of data, and a cumulative pulse profile is displayed at the bottom. The broadband interference spikes have been dedispersed'' by roughly 6% of the pulse period, yielding the smeared-out bumps seen in the left-hand panel. The right panel presents the same observation, but with these broadband power spikes eliminated during processing

Apart from these obvious benefits for basic research, we intend to develop new strategies for interference excision using the proposed data acquisition system. Interference rejection will become the major issue in radio astronomy over the next five to ten years. Fundamental and deep research is necessary to provide useful techniques to keep this unique window on the universe open. This is an important and interesting challenge in the modern era of intensive communications. State-of-the-art receivers installed at radio telescopes provide unprecedented bandwidths, which allow an enormous increase in sensitivity. However, both land-based and satellite-based sources of radio frequency transmissions currently render certain parts of the spectrum unusable. With the coherent dedispersion algorithm, it is possible to implement simple procedures to identify and excise both narrowband and broadband interference. For instance, it will allow us both to avoid the parts of the spectrum containing the legal carrier signals of such transmissions, and to mitigate the effects of any leakage outside the assigned bands (Ellingson et al. 2000). The benefits of broadband excision are demonstrated in Figure \ref{fig:rfi}. There are possible commercial applications for this technology in the broader field of radio communications.

The coherent dedispersion results in reduction in pulse timing errors by a factor of typically 2-5 for millisecond pulsars (Stairs et al. 1999), while the interference rejection results in a similar factor for slow pulsars at the Jodrell Bank site. This is equivalent to increasing the effective area of the Lovell telescope by the same amount. By pushing the frontiers of sensitivity and timing, we open up the possibility of discovering hitherto unrecognised phenomena.

## Bibliography

Camilo, F., Lorimer, D. R., Freire, P., Lyne, A. G., & Manchester, R. N. 2000, Astrophys. J. in press (astro-ph/9911234)

Ellingson, S. W., Bunton, J. D., \& Bell, J. F. 2000, in Astronomical Telescope and Instrumentation 2000 - Radio Telescopes, SPIE conference 4015, Munich

Hankins, T. H. & Rickett, B. J. 1975, Meth. Comp. Phys., 14, 55

Lyne, A. G. 1996, in Pulsars: Problems and Progress, IAU Colloquium 160, ed. S. Johnston, M. A. Walker, & M. Bailes, (San Francisco: Astronomical Society of the Pacific), 73

Lyne, A. G. & Manchester, R. N. 1988, Mon. Not. R. astr. Soc., 234, 477

Lyne, A. G. & Smith, F. G. 1998, Pulsar Astronomy, 2nd ed., (Cambridge: Cambridge University Press)

Stairs, I. H., Arzoumanian, Z., Camilo, F., Lyne, A. G., Nice, D. J., Taylor, J. H., Thorsett, S. E., & Wolszczan, A. 1998, Astrophys. J., 505, 352

Stairs, I. H., Splaver, E. M., Thorsett, S. E., Nice, D. J., & Taylor, J. H. 2000, Mon. Not. R. astr. Soc. in press

Stappers, B. W. et al. 1996, Astrophys. J., 465, L119

Taylor, J. H. & Weisberg, J. M. 1989, Astrophys. J., 345, 434

Thorsett, S. E. & Chakrabarty, D. 1999, Astrophys. J., 512, 288

This document was generated by Duncan Lorimer on November 14, 2001