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Determination of pulsar's binary parameters
A lessandro Corongiu Joseph Foy Vladisl av Kondratiev а ngel G. MuЯoz S. Tutor: Paulo C.C. Freire INA F ­ Oss. A str. di Cagliari, Italy A riz ona State University, USA York University, Canada Univ. de Z ulia, Venezuela

Pulsars
Pulsar s are high ly magnetize d r otating neutron stars. The ma gnetic field is resp onsi ble for : - The ra di o emission itself - The beaming of the e mmitte d radiation The obser ve d p ulse d behavi our is due to the misa ligneme nt of the ma gnetic axis respect to the spin a xis.

NA IC, A recibo Observatory

A recibo Observatory, July 15th, 2005

The Search of a pulsar
The search for a pulsar is the search for periodicities in the o bserved signal. In order to increase the signal to noise ratio, wide band obs ervations are usually performed. W ide band observations are, however, so strongly affected by interstellar disp ersion that is not possible to sum the signal across the bandwidth and, at the sam e tim e, mantain the signatures of the periodicities in the signal. So it's compulsory to dedisperse the signal before adding together al l frequency channels.

The Search of a pulsar
Once the dedispersed time series is obtained, a Fourier transformation is performed. The peak s in the power spectrum indicate the presence of periodicities, which may or may not be a pulsar. The time series is then folded at the frequency of any obtained peak in the power spectrum and the obtained profile is visually inspected. If you're lucky, you may find a pulsar.

Binary parameters
The orbit of a pulsar, as well as any object, in a binary system is determin ed in its geometrical characteristics, space displ acement and syncronised in time by the following parameters: Semim ajor axis Eccentricity Longitude of periastron Binary period Time of ascending node

True spin period and observed spin period
The pulsar is in motion with respect to the Earth, so we don't observe the TRUE spin period, accordingly to DЖppler effect laws:

V POBS = P RUE 1 + R T c
But we k now the pulsar is in a generally elliptical orbit, so we can express the radial velocity VR in terms of the geometrical features of the orbit and, of course, time (circular orbit):

a e PB T0

1 2 2 POBS = PTRUE 1 + c P a sin i cos P (t - t 0 ) B B
Because we deal with VR, we are not able to derive th e semimajor axis of the orbit, but only its projection on the plane containing the line of sight and the line of nodes.

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The observed pulsar and the data set
W e observed PSR J1738+0333, a 5.8ms pulsar in a 8 hr binary system, during the nights of July 13th and 14th. Each observation lasted one hour, the total bandwidth was 400 MHz at a central frequency of 1.4 GHz , and the sampling time was 64 µs. W e also analysed 13 observations already present in the Arecibo observations' archive, so we got a total of 15 measurements of the observed period, covering a data span between MJD 52872 and MJD 53566. 1) 2) 3)

Data reduction
Each dataset has been processed in this way: Dedispersion at the already k nown dispersion measure Scrunched in frequency Fourier transormed

A fter the Fourier transformation has been performed, we pick ed up the period wich could be eff ectively the pulsar period at the binary phase we observed it. Because of the Earth motion around the Sun, an additional contribution to the radial velocity was still present in our measurement, so we had to correct the observed pulsar periods for the Earth's orbital motion.

Data analysis
Because of the high number of fit parameters, we started from reasonable initial values, obtained from a pulsar catalogue. W e put them in the DЖppler formula and introduced variation parameters as follows:

Orbital parameters and more
Combining the initial guess values with the fitted variat ion parameters, we derived the physical param eters of the system:

POBS = в PTRUE

1 2PTRUE 2 2 +в a sin i cos в t - в t0 c PB PB PB

PTRUE = 5.8500959185 ms P B = 0. 3547806 d a sin i = 0.343392ls T 0 = 52902. 41 8037
W e also derived the value for the mass function:

The obtained values for the fitted parameters are: = = = = 1 1 1 . 0002 1 . 0008

f(MC) = 3.429 x 10-4 M
A nd finally, asuming an inclination of 60° and a pulsar mass MP = 1.35 M , we estimated th e mass of the companion:

MC = 0 . 1 0 4 M

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Conclusions
W e analysed the data of the bin ary pulsar PSR J1738+0333. Our data come from the A recibo archive. W e also analysed two observations done by ourselves during this week . The reduction process has been the same th at is used for searching for new pulsar or solve recently discovered ones. W e obtained an orbital solution for this pulsar, and calculated an estimate for the companion mass, which tells us that PSR J1738+0333 is lik ely orbiting around a light white dwarf.

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