Summer Research Fellowship Programme of India's Science Academies 2017
Eight week report for IASc-INSA-NASI Summer
Research Fellowship
on
Pulsar Timing
Name: Pavan U A
Application No.: PHYS253
Guide: Prof. Yashwanth Gupta
At
National Centre for Radio Astrophysics,
TIFR,
Pune
Pune Univerisity Campus, Post Bag 3,
Ganeshkhind Pune 411007, India
Contents
1 Introduction 4
1.1 Overview on Pulsar Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2 Introduction to GMRT 6
2.1 Array Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Receiver Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3 Digital Backends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3.1 GMRT Antenna System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3.2 Design Specifications of GMRT Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4 The GMRT beam mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.5 Coherently vs Incoherently Phased Array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.5.1 Voltage Mode and Coherent Dedispersion Pipeline . . . . . . . . . . . . . . . . . . . . . . . . . 10
3 Radio Frequency Interference 11
4 Introduction to Presto - Pulsar Search Tool 11
4.1 Working with Presto Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2 Analysis using presto . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.3 The Time Domain and Pulse Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.4 Frequency and Sub-bands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.5 Other plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5 GPTool 17
5.1 Running GPTool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.2 Example GPTool.in file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.3 GPTool Output files and plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6 Automation of Analysis 20
7 Pulsar Timing Observation 20
7.1 TOA measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.2 Forming Pulse Emission Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
8 Delays encountered in Pulsar Timing 22
8.1 Atmospheric Delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
8.2 Einstein delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
8.3 Romer Delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
8.4 Shapiro delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
8.6 Tempo2 plot window . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1
8.5 Pulsar Timing Observation with Tempo2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8.7 Results of Timing analysis for the pulsar J1857+0943 . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8.7.1 Pre-fit residuals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.7.2 Post-fit residuals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2
3
1 Introduction
Pulsars are what is known as the ”lighthouses” of the universe they are the rotating neutron stars which emits a
focused beam of electromagnetic radiation that is only visible in line of sight. Known as pulsars, these mysterious
objects get their name because of the way their emissions of electromagnetic waves which appear to be pulsating out
into Universe. Pulsars are highly compact objects that are about the size of a small city but contain more mass than
the any main sequence star.Pulsars are used to study extreme states of matter, search for planets beyond Earth’s
solar system and measure cosmic distances. Pulsars can also be used to indirectly find gravitational waves, which
could point the way to energetic cosmic events like collisions between supermassive black holes. Discovered in 1967,
pulsars are fascinating and mysterious members of the cosmic regime.
1.1 Overview on Pulsar Timing
The process of using observations of radio pulsar pulse profiles to calculate pulse arrival times which are then
used to study the rotational history, kinematics and orbits of pulsars, is called pulsar timing. This is usually carried
out at radio wavelengths, the mean pulse profiles of radio pulsars are usually remarkably stable, and when compared
with the average profile can yield highly accurate times of arrival or ”TOAs”. In the case of millisecond pulsars,
these can be highly accurate in case of the millisecond pulsars.
The process of timing has to be carried out with considerations of the delays which are present in their arrival
times, for example PSR J1909-3744 Pulse arrival times as a function of orbital phase for the binary pulsar PSR
J1909-3744. This pulsar is orbiting a white dwarf every 1.5 days and is nearly edge-on to our line of sight. The
curvature of space-time leads to the distortion of the arrival times seen above after subtracting a simple Keplerian
model of the pulsars orbit from the arrival times. In the absence of the space-time curvature the arrival times would
be flat.
These TOAs can be used to deduce the entire rotational history of a given pulsar. This practice has been used
at many telescopes around the world to examine how pulsars rotate allowing the discovery of pulsar glitches (sharp
discontinuities in their rotation periods) and the study of their neutron superfluid interiors. When a pulsar is in orbit
about another body, pulsar timing can be used to make highly accurate observations that can be used to deduce
companion star masses, and in extreme cases, test theories of relativistic gravity such as Einsteins general theory of
relativity. Observations over many years can provide accurate proper motions and even parallax measurements.
Pulsars can be designated as cosmic clocks or ”clo cks in space”. On long timescales they can match the precision
of atomic clocks. Using a process called pulsar timing, the exact calculation of TOAs can allows a number of
applications, from theories of gravity to detecting gravitational waves. Also universal navigation reference system
suitable for autonomous space navigation can be created by pulsars, using them as natural navigation beacons, and
one which does not require the costly setup of navigation system satellites which can function only up to a certain
designated period and only confined to one planet. By comparing pulse arrival times measured on-board a spacecraft
with predicted pulse arrivals at a reference location, the spacecraft position can b e determined autonomously and
with high accuracy everywhere in the solar system and beyond. There are currently many on-going experiments to
4
use the clock-like nature of pulsars to produce a huge gravitational wave detector for low-frequency gravitational
waves.
1.2 Motivation
Though much research is already been carried out at various parts in the world in the study of pulsars, through
its discovery in the year 1967 by accident while Jo celyn Bell and Antony Hewish were looking for twinkling sources
of radio radiation, many aspects still remain a large mystery such as the emission mechanism of the pulsars and their
potential use in the future in the Galactic positioning (see Becker Lange et al., 2013) made me choose this topic for
my Summer Research Fellowship, in the two Nobel prizes which have been conferred to the pulsar related field, one
Nobel prize was presented for the indirect detection of the Gravitational waves by Princeton University astronomers
Russell A. Hulse and Joseph H. Taylor in 1974 for their work on PSR 1913+16 by the Pulsar Timing. PSR 1913+16
emits radiation every 59 milliseconds. It orbits another star, which is likely another neutron star. These stars orbit
each other at super-high speed every eight hours.
Four years after first discovering PSR 1913+16 and after some careful timing measurements of the pulsar, Hulse
and Taylor found that the two stars move closer to each other by about three millimeters per orbit. That could only
happen if something was pulling energy out of the system which was precisely the detection of the Gravitational
waves.
The pulsar timing also has many other advantages with respect to reducing the time residual in the observation
of the pulsar in GMRT, which is currently been carried out by calculating the Time of Arrival (TOAs) of the pulses.
5
2 Introduction to GMRT
The Giant Metrewave Radio Telescope (GMRT) consists of an array of 30 antennas. Each antenna is 45 m in
diameter, and has been designed to operate at a range of frequencies from 50 MHz to 1450 MHz. The antennas have
been constructed using a novel technique (nicknamed SMART) and their reflecting surface consists of panels of wire
mesh. These panels are attached to rope trusses, and by appropriate tensioning of the wires used for attachment the
desired parabolic shape is achieved. This design has very low wind loading, as well as a very low total weight for
each antenna. Consequently it was possible to build the entire array very economically.
2.1 Array Configuration
GMRT has 30 parabolic radio dishes each of diameter 45m. Owing to the interferometric arrangement GMRT
has achieved enough sensitivity to study the profile of single pulses. The antennae dishes are arranged in a hybrid
configuration. 14 of the dishes are packed in a region called the central square, which is 1 Km across and they are
arranged randomly. This random arrangement is intentional to avoid grating lobes. The antennas in the central
square are numbered as C00 to C14, (C07 being absent) . Then the remaining of the antennas are arranged in a
‘Y’ configuration the length of each arm being 14 Km. The arms are caller the East, West and South arms and are
numbered as E02 to E06, S01 to S06 (S05 not present), W01 to W06. This makes up the total 30 dishes
1
2.2 Receiver Systems
The GMRT currently operates at 5 different frequencies ranging from 150 MHz to 1420 MHz. Some antennas
have been equipped with receivers which work up to 1750 MHz. Above this frequency range however, the antenna
performance degrades rapidly both because the reflectivity of the mesh falls and also because of the rapidly increasing
aperture phase errors because of the deviations of the plane mesh facets from a true parabola.
The GMRT feeds, (except for the 1420 feed), are circularly p olarized. The circular polarization is achieved by means
of a polarization hybrid inserted between the feeds and the RF amplifiers. No polarization hybrid was inserted for
the 1420 MHz feed, in order to keep the system temperature low. None of the receivers are cooled, i.e. they all
operate at the ambient temperature. The feeds are mounted on four faces of a feed turret placed at the focus of the
antenna. The feed turret can be rotated to make any given feed point to the vertex of the antenna. The feed on one
face of the turret is a dual frequency feed, i.e. it works at both 233 MHz as well as 610 MHz.
After the first RF amplifier, the signals from all the feeds are fed to a common second stage amplifier (this amplifier
has an input select switch allowing the user to choose which RF amplifiers signal is to be selected), and then converted
to IF. Each polarization is converted to a different IF frequency, and then fed to a laser-diode. The optical signals
generated by the laser-diode are transmitted to a central electronics building (CEB) by fiber optic cables. At the
central electronic building, they are converted back into electrical signals by a photo-diode, converted to baseband
1
The array was originally meant to have 34 antennas but due to escalating costs it was reduced to 30 antennas.
6
frequency by another set of mixers, and then fed into a suitable digital backend. Control and telemetry signals
are also transported to and from the antenna by on the fiber-optic communication system. Each antenna has two
separate fibers for the uplink and downlink.
2.3 Digital Backends
There are a variety of digital backends available at the GMRT. The principle backend used for interferometric
observations is a 32 MHz wide FX correlator. The FX correlator produces a maximum of 256 spectral channels for
each of two polarizations for each baseline. The integration time can be as short as 128 ms, although in practice
2 sec is generally the shortest integration time that is used. The FX correlator itself consists of two 16 MHz wide
blocks, which are run in parallel to provide a total instantaneous observing bandwidth of 32 MHz. For spectral line
observations, where fine resolution may be necessary, the total bandwidth can be selected to be less than 32 MHz.
The available bandwidths range from 32 MHz to 64 kHz in steps of 2. The maximum number of spectral channels
however remains fixed at 256, regardless of the total observing bandwidth. The GMRT correlator can measure all
four Stokes parameters, however this mode has not yet been enabled. In the full polar mode, the maximum number
of spectral channels available is 128. Dual frequency observations are also possible at 233 and 610 MHz, however in
this case, only one polarization can be measured at each frequency. The array can be split into sub-arrays, each of
which can have its own frequency settings and target source. The correlator is controlled using a distributed control
system, and the data acquisition is also distributed. The correlator output, i.e. the raw visibilities are recorded in a
GMRT specific format, called the LTA format. Programmes are available for the inspection, display and calibration
of LTA files, as well as for the conversion of LTA files to FITS.
The first block of the GMRT pulsar receiver is the GMRT Array Combiner (GAC) which can combine the signals
from the user-selected antennas (up to a maximum of 30) for both incoherent and coherent array operations. The
input signals to the GAC are the outputs of the Fourier Transform stage of the GMRT correlator, consisting of 256
spectral channels across the bandwidth being used, for each of the two polarization from each antenna. The GAC
gives independent outputs for the incoherent and coherent array summed signals, for each of two polarizations. For
nominal, full bandwidth mode of operation, the sampling interval at the output of the GAC is 16µsec.
Different back-end systems are attached to the GAC for processing the incoherent and coherent array outputs. The
incoherent array DSP processor takes the corresponding GAC output signals and can integrate the data to a desired
sampling rate (in powers of 2 times 16 microsec). It gives the option of acquiring either one of the polarizations or
the sum of both. It can also collapse adjacent frequency channels, giving a slower net data rate at the cost of reduced
spectral resolution. The data is recorded on the disk of the main computer system.
The coherent array DSP processor takes the dual polarization, coherent (voltage sum) output of the GAC and can
produce an output which gives 4 terms the intensities for each polarization and the real and imaginary parts of the
cross product from which the complete Stokes parameters can be reconstructed. This hardware can be programmed
to give a sub-set of the total intensity terms for each polarization or the sum of these two. The minimum sampling
interval for this data is 32 microsec, as two adjacent time samples are added in the hardware. Further preintegration
7
(in powers of 2) can be programmed for this receiver. The final data is recorded on the disk of the main computer
system. There is another independent full polarimetric back-end system that is attached to the GAC. This receiver
produces the final Stokes parameters, I,Q,U & V. However, due to a limitation of the final output data rate from this
system, it it can not dump full spectral resolution data at fast sampling rates. Hence, for pulsar mode observations
the user needs to opt for online dedispersion or gating or folding before recording the data (there is also a online
spectral averaging facility for non-pulsar mode observations). In addition, there is a search preprocessor back-end
attached to the incoherent array output of the GAC. This unit gives 1-bit data, after subtracting the running mean,
for each of the 256 spectral channels. Either one of the polarizations or the sum of both can be obtained.
Most sub-systems of the pulsar receiver can be configured and controlled with an easy to use graphical user interface
that runs on the main computer system. For pulsar observations, since it is advisable to switch off the automatic
level controllers at the IF and baseband systems, the power levels from each antenna are individually adjusted to
ensure proper operating levels at the input to the correlator. The format for the binary output data is peculiar to
the GMRT pulsar receiver. Simple programs to read the data files and display the raw data - including facilities for
dedispersion and folding - are available at the observatory and can be used for first order data quality checks, both
for the incoherent mode and coherent mode systems.
2.3.1 GMRT Antenna System
A radio telescope in its simplest form consists of three components, (i) an antenna that selectively receives
radiation from a small region of the sky, (ii) a receiver that amplifies a restricted frequency band from the output of
the antenna and (iii) a recorder for registering the receiver output. In this chapter we focus on the antenna, and in
particular the antennas used for the GMRT. The GMRT antennas are parab olic reflector antennas. The first reflector
antenna was invented by Heinrich Hertz in 1888 to demonstrate the existence of electromagnetic waves which had
been theoretically predicted by J.C.Maxwell. Hertzs antenna was a cylindrical parabola of f/D = 0.1 and operated
at a wavelength of 66 cm.(450 MHz). The next known reflector antenna was that constructed in 1930 by Marconi for
investigating microwave propagation. After that, in 1937, Grote Reber constructed the prototype of the modern dish
antenna - a prime-focus parabolic reflector antenna of 9.1 m. diameter, which he used to make the first radio maps
of the sky. During and after World War II, radar and satellite communication requirements caused great advances
in antenna technology
2.3.2 Design Specifications of GMRT Antennas
The f/D ratio for the GMRT antennas was fixed at the value 0.412 based both on structural design issues as
well as preliminary studies of various feeds radiation patterns. Since the antennas are to work at meter wavelengths
prime focus feeds were preferred. Cassegrain feeds at meter wavelengths would result in impractically large secondary
mirrors (the mirror has to be several across) and concomitant large aperture blockage. Six bands of frequencies had
been identified for the GMRT observations. It was deemed essential to be able to change the observing frequency
rapidly, and consequently the feeds had to mounted on a rotating turret placed at the prime focus. If one were to
mount all the six feeds on a rotating hexagon at the focus, the adjacent feeds will be separated by 60 . If one wants
8
to illuminate the entire aperture, then one has to have a feed pattern that extends at least up to the subtended
angle of the parabola edge, which is 0 = 62.5
(Note that cot(
/2) = 4f/D). Hence this arrangement of feeds would
cause the one feed to see the feeds on the adjacent faces. It was decided therefore to mount the feeds in orthogonal
faces of a rotating cube. Since one needs six frequency bands, this leads to the constraint that at least two faces
of the turret should support dual frequency capability. For astronomical reasons also dual frequency capability was
highly desirable. One specific aspect of GMRT design is the use of mesh panels to make the reflector surface. Since
the mesh is not perfectly reflective, transmission losses thorough the mesh have to be taken into account. Further,
the expected surface errors of the mesh panels was 5 mm. This implies that the maximum usable frequency is (see
Section 19.2) 3000 MHz, independent of the transmission losses of the mesh. (Incidentally, since the mean-spacing
of feed-support legs, L = 23.6 m, the lowest usable frequency is around 6 MHz).
2.4 The GMRT beam mode
The GMRT Software Backend (GSB; Roy et al., 2010) consists of a beam former, wherein the signals from the
antennas can be appropriately combined for high time resolution studies of radio transients like pulsars (Gupta et
al., 2000). Depending on the specific science requirement, the signals can be combined to form a incoherently phased
array or a coherently phased array for transient observations.
2.5 Coherently vs Incoherently Phased Array
Normally, the signals from an n-element phased array are combined by adding the voltage signals from the different
antennas after proper delay and phase compensation. This summed voltage signal is then put through a square-law
detector and an output proportional to the power in the summed signal is obtained. For identical elements, this
phased array gives a sensitivity which is n times the sensitivity of a single element, for point source observations. The
beam of such a phased array is much narrower than that of the individual elements, as it is the process of adding the
voltage signals with different phases from the different elements that produces the narrow beam of the array pattern.
For some special applications, it is useful to first put the voltage signal from each element of the array through a
square-law detector and then add the powers from the elements to get the final output of the array. This corresponds
to an incoherent addition of the signals from the array elements, whereas the first method gives a coherent addition.
In the incoherent phased array operation, the beam of the resultant telescope has the same shape as that of a single
element, since the phases of the voltages from individual elements are lost in the detection process. This beam width
is usually much more than the beam width of the coherent phased array telescope. The sensitivity to a point source
is higher for the coherent phased array telescope as compared to the incoherent phased array telescope, by a factor
of
n. The incoherent phased array mode of operation is useful for two kinds of astronomical obervations. The first
is when the source is extended in size and covers a large fraction of the beam of the element pattern. In this case,
the incoherent phased array observation gives a better sensitivity. The second case is when a large region of the sky
has to be covered in a survey mode (for example, in a survey of the sky in search for new pulsars). Here, the time
taken to cover the same area of sky to equal senstivity level is less for the incoherent phased array mo de. Only for
9
a filled aperture phased array telescope are these times the same. For a sparsely filled physical aperture such as an
earth rotation aperture synthesis telescope, this distinction between the coherent and incoheret phased array modes
is an important aspect of phased array operation.
2.5.1 Voltage Mode and Coherent Dedispersion Pipeline
The PA mode has provision for recording the raw voltage stream from a single phased array beam on a disk,
where the data is stored with the real and imaginary parts from the output of a Fast Fourier Transform (FFT) on the
sampled voltage time series, with 256 or 512 channels. The voltage recording mode has many powerful applications,
such as coherent dedispersion of pulsar signals, but is often limited due to the large disk storage requirements for
Nyquist sampled voltage data.
2
The prohibitive requirements for disk storage of voltage data necessary for offline coherent dedispersion can be
avoided by implementing a real- time coherent dedispersion pipeline. Such a pipeline has recently been developed
at the GMRT for the currently available maximum bandwidth of 32 MHz (De & Gupta, 2016). The pipeline uses a
hybrid CPU-GPU based signal processing system for real-time co- herent dedispersion of dual polarisation voltage
signals from a single phased array beam, followed by recording of the power signals (with desired integration) on
disks.
2
Independent samples are obtained if sampled at Nyquist Rate see Shannon Sampling Theorem
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3 Radio Frequency Interference
Pulsars emit much of their radiation in the radio region. The radio waves emitted by the Pulsar has to travel
through interstellar space to reach us, because of this the power of the signal will be already very low. These signals
are then collected by our telescopes, more over there is tremendous amount of man made signal in todays world,
from the small smart watch to that of the large planes and Satellites, all of them create humongous amount of
problems in the pulsar observation. Even the spark plugs in a gasoline-powered car give off radio waves! There are
cosmic sources of radio waves too, like our very own Sun. Many sources such as our FM radios, radio broadcasts
which are highly important for a human being in todays world are from the point of view of a radio astronomy is
all very bad. These sources of radio waves get in the way, and make some observations almost impossible. For this
reason, astronomers refer to these man-made sources of radio waves as Radio Frequency Interference, or RFI. RFI
is similar to light pollution. In large cities, light pollution can make it impossible to see faint stars, and RFI can
make it similarly difficult to see faint sources of radio waves. There are many methods for RFI mitigation such as
using a filter to block the RFI and to build the Telescopes outside the cities to point out some.Filters can be used
when ever we definitely know about the sources but even that has certain draw backs such as , if a radio source is
present in that particular band width we are effectively removing the whole band! And sometimes computers can
be used to try to identify obvious sources of RFI and then ignore it in a given set of data, which is what the below
mentioned software does. RFI is especially problematic for pulsar searches because it can often look like a pulsar
signal in the Fourier transformed data because the the Fourier transform is essentially helping us find the repeating
signal of the pulsar. But these man-made sources of RFI often repeat as well. For example, the alternating current
(AC) that supplies your home with power changes polarity at a frequency of 60 Hz. Power lines that carry this
current therefore give off RFI at a frequency of 50 Hz if they are not shielded. For this reason, we always see a large
signal in our Fourier transformed data at 60 Hz. Luckily, we can simply tell a computer to ignore that frequency.
Other sources of RFI may not be as easy to handle, because we dont always know what frequency they will show up at.
4 Introduction to Presto - Pulsar Search Tool
PRESTO is a software package for pulsar search and analysis written by Scott Ransom of NRAO. It was primarily
designed to efficiently search for binary millisecond pulsars from long observations of globular clusters although it is
in use for various surveys with short integrations and to process a lot of X-ray data. It is written primarily in ANSI
C, with many of the recent routines in Python such as the get TOAs routine. The files generated by presto is used
for estimation of the time of arrival of a pulse using the get TOA routine which is also written by Scott Ransom.
4.1 Working with Presto Software
The input to the presto is the binary raw file which is generated by observation carried out by the Observatory
along with certain other header file (For GMRT it has the extension of . gmrt hdr) and a parameter file which is
obtained from the ATNF Catalog maintained by the Australia Telescope National Facility. The sample .gmrt hdr
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file is as shown below.
#DATA FILE HEADER #
Site : GMRT
Observer : Gupta
Proposal : 30_043
Array Mode : PA
Observing Mode : Timing
Date : 25/04/2017
Num Antennas : 13
Antenna List : C00 C01 C03 C04 C05 C06 C08 C09 C10 C11 C12 C13 C14
Num Channels : 2048
Channel width : -0.09765625
Frequency Ch.1 : 300.1
Sampling Time : 81.92
Num bits/sample : 16
Data Format : integer binary, little endian
Polarizations : Total I
MJD : 57868.63887927306
UTC : 15:19:59.169192001
Source : PSRJ1022+1001
Coordinates : 10:22:58.0007, +10:01:52.77
Coordinate Sys : J2000
Drift Rate : 0.0, 0.0
Obs. Length : 2621440
Bad Channels : 2:1-20,2020-2048
Bit shift value : 8
The hdr file consists of parameters such as the Sampling time , No of bits per sample, MJD, UTC etc. which are
explained below.
Site: Gives the name of the observatory.
Array Mode: In Phased Array arrangement the voltage outputs from the antennas are combined by adding phases
to obtain a total power signal.
Frequency Ch.1 : It is the lower limit in the frequency of observation where the 300.1MHz represents the observation
in the band 300MHz - 500MHz with 200Mhz bandwidth by the GMRT Observatory.
Antenna List: Denotes the names of the antenna which were used for t he observation.
Sampling Time: The time interval in which the data is collected for digitalization.
Source: It gives the name of the Pulsar of which the observation was.
Coordinates: This gives the coordinates of the Pulsar in the J2000 coordinate system.
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Bad-channels: This used to specify the presto to ignore certain channels during the folding process.
4.2 Analysis using presto
The work carried out in the last 8 week period was primarily on Presto, GPTool and Tempo2, using the data
generated in the uGa-10 ,cycle 30 047, 31 057 and 32 032. The various pulsars observed in these cycles were as
follows
J06130200
J0645+5158
B0740-28
J0751+1807
B0950+08
J1012+5307
J1022+1001
1024-0719
10454509
B1133+16
J14553330
J1614-2230
J1640+2224
B1642-03
J1713+0747
J1730-2304
J1738+0333
J1857+0943
J19093744
J2302+4442
JB1508+55
J16003053
The data obtained from the pulsar observation will be in the binary format which had to linked to dat file in order
to run the prepfold command for the Presto analysis which also requires a parameter file containing the details of
the various parameters of the pulsar such as the Barycentric period of the pulsar (s),Time derivative of barcycentric
period, Barycentric rotation frequency (Hz),Time derivative of barycentric rotation frequency,Second time derivative
of barycentric rotation frequency etc which is obtained from the ATNF catalog
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.
A typical result form the presto analysis of the pulsar J1909-3744 is shown in the figure 1.
3
http://www.atnf.csiro.au/people/pulsar/psrcat/
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Figure 1: A prepfold plot or a presto output plot of J1909-3744. This pulsar has a period of 0.002947108069160717
seconds.
4.3 The Time Domain and Pulse Profile
Figure 2 above shows one of the most important plots that is generated through the prepfold routine for which the
analysis was carried out during this last 8 weeks. The time domain plot shown above gives details regarding the
strength of the signal collected by any telescope which varies during the course of the observation. The darker gray
segments represents the strength of the signal in some small piece of the data, which is also know as bin in the
terminology. The darker the bin indicates higher signal strength where as the white parts indicates that no signal
was detected in that place or bin. Each of the row on the Time vs Phase plot is a sub-fold, i.e. a fold of a small part
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