Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.naic.edu/~tghosh/now/dynspec_20_50.desc Дата изменения: Sun Mar 8 01:10:36 2009 Дата индексирования: Sun Apr 10 12:18:46 2016 Кодировка: Поисковые слова: m 8

Hi Tapasi,

The above plots are the dynamic spectra for PSR B1737+13. This is a
bright normal pulsar that we observed in September. As you can see,
there is a lot of intensity variations. Also, as the pulsar is very
bright, you can signal right down the edges of the front-end bandpass.
The "narrow" plot shows a zoom-in of a narrower portion of the band to
show some of the more detailed structure. As can be seen, there is
definitely structure at even smaller scales too. This indicates that
the scattering region is quite large.

Here are some general thoughts about the role of Mark5 recording in
pulsar experiments.

Pulsar work with the Mark5 Backend:

Advantages:

Flexibility: Set frequency time resolution after observations,
enabling multiple analyses of the data. Allows for off-line coherent
dedispersion.
Ease of Use: Subroutines have been developed by Walter Brisken to
enable easy decoding of data.
Bandwidth Complementarity: The Mark5a system can cover the entire
front-end frequency bandwidth of 327 and 430 MHz receivers.
Time accuracy: VLBI observations require stringent time accuracy. We
can use the Mark5 recorders to gauge stability of the other pulsar
timing backends: at multiple telescopes, and multiple epochs).
Raw Data Rate: In 64 MHz bandwidth mode, we are recording 64
MBytes/s. It is now feasible to store and analyze data at this high
rate.

Disadvantages:

Remote Observing Capabilities: Not incorporated into CIMA
2-bit sampling: More susceptible to RFI in single-dish experiments.

Applications:

Interstellar Scintillation: Multi-path scattering in the ISM causes
self-interference of the pulsar signal, resulting in intensity
variations. The variations are observed in pulsar dynamic spectra and
can impose very narrowband features in frequency. This necessitates
high frequency resolution. However, due to the subtle nature of these
variations (> 40 dB below the main signal) large telescopes with large
observation bandwidths are required. The properties of these
features changes as the scattering medium changes on day-month time
scales. Dan Stinebring (P2419) and Ryan Shannon (P2395) have been
using this to study the ISM in the direction of bright pulsars. As
the extent of the scattering region can be greater than 1 AU, there is
information encoded in the intensity variations about the structure of
the background pulsar. This technique has given resolving power of <
1 nano-arcsecond in the past. Ryan Shannon (P2395) is using the ISM
as an interferometer and trying to resolve the pulsar magnetosphere.

Pulsar timing systems: Long term stability of backends and frontends
is necessary if the gravitational waves are going to be detected using
pulsar timing. Additionally, understanding the delays in the
telescopes is necessary, as well. We can use the VLBI backend to
compare to timing machines (ASP and GASP). Recently, the Nanograv
collaboration observed a few pulsars simultaneously at Arecibo and the
GBT in a modern attempt to measure these delays. Paul Demorest (
gbt08a80) is leading this effort.

Timing and Interstellar Propagation:
Current analysis methods only take into account changes in gross
properties of the ISM (changes in the total column density along the
line-of-sight). However, more subtle effects are likely to be
manifested causing 10 – 100 ns variations in the time of arrival of
pulses from pulsars. Identifying these perturbations (and hopefully
correcting them) will be important for long-term timing projects and
gravitational wave detection. Again the Mark5 recorder can be used to
study propagation effects. Ryan Shannon (P2445) is currently leading a
study of PSR J1713+0747 to estimate the strength of propagation
effects toward this well-timed pulsar.

Future of Baseband Recording:

Remote Observation Capability:

Mock Spectrometers: A baseband recording mode is available in the
Mock Spectrometers, with sampling using a larger number of levels.


On Fri, Mar 6, 2009 at 9:41 PM, wrote:
> Hi Ryan,
>
> Would it be possibile fro your send me the quite/short writeup by
> tomorrow ? My talk is on Monday. I am begining to get the slides
> together, so it will be o fgreat help.
>
> Thanks,
> Tapasi
>