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VLBI PROPOSAL COVERSHEET
DEADLINES: 1st of Feb., June., Oct.
rcvd:
(1) Date Prepared: 29 September 1996
(2) Proposal Title: VLBA monitoring observations of the intra­day variable PKS 0405--385 For Grad Students
Only
Observations
(3) AUTHORS INSTITUTION/LOCATION For Ph.D. Anticipated
Thesis? Ph.D. Year
D.L. Jauncey, J.E. Reynolds ATNF, Epping
L. Kedziora­Chudczer, M.A. Walker RCfTA, University of Sydney yes (LK) 1997
G. Nicolson HartRAO, RSA
M.H. Wieringa, E.A. King, A.K. Tzioumis ATNF, Epping
P. McCulloch University of Tasmania
D. Gabuzda Lebedev Physical Institute
(4) Related previous or current VLBI proposal(s): bj024
(5) Contact author for scheduling: J. Reynolds c/­ ATNF
Telephone: +61 2 9372­4165 Address: PO Box 76
E­Mail: jreynold@atnf.csiro.au Epping 2121
Fax: +61 2 9372­4176 AUSTRALIA
(6) Scientific Category (indicate all that apply):
fl astrometry & geodesy, fl solar, fl propagation, fl planetary, fl stellar, fl pulsar, fl ISM, fl masers,
fl normal galaxies,
N
active galaxies, fl cosmology
(7) Wavelength(s) requested (those not available on the global network are indicated with a small circle):
fl 90cm, fl 50cm, ffi 21cm, fl 18cm, fl 13cm, fl 6cm,
N
3.6cm, fl 3.6/13cm,
N
2cm, fl 1.3cm, fl 7mm
N
Global Network standard bands fl Special frequencies
(8) Recording format:
N
VLBA fl MkIV fl MkIII (Mode )
Aggregate bit rate 128 ( 4 BB channels at 16 MSamples/sec of fl 1 bit, N 2 bit samples)
(9) N
Multi­epoch observation: 4 epochs of 7 hours each, separated by 1 month
(10) Network Requested antennas Total time requested
EVN
VLBA ALL 28
US
Unaffiliated
(11) ABSTRACT (Do not write outside this space. Please type.)
Scheduler use only
(09/96)
We propose regular VLBA monitoring at 8 and 15 GHz of the remarkable intra­hour variable radio source PKS
0405--385. This source has recently been discovered to exhibit episodes of deep and rapid intensity fluctutations,
several times stronger and an order of magnitude faster than that observed in any previous intra­day variable (IDV).
We propose regular monthly observations in order to correlate the changes in source structure and polarization with
both the episodic and rapid variability.

(12) Observation type:
N
Interferometry, fl Spectroscopy, fl Pulsar, fl Phase referencing, fl Single dish
(13) Polarization: fl IEEE RCP fl IEEE LCP
N
Dual Circular
Global network standard for single polarization is LCP for all –s except 13cm (RCP) and 3.6cm (RCP).
(14) Tape usage (Show !recording time?/!total time?): 7/7
(15) Assistance required:
Observation Setup: fl Consultation, fl Extensive help,
N
Control file preparation
Correlation: fl Consultation, fl Extensive help, fl Control file preparation
Postprocessing: fl Consultation, fl Extensive help, fl Control file preparation
(16) Processor: Socorro
Special processing:
N
XPol, fl Pulsar gate, fl Multiple Fields:
Averaging time: Spectral channels per baseband channel: 16
fl Other special processing:
(17) Postprocessing Location: ATNF
(18) Source list: N J2000 fl B1950
If more than 4 sources, please attach list. If more than 30, give only selection criteria and GST range(s)
Source 1 Source 2 Source 3 Source 4
Name(s) PKS 0405--385
RA (hh mm) 04 07
Dec (dd.d) ­38.4
GST range (Europe)
GST range (US) 8­15
GST range (Other)
Band(s) 3.6cm & 2cm
Flux density (Total, Jy) ¸2Jy
Flux (correlated, mJy) ¸2Jy
RMS needed (mJy/beam) 4mJy
Peak/RMS needed ¸500:1
(19) Preferred VLBI session or range of dates for scheduling, and why:
(20) Dates which are NOT acceptable, and why:
(21) Attach a self­contained scientific justification, not in excess of 1000 words.
Preprints or reprints will not be forwarded to the referees.
A brief description of the capabilities of the VLBA may be obtained by anonymous ftp from
ftp.aoc.nrao.edu (146.88.1.103 if you do not use a name server), file pub/obssum.vlba.ps. Or, if
you have the mosaic software available, use URL http://info.aoc.nrao.edu.
A brief description of the capabilities of the EVN may be obtained by ftp from astbo1.bo.cnr.it
(137.204.51.1 if you do not use a nameserver), login VLBINFO, file EVN.STS.
Please include the full addresses (postal and e­mail) for first­time users or for those that have
moved (if not contact author).

Monitoring observations of the structure and polarization of PKS 0405--385
Background
We have discovered intra­hour flux density variations in the z=1.285 quasar PKS 0405--385 during
Australia Telescope Compact Array (ATCA) flux density monitoring observations carried out in June this
year (Kedziora­Chudczer et al. 1996a,b). Monitoring continued for three consecutive days 8--10 June at
4 frequencies, revealing remarkable large­amplitude, highly­correlated, quasi­periodic fluctuations (Figure
1). Such large, rapid changes in the radio flux density of an extragalactic source are, to our knowledge,
unprecedented.
If such changes are intrinsic to the source, then a brightness temperature in excess of 10 20 K is implied,
which is difficult to reconcile with the inverse Compton limit of 10 12 K. For beaming models, a Doppler factor
of ¸500 is needed, well in excess of the factors of up to ¸10 seen in superluminals (cf. Vermeulen, 1995).
Preliminary measurements of linear polarization show it to be variable at around 1.5%.
There are similarities in the characteristics of the variations shown in Figure 1 and those found for
0917+624 (Quirrenbach et al., 1989, Qian et al., 1991). The variations of PKS 0405--385 are, however, larger
(¸1 Jy peak­to­peak), occur at a higher frequency (5¸8GHz), and more rapidly (t peak\Gammato\Gammapeak ¸2.5 hours)
than in the former (with corresponding values ¸0.3Jy and ¸25 hours respectively).
Continued monitoring of the flux density of PKS 0405--385 with the ATCA showed that the rapid
variations persisted through the end of June, but then were entirely absent from observations taken on 27
July. Analysis of archival ATCA data indicates that the rapid variability is episodic, coming and going on
a time­scale of weeks to months, with no large associated changes to the mean flux density.
Proposed VLBA observations
We propose monitoring the structure and polarization structure of PKS 0405--385 with the VLBA at 8
and 15 GHz at monthly intervals. Since the source shows episodic behaviour, we propose to observe every
month for the next year so that we can sample both the quiescent and variable periods of source behaviour.
The two observing frequencies, 8 and 15 GHz, were chosen as;
ffl the amplitude of the flux density variations is strongest near 8 GHz, so the effect is seen most clearly
at this frequency and,
ffl imaging at a second frequency with equal (or better) resolution will provide spectral­index information
which is most important for interpreting the 8GHz image. Flux density monitoring with the VLA in June
1996 (under a ``Target of Opportunity'' proposal) showed diminished amplitude of the fluctuations at 22GHz,
but substantial fluctuations are still inferred for 15GHz under any conceivable model.
Rapid variability on timescales of a day or less violates one of the principal assumptions of earth­
rotation synthesis and will require special care during the imaging process. Consideration of a number of
plausible classes of mechanism (Kedziora­Chudczer et al., op. cit.) leads to the conclusion that the variable
component is most unlikely to be resolved at ¸mas resolution. Under this simplifying assumption, it should
be straightforward to modify existing self­calibration software (e.g. DIFMAP) to add into the source model
a time­variable unresolved component, with a flux history determined from the short­spacing visibilities.
Although we anticipate no problems with this approach, the selection of our second imaging frequency
is motivated partly by the reduced amplitude of the variability expected at 15GHz, which should alleviate
any variability­related problems arising during the imaging.
The mechanism for the rapid variability in this source is clearly of great interest and a number of
competing models can be advanced. Qian et al. (1991), for example, propose a shocked­jet model for
0917+624, in which a relativistic shock moves along the line of sight through inhomogeneous, small­scale
structures distributed along the jet. In this model, the VLBA observations could be expected to reveal
displaced relative positions and possibly relative motion of the flat­spectrum ``core'' and the time­variable
component embedded in the ``jet''. In any case, it is clear that regular high­resolution imaging of the source
structure is necessary step towards a better understanding of this remarkable source.
We anticipate that polarization changes accompanying the amplitude variability (or the absence of) may
well furnish an important clue to the physical processes responsible for the variability, and full polarization
imaging will be undertaken at both frequencies.

References
Kedziora­Chudczer, L. et al. 1996a IAU Circ. 6418, 1
Kedziora­Chudczer, L. et al. 1996b, in preparation.
Quirrenbach et al. 1989, Nature, 337, 442.
Qian et al. 1991, A&A, 241, 15.
Vermeulen, R. C., 1995, Proc. Nat. Acad. Sci. USA, 92, 11385.
jreynold 29­Sep­1996 00:28 [1]
Figure 1. Flux­density curves for PKS 0405--385 at frequencies of (from top to bottom) 8.6GHz,
4.8GHz, 2.4GHz and 1.4GHz over the three­day period 7--9 June 1996. The four­frequency data (circles)
were measured by the AT Compact Array; the triangles at 8.6 and 4.8GHz were measured at Hartebeesthoek.