VSOP monitoring of the quasar 1928+738

Adv. Space Res. Vol. 26, No. 4, pp. 665-668,2000

0 2000 COSPAR. Published by Elsevier Science Ltd. AII rights reserved

Pergamon

Printed in Great Britain 0273-1177/00$20.00 + 0.00

www.eIsevier.nI/Iocate/asr

VSOP D.W.

PII: SO273-1177(99)01186-2

MONITORING

Murphy’,

S.J. Tingay’,

R.A. Preston’,

J.E. Conway4, H. Hirabayashi5,

’ Jet Propulsion

Laboratory,

2 Dpto. Astronomia Valencia,

OF THE QUASAR

4800

y Astrofisica,

H. Kobayashi5,

D.L. Meierl,

J.C. Guirado2,

A. Polatidis3t4,

and Y. Murata5

Oak Grove Drive, Pasadena, Universidad

1928+738

de Valencia,

CA 91101,

Dr. Molines

USA

50, E-46100

Burjassot

Spain

3 Joint Institute for VLBI in Europe, Postbus 2, 7990 AA, Dwingeloo, Netherlands 4 Onsala Space Observatory, Chalmers Technical University, Onsala, S-43992, Sweden 5 Institute

of Space and Astronautical

Science,

3-l-l

Yoshinodai,

Kanagawa

229, Japan

ABSTRACT In the paper we describe the first results from a campaign to monitor the relatively low redshift (z=O.3) circumpolar superluminal quasar 1928+738 with VSOP. The three epochs of data that we have analyzed show that there have been substantial structural changes in this source near the core 0 2000 COSPAR. Published by Elsevier Science Ltd. on the time-scale of a few months.

INTRODUCTION One limitation of the VSOP mission is that several famous superluminal sources such as 3C273 cannot be monitored with good (u,v) coverage throughout the lifetime of the VSOP mission at regular intervals that are spaced closely enough to follow the evolution in the fine-scale source structure. The reason for this is that HALCA spacecraft cannot observe sources outside certain restricted ranges of sun angles, where the sun angle is the time variable angle between the source and the sun. However, sources that lie within 10” of the ecliptic poles can be observed throughout the year and observations are not restricted to narrow temporal windows. Furthermore, the best ground-based (u,v) coverages are obtained for circumpolar sources and consequently these will be the sources for which the maximum amount of space VLBI data will be obtained with a given ground array. During the first A0 period we began a monitoring campaign at 5 GHz on the relatively low redshift (z=O.3) superluminal quasar 1928+738 which is both a circumpolar source and lies 10” away from the ecliptic pole. In this paper we present images for our first three epochs of observations. For cosmological calculations we assume Ho = 100 km/s/Mpc and qs = i.

SOURCE

OVERVIEW

1928+738 is in the S5 polar cap sample and has been well studied both on the arcsecond-scale (Johnston et al. 1987, Hummel et al. 1992, and Murphy et al. 1993) and mas-scale (Hummel et al. 1992 and Ros et aI 1998). 1928+738 is quite unusual among core-dominated in quasars in that it

665

666

D. W. Murphy et al.

has a lot of extended that

goes from

and

proper

range

motions

0.29-0.37

Hummel

emission have

mas/yr

been

linear

expansion

et al.

sinusoidal plane

along

this short

within

that

relativistic (KH)

is then

jet model

instabilities

1928+738

angle

components

hole (MBBH)

by geodetic

extremely

is to assume

which

short

that

implies

(-10

the observed

by the orbital

wiggles

motion

system

(Roos

!). A more

orbital

Indeed,

this

work, for the

of this sinusoid,

in the

et al.

black

of the quasar

1993).

A period

hole as the implied scenario

An alternative

in the jet are caused

of the MBBH

1928+738

upon

is responsible

realistic

period.

lie in the

was not consistent

Expanding

of the primary

years

that

in the rest frame

jet model

precession

with the binary

values

The phase

a period

relativistic

jet

c. The 22 GHz work of

in 1928+738

was proposed.

black

of a ballistic

with

3.4-4.1

one-sided

on the mas-scale

(PA) for all components.

jet motion

binary

curved

well studied

in the range

at 22 GHz over a 5 year period.

is associated

also driven

VSOP

helical

by M 0.28 mas yr-‘,

period

also been

distinct

speeds

a fixed position

to be caused

lifetime

the observed

source

of the VLBI components

a massive

the framework

is unlikely

gravitational

that

line observed

of the sky, varies

of 2.9 years

This

in several

the motion

for which

1993 proposed

jet ridge

lobe.

leads to apparent

that

was one of the first sources Roos

sides of the core with a very prominent

detected

which

et al. 1992 showed

with simple

on both

the core to the southern

is to assume to the ballistic

by Kelvin-Helmholtz

system.

IMAGES

Figure 1 shows 5 GHz images of the core region of our first three epochs of data along with their respective (u?v) coverages. The (u,v) data were fringe-fit and amplitude calibrated using the AIPS package. The 1024 by 1024 images were made using the Caltech Difmap package with a cell-size of 0.05 mas and the uvweight but there was no weighting are very distinct structural

parameter had the value of 2,0. That is, uniform weighting was applied based on the noise level attached to each visibility. As can be seen, there changes in the core region. The core is located in the northern most part

of the image and is not the brightest feature on the images. Examining our first epoch of data, we see that the initially the jet has a position angle (PA) of about 150”. However, at a distance of roughly 2.3 mas (6.3 pc) from the core the jet goes through

a apparent

bend

of 93”. This

behavior

has not

been in seen in other other observations of this well-studied source (Hummel et al. 1992, and Ros et al 1998). One might question the validity of VSOP images given the large size of the holes in the (u,u) plane.

However,

we are monitoring

with the CMVA.

A 43 GHz image

right

in jet.I

index

angle

bend

43 GHz image

both

taken

also enables

at 43 GHz with

on 07 July

the VLBA

and

86 GHz

1997 also shows the pronounced

the core to be identified

based

on spectral

arguments.

However, 0.32 years

in the second there

one interprets than

This

this source

made from data

the speed

epoch

of data

has been

a substantial

this change

an apparent

the jet bends change

by only 41” 2.3 mas from the core.

in the jet position

speed of about

at this point.

30 c can be derived.

Thus,

Depending

This speed is much

in only on how higher

components in this source which have apparent speeds of about 4 c (Hummel but the situation is made even more intriguing et al. 1992, Ros et al 1998)). Th is is all very unusual by the third epoch of data which shows that the structure following the bend at 2.3 mas has a more complex morphology which is somewhat intermediate between the the first and third epochs of data. Interpreted It should

of other

naively, be pointed

this might

provide

out that

the images

evidence presented

for superluminal

contraction

here are preliminary

in this source.

and more work is needed

to

produce them in their final form. The ground array that observed with HALCA for all 3 epochs was primarily the VLBA. However the VLBA was augmented by Effelsberg for all three epochs and by the Usuda 64m and Noto for the third epoch. The observation length was 12, 8, and 15 hours for

VSOP Monitoring of 1928+738

667

the three epochs respectively.

Fig. 1. 5 GHz images (left) and (u,v)-coverages (right) f rom the 3 epochs of 1928+738 monitoring. Contours for each image are at -1.5, 1.5, 3, 6, 12, ,624, 48, 96 % of the peak flux density. Top: 22 August 1997 image and (u,v)-coverage (peak flux density = 0.908 mJy/beam, beam = 0.588 Middle: 16 December 1997 image and (u,v)-coverage (peak flux density = 0.488 Bottom:

mJy/b earn, beam = 0.474 29 April 1998 image and (u,v)-coverage

(peak flux density = 0.508

mJy/beam,

beam = 0.497

x 0.373

(mas) at -52.90)

x 0.223

(mas) at -72.P)

x 0.245

(mas) at 52.7”)

668

D. W. Murphy et al.

SUMMARY We have imaged observed image

1938+378

structural

is confirmed

changes

at 3 epochs

that

of a type

never

at one epoch

span

a time range

before

by a complementary

6.3 pc from the core is seen to vary

dramatically

of 8.5 months.

seen in this

source.

43 GHz VLBA

The

In that

time,

validity

of our VSOP

observation.

we have

The jet bend

angle

with time.

ACKNOWLEDGMENTS We gratefully

acknowledge

Astronautical

Science

the VSOP

in cooperation

Project, with many

which

is led by the Japanese

organizations

and radio

Institute

telescopes

of Space

around

and

the world.

REFERENCES Hummel, ture,

C. A., et al., The Jets of Quasar

A&A,

Johnston, Murphy, MNRAS,

K.J.,

et al., A Superluminal

E.,

Universitat

Source

D. W., et al., VLA Ob servations

- Superluminal

Motion

and Large

Scale Struc-

with

Large

of a Complete

Scale Structure, Sample

ApJL,

of Core-Dominated

313, L85 (1987). Radio

Sources,

264, 298 (1993).

ROOS, N., et al., A Massive Ros,

1928+738

257, 489 (1992).

Astrometria de Valencia

Binary

Diferencial (1997).

Black

Hole in 1928+738,

de Precision

con VLBI

AJ, 409, 130 (1993). en el Triangulo

de Draco,

PhD

thesis,