VSOP Space VLBI images of 1413+135 at C Band

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Figure 1:

Plot of the X and S band flux densities of 1413+135 versus time. The nearly constant line at 0.8 Jy is the S band Flux density, and the variable line is the X band flux density. Note there is significant variation with in a 3 month interval.


We present VLBI observations of the remarkably variable radio source 1413+135. (See Figure 1). We began our studies of 1413+135 to discover why is 1413+135 so variable at high frequencies, yet nearly constant at low frequencies. Answer: The compact core of 1413+135 is faint at lower frequencies, while at higher frequencies the lobes are. At frequencies lower than 3 GHz, the parsec scale jets of 1413+135 produce most of the emission. The outer regions of the jets are not rapidly variable in intensity, while the core is highly variable.

Recently obserservations of 1413+135 were make using ISAS's VSOP satellite and NRAO's VLBA. A schmatic diagram of the observing system is shown in Figure 2. These observations utilized the NRAO Green Bank Tracking station shown in Figure 3. This page presents work towards progress to understand the region near the central engine of these sources.

Figure 2: :Figure 3

Figure 2: Graphical representation of the VSOP Satellite co-observing with Ground Antennas. The satellite receives a maser based reference tone from the tracking station, and transmits the sampled astronomical data to the tracking station for recording.

Figure 3: NRAO Green Bank Space VLBI Tracking Station. Designed by NRAO for NASA to support the Japanese ISAS VSOP and Russian RadioAstron missions.

Figure 4: : Figure 5

Figure 4: Hubble Space Telescope Archive Image of the host galaxy containing 1413+135. The HST image was taken with filter F555W and has pixel size is 0.044 x 0.044 arc seconds. Notice the "spiral" shape of the galaxy. The vertical strip is a bad column on the HST Wide field Planetary Camera. There is a dust lane running across the center of the galaxy. This image has been processed with a 0.2 x 0.2 arc second gaussian convolving function to show the galaxy more clearly.

Figure 5: Image of 1413+135 at C band based only on the VLBA observations. The peak flux density is 0.765 Jy/Beam for a synthesized beam 0.00361 x 0.00191 arc seconds with position angle 16.79 degrees. The entire VLBI image of 1413+135 is approximately the size of a single pixel of the HST image. The image is available in FITS format.


The radio source 1413+135 is remarkable for a number of reasons; 1) it is 18 times more variable at 3.6 cm (X band) wavelengths than at 13 cm (S band), 2) it is one of the few (5) sources where radio absorption lines of (Carilli et al. 1992), CO, HCN and other molecules (Wiklind and Combes 1994) have been detected in absorption, and 3) it appears to be a young radio source ( years, Perlman et al. 1996).

Radio source 1413+135 is one of 5 sources in our sample. The properties of these 5 sources are summarized in Table 1.

The radio source 1413+135 is compact, < 0.1 arc seconds in extent, and is a VLA calibration source. The source was monitored daily by the Green Bank Interferometer (GBI), between October 1989 and June 1994. It was the remarkable GBI (Waltman et al. 1992, Figure 1) data which brought our attention to this source. Perlman et al. 1996 have published excellent images of 1413+135. These images did not resolve the central component of 1413+135, but did show the spectral properties of the outer regions which explain the nearly constant brightness of the 1413+135 at lower frequencies.

The high time variability of 1413+135 at X band, a 50 % increase in 0.3 years, indicates it has components smaller than 0.08 pc. This corresponds to an angular size of 35 milli-arc seconds, assuming Hubble's Constant = 100 km/sec/Mpc and q_O = 0.5.

The optical host of 1413+135 appears to be a spiral galaxy, as shown by the HST image in Figure 4. The VLBA image of 1413+135 at C band is shown in Figure 5. The VLBA plus VSOP image is shown in Figure 6. The VLBA + VSOP data resolve the more extended ``lobe'' components, and show the jet components close to the core. Perlman et al. 1996 find no evidence for motion of the components of 1413+135 on scales larger than 2 mas. Their measurements of the spectra of the jets, along with limits on the temperature of the gas surrounding 1413+135, are consistent with a young or recurrent model for the source.

The VLBA image in figure 5 is approximately the size of one pixel of the HST image in Figure 4. The VSOP+VLBA image has approximately 3 times the resolution of the VLBA only image, as expected. The more extended components of 1413+135 visible in the VLBA only image are resolved out in the VSOP image.

Still higher resolution and more frequent observations of 1413+135 are needed to explain the high core variability. Planed future Space VLBI missions will provide sufficient sensitivity and resolution to resolve the core of 1413+135. GBI monitoring of 1413+135 has resumed, and the data suggest that 1413+135 appears to have begun a quiescent period.

Figure 6: HALCA plus VLBA image of 1413+135 at C band. Notice the two components of the jet visible to the south west, which are not resolved in the VLBA only image. The synthesized beam for this image is 0.00205 x 0.00047 arc seconds with position angle -26.31 degrees. The entire image above is smaller than a single pixel of the HST image. The image is available in FITS format.

The Space VLBI image has a factor of 3 angular resolution over the ground based image, as expected. The ground based image shows the more extended jet emission to the north east. This emission is barely visible in the Space VLBI image, which is more sensative to the compact emission near the core of 1413+135.

Sources in High Frequency Variable Sample

The table 1, below, lists the radio sources which are part of the High Frequency Variable sample from the Green Bank Interferometer monitoring experiment.

IAU Name Survey Name Source Type Red Shift Note
0201+113 OD+101 AGN 3.560
0923+392 4C39.25 QSO 0.699 Alberdi 1993
1116+128 4C12.8 AGN 2.118
1413+135 OQ+122 BL Lac 0.247 Highly Variable
2344+094 OZ+073.5 Seyfert I 0.673

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Glen Langston
February 11, 1998