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THE ASTROPHYSICAL JOURNAL, 467 : L37–L40, 1996 August 10 q 1996. The American Astronomical Society. All rights reserved. Printed in U.S.A.

8.35 AND 14.35 GHz CONTINUUM OBSERVATIONS OF COMET HYAKUTAKE C/1996 B2 ANTHONY H. MINTER

AND

GLEN LANGSTON

National Radio Astronomy Observatory, Green Bank, WV 24944 Received 1996 April 26; accepted 1996 May 30

ABSTRACT We present radio continuum observations of comet Hyakutake C/1996 B2 at 8.35 and 14.35 GHz with the NRAO OVLBI Earth Station at Green Bank. Three sets of observations were made from 22;50 UTC 1996 March 27 to 19;09 UTC 1996 March 28, from 18;30 to 20;30 UTC on 1996 April 17, and from 15;40 to 20;30 UTC on 1996 April 18. No radio continuum emission from comet Hyakutake C/1996 B2 was detected during any of these observations. The lack of radio continuum emission from comet Hyakutake C/1996 B2 at 8.35 and 14.35 GHz allows us to constrain the radio flux in the X-ray– emitting region of comet Hyakutake C/1996 B2 to be S8.35 GHz # 5.3 3 10 227 W m 22 Hz 21 and S14.35 GHz # 2.8 3 10 227 W m 22 Hz 21 . The angular size of the NRAO OVLBI Earth Station’s primary beam is well matched to the size of the region of the diffuse X-ray emission from comet Hyakutake C/1996 B2. Subject headings: comets: individual: (Hyakutake C/1996 B2) — radio continuum: solar system comet Hyakutake C/1996 B2 and to observe the comet once an orbit file for the comet was obtained. The NRAO OVLBI Earth Station is an off-axis Cassegrain telescope capable of observing simultaneously at 8.35 and 14.35 GHz. The X-band (8.35 GHz) receiver is located at the secondary focus, while the Ku-band receiver is located off the subreflector to vertex axis. A frequency selective surface (FSS) is located in front of the X-band receiver. The FSS allows the X-band signal to pass through the FSS into the X-band receiver, while the Ku-band signal is reflected by the FSS. The Ku-band signal reflected from the FSS is then reflected off of an ellipsoid at the vertex into the Ku-band receiver. Both the X- and Ku-band receivers are cooled cryogenic receivers with a 500 MHz bandwidth and dual circular polarized feeds. Typical system temperatures for the NRAO OVLBI Earth Station at X-band are 45 K, while they are 55 K at Ku-band. These receivers are identical to the VLBA X- and Ku-band receivers. The resolution of the NRAO OVLBI Earth Station at 8.35 GHz is 1109 and is 179 at 14.35 GHz.

1. INTRODUCTION

Comet Hyakutake C/1996 B2 provided a unique opportunity for radio continuum observations with the NRAO1 OVLBI Earth Station at Green Bank. Comet Hyakutake C/1996 B2 approached the Earth to within 10.1 AU and was expected to be a very active comet. These conditions led us to believe that it may be possible to observe radio continuum emission originating from either thermal emission from the coma and the nucleus of the comet or from bremsstrahlung emanating from the interaction of the comet’s coma with the solar wind. The NRAO OVLBI Earth Station was designed to track orbiting VLBI radio telescope satellites, which means that the NRAO OVLBI Earth Station can easily follow a comet’s motion across the sky without modification of any software once the comet’s orbit is known. The nearness of comet Hyakutake C/1996 B2 and the capabilities of the NRAO OVLBI Earth Station led us to observe the comet from 22;50 UTC 1996 March 27 to 19;09 UTC 1996 March 28, from 18;30 to 20;30 UTC on 1996 April 17, and from 15;40 to 20;30 UTC on 1996 April 18. In § 2 we briefly describe the NRAO OVLBI Earth Station. The observations and data reduction are described in § 3, while the results and conclusions are presented in § 4.

3. OBSERVATIONS AND DATA REDUCTION

We observed comet Hyakutake C/1996 B2 from 22;50 UTC 1996 March 27 to 19;09 UTC 1996 March 28, from 18;30 to 20;30 UTC on 1996 April 17, and from 15;40 to 20;30 UTC on 1996 April 18 with the NRAO OVLBI Earth Station. An orbit prediction file for the comet was obtained (M. Ryne 1996, private communication) so that the comet could be tracked as it moved across the sky without implementing any new software at the NRAO OVLBI Earth Station. “On-off” pointing observations of the comet were carried out such that an “eight-point star” pattern centered on the comet with a radius of 459 was mapped out. Each pointing scan lasted a total of 2 minutes, integrating each point in the scan for 1/8 s. The orientation of the star pattern was fixed in altitude-azimuth coordinates so that during the course of the observations a complete map of the sky within 459 the comet was produced. Before saving the data for a pointing scan, the data were median filtered to remove the observed emission due to the Earth’s atmosphere, the Galactic background radiation and the 3 K background radiation. This median filtering was

2. THE NRAO OVLBI EARTH STATION

The NRAO OVLBI Earth Station is a 13.7 m (45 ft) antenna located in Green Bank, WV, and is one of four NASA tracking stations dedicated to the support of very long baseline interferometry satellites. The NRAO OVLBI Earth Station is designed to transmit a reference maser signal at either X band or Ku band to the VLBI satellite and to also receive the downlink data from the satellite in the same two bands. The antenna is designed to track the satellite as it moves across the sky, deriving its pointing commands from an “orbit file” describing the orbit of the satellite. The antenna is also capable of performing radio astronomical observations. This capability allowed the NRAO OVLBI Earth Station to track 1 The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

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performed with a time interval of 15 s. Following the filtering, each 1/8 s integration was then saved in near real time in a FITS table along with the azimuth, elevation, local sidereal time, and date of the observation. After the completion of the observations, the data were median filtered with a spatial resolution of 309 and were then converted into “body-centered” coordinates. The data were also flagged to remove the effects of interference. The body-centered coordinate system that is used is defined in the following manner: the center of the coordinate system is defined as the position of the comet, with the direction of the comet’s motion defining the equator. The data were then saved in a FITS file as single dish data, which could then be processed by the Astronomical Image Processing System (AIPS). In AIPS the data were converted into a map using the task SDGRD. In order to test the observing procedures and the data reduction scheme, we observed Saturn on 1996 April 12 with the NRAO OVLBI Earth Station. The observations of Saturn lasted for 50 minutes and were reduced in the same manner as the observations of comet Hyakutake C/1996 B2. We plot the resultant images of Saturn in Figure 1, with the 8.35 GHz image being shown in Figure 1a and the 14.35 GHz image shown in Figure 1b. Figure 1c shows the 14.35 GHz image of comet Hyakutake C/1996 B2 for the observation performed from 22;50 UTC 1996 March 27 until 19;09 UTC 1996 March 28. It can be easily seen from the observations of Saturn that the conversion from altitude-azimuth to “body-centered” coordinates works properly. The direction of the object’s motion is horizontally toward the right in the images in Figure 1. The eight-point star pattern is also clearly visible in the observa-

Vol. 467 TABLE 1

OBSERVED 3 s DETECTION LIMITS FOR OBSERVATIONS COMET HYAKUTAKE C/1996 B2

OF

Begin UTC

End UTC

^D& (AU)

GHz T 38.35 s (mK)

22;50 Mar 27 . . . . . . . . . . . 18;30 Apr 17 . . . . . . . . . . . . 15;40 Apr 18 . . . . . . . . . . . .

19;09 Mar 28 22;30 Apr 17 20;30 Apr 18

0.14 0.78 0.81

3.0 H 0.4 6H1 4H1

GHz T 314.35 s (mK)

2.4 H 0.4 4H1 3H1

tions of Saturn. Saturn was found to have a brightness temperature of 19 mK at 8.35 GHz and 46 mK at 14.35 GHz. For the NRAO OVLBI Earth Station, 1 mK 1 57 mJy at 8.35 GHz and 1 mK 1 77 mJy at 14.35 GHz. 4. RESULTS AND CONCLUSIONS

No radio continuum emission from comet Hyakutake C/1996 B2 was detected above the 3 s rms level during the three sets of observations with the NRAO OVLBI Earth Station. In Table 1 we show the 3 s detection limits for each frequency during each observation. In columns (1) and (2) we list the starting and ending times for each observation. The average geocentric distance of the comet during the observations is listed in column (3). In column (4) we give the 3 s detection limit at 8.35 GHz, and the 3 s detection limit at 14.35 GHz is shown in column (5). The 3 s detection limits were determined from the measured rms in each of the resulting maps. From the 3 s detection limits, we can place limits on various properties of comet Hyakutake C/1996 B2. For a given region

FIG. 1.—Observations of Saturn with the NRAO OVLBI Earth Station on 1996 April 12. (a) 8.35 GHz observations; (b) 14.35 GHz observations. Saturn was found to have a brightness temperature of 19 mK at 8.35 GHz and 46 mK at 14.35 GHz. (c) 14.35 GHz image of comet Hyakutake C/1996 B2 for the observation performed from 22;50 UTC 1996 March 27 until 19;09 UTC 1996 March 28. The position of the comet’s nucleus is marked with a cross. The approximate region of the X-ray emission is outlined by the ellipse (Lisse et al. 1996). The direction of the comet’s motion (horizontally to the right) and the direction toward the sun (toward the upper right) are shown by the arrows. The gray-scale flux range of each image in is shown in millikelvins at the top of each image.

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FIG. 2.—Limits of bremsstrahlung emission temperature vs. optical depth for the region of observed X-ray emission. The shaded region in the figure is excluded by our observations.

of thermal emission with an angular diameter f, we constrain the temperature (T ) in the emitting region to be T3 s $ T~1 2 e 2t!

SD f u

2

,

(1)

where T3 s is the 3 s detection limit in units of kelvins, t is the optical depth of the emitting region, and u is the diameter of the NRAO OVLBI Earth Station beam. Using equation (1) for the results from the March 27–28 observations of comet Hyakutake C/1996 B2, we find T~1 2 e 2t! # 0.0037 K .

(2)

The March 27–28 observations of comet Hyakutake C/1996 B2 occurred simultaneously with ROSAT observations that detected X-ray emission from the comet’s coma (Lisse et al. 1996; Lisse 1996). The region of the X-ray emission was found to be of the order of 49 3 89 (Lisse et al. 1996), filling 157% of the NRAO OVLBI Earth Station beam at 8.35 GHz and 180% of the beam at 14.35 GHz. The NRAO OVLBI Earth Station was thus well suited to observe any radio continuum emission from the X-ray– emitting region, since the entire X-ray– emitting region filled a large fraction of the antenna’s beam. We are thus able to put the following 3 s limits on the total radio continuum flux from the X-ray– emitting region: S8.35 GHz # 5.3 3 10 227 W m 22 Hz 21 , S14.35 GHz # 2.8 3 10 227 W m 22 Hz 21 .

(3)

For thermal blackbody emission (t .. 1) arising from the region of the X-ray emission, we limit the blackbody temperature to be T , 3.7 mK. Thermal blackbody emission from the X-ray– emitting region is thus effectively ruled out by our observations. If the X-ray emission were to arise from the interaction of the comet’s coma with the solar wind, then we might expect the X-ray emission to arise from thermal bremsstrahlung with t , 1. Since the X-rays are seen as a diffuse source (Lisse et al. 1996) with an angular size equivalent to half the size of our beamwidth or larger, we can put strong limits on the temperature of the bremsstrahlung– emitting region with an optical depth t. For example, we limit the temperature of the X-ray– emitting region to be T , 105 K if it arises from bremsstrahlung with an optical depth of t 1 3.7 3 10 28 . In Figure 2 we plot the allowed temperature versus optical depth from our observations for thermal radio continuum emission arising from bremsstrahlung. The shaded area in Figure 2 is the region excluded by our observations. From Figure 2 we see that a small optical depth is needed for realistic temperatures (T 1 104 – 6 K) that should occur if the interaction of the comet’s coma with the solar wind generated the X-rays.

We thank Mark Ryne for kindly providing the comet and planet orbit prediction files.

REFERENCES Lisse, C. 1996, IAU Circ. 6350

Lisse, C., Mumma, M., Petre, R., Dennerl, K., Schmitt, J., Englhauser, J., & Truemper, J. 1996, IAU Circ. 6373