E. I. Robson

SCUBA: A common-user submillimetre camera operating on the James Clerk Maxwell Telescope

SCUBA, the Submillimetre Common-User Bolometer Array, built by the Royal Observatory Edinburgh for the James Clerk Maxwell Telescope, is the most versatile and powerful of a new generation of submillimetre cameras. It combines a sensitive dual-waveband imaging array with a three-band photometer, and is sky-background-limited by the emission from the Mauna Kea atmosphere at all observing wavelengths from 350 μμto 2 mm. The increased sensitivity and array size mean that SCUBA maps close to 10 000 times faster than its single-pixel predecessor (UKT14). SCUBA is a facility instrument, open to the world community of users, and is provided with a high level of user support. We give an overview of the instrument, describe the observing modes, user interface and performance figures on the telescope, and present a sample of the exciting new results that have revolutionized submillimetre astronomy.

STRUCTURE IN THE e ERIDANI DEBRIS DISK

New submillimeter images have been obtained of the dust disk around the nearby K2 V star e Eridani, with the total data set now spanning 5 yr. These images show the distribution of dusty debris generated by comet collisions, reflecting clearing and perturbations by planets, and may give insights to early conditions in the solar system. The structure seen around e Eri at 850 mm and published in 1998 is confirmed in the new observations, and the same structure is also seen in an image obtained for the first time at 450 mm. The disk is inclined by ≈25 to the sky plane, with emission peaking at 65 AU, a 105 AU radius outer edge, and an inner cavity fainter by a factor of ≈2. The structure within the dust ring suggests perturbations by a planet orbiting at tens of AU, and long-term tracking of these features will constrain its mass and location. A preliminary analysis shows that two clumps and one arc appear to follow the stellar motion (i.e., are not background objects) and have tentative evidence of counterclockwise rotation of ∼1 yr 1 . Within the ring, the mass of colliding comets is estimated at 5–9 M, similar to the primordial Kuiper Belt, and so any inner terrestrial planets may be undergoing an epoch of heavy bombardment. Subject headings: circumstellar matter — planetary systems: formation — stars: individual (e Eridani)

Submillimeter Observations of an Asymmetric Dust Disk around Fomalhaut

New submillimeter images of the cold dust emission around the nearby main-sequence star Fomalhaut are presented. Observations at a wavelength of 450 lm, where the telescope beam size is equivalent to a resolution of 50 AU, reveal that Fomalhaut is encircled by a significantly nonaxisymmetric inclined ring. Smooth axisymmetric models of the ring images suggest the existence of a least one ‘‘ clump ’’ with an estimated flux of about 5% of the total from the disk, thus implying a clump mass of 0.075 lunar masses. At the resolution of the data, this clump could instead be a ring arc. The most plausible explanation is that this feature is produced by dust trapped in a resonance with a large planet. The observed structures around Fomalhaut and other Vega-excess stars qualitatively resemble features seen in numerical simulations with a gas giant perturber. Subject headings: circumstellar matter — planetary systems — stars: individual (Fomalhaut)

Multiwavelength Observations of Markarian 421 During a TeV/X-Ray Flare

A TeV flare from the BL Lac object Mrk 421 was detected in May of 1994 by the Whipple Observatory air Cherenkov experiment during which the flux above 250 GeV increased by nearly an order of magnitude over a 2-day period. Contemporaneous observations by ASCA showed the X-ray flux to be in a very high state. We present these results, combined with the first ever simultaneous or nearly simultaneous observations at GeV gamma-ray, UV, IR, mm, and radio energies for this nearest BL Lac object. While the GeV gamma-ray flux increased slightly, there is little evidence for variability comparable to that seen at TeV and X-ray energies. Other wavelengths show even less variability. This provides important constraints on the emission mechanisms at work. We present the multiwavelength spectrum of this gamma-ray blazar for both quiescent and flaring states and discuss the data in terms of current models of blazar emission.

Submillimeter and millimeter observations of jupiter

We report narrowband photometry of the Jovian disk in 10 passbands covering the range from 0.35 to 3.3 mm wavelength. Absolute calibration was referenced to Mars. The derived brightness temperature spectrum is analyzed in the context of existing contraints on the atmospheric temperature structure and composition from ground-based studies at shorter wavelengths and from various spacecraft measurements. Our results for wavelengths between 0.35 and 0.45 mm suggest that the radiances can be matched by models which include NH3 ice particles which are between 30 and 100 μm in size, regardless of the scale height characterizing the cloud. It is difficult however, to model the relatively cool observations longward of 0.7 mm unless additional absorbers are assumed in the atmosphere or a different NH3 lineshape is assumed. If the absolute calibration scale were increased by 5%, the results would be fit by a clear atmosphere (or a small particle cloud) model, with no need to invoke additional absorption in the Jovian atmosphere.

Submillimeter and millimeter observations of Uranus and Neptune

We have made narrowband photometric measurements of Uranus and Neptune covering the wavelength range from 0.35 to 3.3 mm. The observations provide accurate comparative radiometry of these planets. Absolute calibration was referenced to Mars, and to Jupiter as a secondary standard. The results establish Uranus and Neptune as reliable secondary calibrators in their own right. We have combined our observations with other measurements made in the period 1978 through 1984 in the spectral range of 17 μm through 3 mm to form models for atmospheric temperature structure in the vertical range from 100 mbar to 8 bar. The simplest models imply that the tropospheres of both planets are consistent with “frozen” equilibrium H2 and a mixing ratio of CH4 of about 2% by volume in the deep atmosphere. There is some evidence in the Uranus data which implies the presence of discrete spectral lines. These could be due to CH4 pure rotational or dimer transitions or to minor constituents such as CO, which remain uncondensed even at the cold temperatures in the atmosphere of Uranus.

Dust, Gas, and the Evolutionary Status of the Radio Galaxy 8C 1435+635 at z = 4.25

We present the results of new rest-frame far-IR observations of the z = 4.25 radio galaxy 8C 1435+635, which not only confirm that it contains an enormous quantity of dust (as first inferred from its millimeter-wave detection by Ivison in 1995), but also allow the first meaningful constraints to be placed on the mass of this dust and associated gas. The new measurements consist of (1) clear detections of submillimeter continuum emission at λobs = 450 and 850 μm obtained with the new submillimeter bolometer array, SCUBA, on the James Clerk Maxwell Telescope, (2) continuum upper limits at λobs = 350, 750, and 175 μm obtained with SCUBA and the PHT far-IR camera aboard the Infrared Space Observatory, and (3) a sensitive upper limit on the CO (4-3) line flux obtained with the IRAM 30 m Millimeter Radio Telescope. The resulting rest-frame 33-238 μm continuum coverage allows us to deduce that 2 × 108 M☉ of dust at a temperature of 40 ± 5 K is responsible for the observed millimeter/submillimeter emission. Using our CO upper limit, which constrains MH2/Md to less than 950, we go on to calculate robust limits on the total gas reserves (H2 + H I), which are thereby constrained to between 4 × 1010 and 1.2 × 1012 M☉. The submillimeter properties of 8C 1435+635 are thus strikingly similar to those of the z = 3.80 radio galaxy 4C 41.17, the only other high-redshift galaxy detected to date at submillimeter wavelengths whose properties appear not to be exaggerated by gravitational lensing. The inferred gas masses of both objects are sufficiently large to suggest that the formative starbursts of massive elliptical galaxies are still in progress at z simeq 4. Observations of complete samples of radio galaxies spanning a range of redshifts and radio luminosities will be required to determine whether the spectacular far-IR properties of 8C 1435+635 and 4C 41.17 are primarily due to their extreme redshifts or to their extreme radio luminosities.

Multifrequency observations of blazars. III - The spectral shape of the radio to X-ray continuum

Multifrequency, quasi-simultaneous spectra for a sample of 11 blazars are presented. The spectral shape of the violently variable millimeter to ultraviolet flux is consistent with emission from a very compact single component which becomes self-absorbed at wavelengths longer than about 3 mm. The centimeter emission can be attributed to a separate, more slowly varying component. Three out of four optically violent variable quasars also exhibit evidence of a UV excess component. Values for the size of the flaring regions of 0.001-0.1 pc and magnetic fields of order 1 G are deduced. It is found that photon energy densities may dominate over magnetic field energy densities, in which case inverse Compton scattering may be the dominant energy loss mechanism in flaring components.

Multifrequency observations of blazars. IV - The variability of the radio to ultraviolet continuum

The results of analysis of the variability properties of the high-frequency radio to UV continua of a sample of nine blazars are presented. The IR and optical variations of the three sources for which detailed comparisons can be made between the light curves obtained at the various wavebands are found to be well correlated. A strong correlation is observed beween the near-IR flux levels and the near-IR spectral slopes of the BL Lac objects, in the sense that the spectra are steeper when the sources are fainter. A similar correlation is observed to hold in optically violent variable (OVV) quasars during large-amplitude flare events; in general, however, the spectra of the OVV quasars exhibit no significant correlation between flux level and spectral slope. An additional nonvariable 1-5 micron component may be present in the near-IR spectra of OVV quasars.

EGRET observations of the 1993 March gamma-ray flare from PKS 0528+134

EGRET observation of PKS 0528 + 134 during the 1993 March flare in gamma-rays is presented here. During the flare gamma-rays were detected at a level approximately three times greater than the observed intensity in earlier observations. Spectra of the source during the flare, as well as during the nonflaring state are presented. The flux history of PKS 0528 + 134 as observed from the start of the Compton Gamma Ray Observatory (CGRO) mission up to the end of Phase 3 (1993 October) is given. Multiwavelength observations of PKS 0528 + 134 during the flare are also presented. A detailed relativistic SSC jet model agrees well with the 1993 March Multiwavelength spectrum; the data, however, are insufficient to discriminate between this and other emission models.