Analyses of the Light Variations of OJ 287, 0716+714 and BL Lacertae

OJ 287 is a BL Lac object which exhibits intense activities of low peak-frequencies. Its energy spectrum in low frequency band is quite similar with those of two other TeV BL Lac objects (i.e., 0716+714 and BL Lacertae). However, the Cerenkov telescope did not detect its TeV rays. By using the observational data of these three heavenly bodies and comparing the discrepan- cies of their minimal periods of light variations and delays at 22 GHz, 37 GHz and B-waveband, we have further investigated the possible reason why the TeV gamma-rays of OJ 287 have not been observed. The results of analyses are as fol- lows. (1) For the minimal periods of light variations, the periods of OJ 287 at 37 GHZ and B-waveband are short. At 22 GHz the results of OJ 287 and 0716+714 are comparable, but the period of OJ 287 is much shorter in comparison with that of BL Lacertae, and this shows that its activity is more intense. However, the TeV gamma-rays of OJ 287 have not been detected, which implies that the radiation of OJ 287 in TeV waveband may have no connection with the minimal periods of light variations in these three low-energy wavebands. (2) In respect of delays, the delay of OJ 287 in the B waveband with respect to 37 GHz is longer than that of 0716+714, but shorter than that of BL Lacertae. Its delay at 37 GHz in respect to 22 GHz is shorter than that of 0716+714. Meanwhile, the delay of BL Lacertae at 37 GHz in respect to 22 GHz is negative, which implies that 22 GHz precedes 37 GHz. Via the comparison and analysis of delays, no obvious differences between OJ 287 and 0716+714 as well as BL Lacertae have been found. On the side of energy spectra, it is quite possible that due to the steep energy spectrum of OJ 287 in TeV waveband, the Cerenkov telescope has not detected the gamma radiation of OJ 287. However, nowadays it is still not clear whether the steep energy spectrum in TeV energy range has some influence on the light variations in low energy realm.     

An Observational Study of the Strong Single Pulses of PSR J0034-0721

Using the 25m radio telescope of the Urumqi Observatory, the strong single pulses of the pulsar PSR J0034-0721 were observed at 1.54 GHz on 6th Aug. 2007. With the technique of single-pulse detection, 116 single pulses with the signal-to-noise ratios of R_SN≥5 were detected from the observed data of 1 h. At 1.54 GHz, the signal-to-noise ratios of the detected single pulses are in the range from 5 to 10.5, and the peak flux densities of these pulses are approximately 14∼29 times that of the average pulse (AP), much less than the ratios between the intensities of typical giant pulses and the intensity of AP. The cumulative distribution of the intensities of these pulses is basically a powerlaw distribution with the spectral index α = −4.3 ± 0.4. The detection rates for the pulses of R_SN≥5 and R_SN≥10 are 3% and 0.08%, respectively. For these pulses, the half-peak width W₅₀ ranges from 1.6 ms to 8 ms, 3.9 ms in average. The phases of the vast majority of the strong single pulses are concentrated around the peak position of AP, but 2 strong pulses of R_SN≥8.5 are detected at the phases about 33 ms earlier than the phase of the AP peak. This implies that there probably exist two emission regions of strong pulses, and this is consistent with the previously observed results at 40 MHz and 111 MHz, except that at 1.54 GHz the profile of AP exhibits only one component.     

A Research on the Characteristics of Light Variation of Blazar 3C 273 in 2-10 keV

From the original observed data of RXTE (Rossi X-ray timing explorer), the data of 3C 273 in the X-ray band of 2-10 keV from February 2, 1996 (MJD 50115) to August 27, 2007 (MJD 54339) are analyzed. The photon spectral indexes and corresponding fluxes of the observational data from 1010 observations are obtained in total. The average time of each observation is 1666.76 seconds. By analyzing the spectral variation and behavior of the light variation of 3C 273 in the hard X-ray band of 2-10 keV during observational periods, it is found that there was a significant anti-correlation between the photon spectral index Γ and flux lgF_{2∼10 keV} in February 2000, March and April 2003, February, March and July 2004, as well as 2006 and 2007. During the observational periods in 1999 and from 2000 to 2007, there was also a significant anti-correlation between the monthly average behaviors of Γ and lgF_{2∼10 keV}. From the quantitative analysis of the behavior of light variation in the entire observational period, several relatively large light variations are found. The evidence of the existence of the Fe emission line with average width of 93.85±21.49 eV is also found by fitting a part of the spectra. Through analyzing the intensity of the light variation and features of Fe emission line, it is found that 3C 273 has some characteristics of blazars and Seyfert galaxies simultaneously.     

An RFI investigation for setting up a VLBI station below 2.8 GHz in Malaysia

In this paper, we investigated the radio frequency interference (RFI) that future Very Long Baseline Interferometer (VLBI) observations in Malaysia may encounter. Four frequency windows below 2.8 GHz were chosen for this study and their spectra were measured at four sites. The frequency windows are 322-328 MHz, 608-614 MHz, 1660-1660.5 MHz and 1660.5-1668.4 MHz. The measured averaged RFI floor noise levels in these windows are −99.992 (±0.570) dBm, −99.907 (±0.639) dBm, −100.220 (±0.4941) dBm and −100.359 (±0.110) dBm, respectively. We found that only two bands below 2.8 GHz are permitted for the purpose of radio astronomy in Malaysia. They are 608-614 MHz and 1660-1660.5 MHz. The RFI levels in these permissible bands at the best site (Langkawi) were also measured and concluded to be relatively low. Main sources of RFI in these bands in Malaysia were identified. We also reviewed several current VLBI observations in these two bands.     

Optical intra-day variability timescales and black hole mass of the blazars

In this paper, we have used optical intra-day variability archive data to calculate the central black hole masses and Eddington luminosities for nine blazars: 3C 66A, AO 0235+164, S5 0716+714, PKS 0735+178, OJ 287, 1215+303, 1216−010, 1308+326, PKS 1510−089, Mrk 501 and BL Lac using intra-day variability timescales and periodicity (if present). The calculated central black hole mass of these nine blazars using intra-day variability timescales are found to be in the range of 1.22-25.30 × 10⁷ M_⊙ and corresponding Eddington luminosity in the range of 1.58-32.88 × 10⁴⁵ erg s⁻¹. The black hole mass and Eddington luminosity are in the range of 0.32-31.23 × 10⁸ M_⊙ and 1.23-31.20 × 10⁴⁶ erg s⁻¹, respectively when optical Doppler factor is taken into account. The comparison show, our estimated values of black hole mass are consistent with earlier reported values. Periodicity were present in two blazars OJ 287 and 1216−010 which give the central black hole mass of these blazars in the range of 1.32-14.6 × 10⁷ M_⊙ and corresponding Eddington luminosity in the range of 1.60-19.0 × 10⁴⁵ erg s⁻¹, respectively.     

The Black Hole Masses and Radio Characteristics of Radio Quasars

In this paper, we collect the redshift, bolometric luminosity, the full- width at half maximum of the H_β emission line, the monochromatic luminosity at 5100 Å and the radio loudness for the sample of 117 quasars, including 20 radio-quiet quasars (RQQs) and 97 radio-loud quasars (RLQs). With the reverberation mapping method we calculate the black hole mass and Eddington ratio for this sample, as well as the radio luminosity from the total 5 GHz ?ux density. By analyzing the correlations among them, we obtain the following conclusions: (1) The black hole mass has weak correlations with the bolometric luminosity, radio loudness and radio luminosity for the RQQs, and has strong correlations with the bolometric luminosity, radio loudness and radio luminosity for the RLQs; (2) For the RQQs, the bolometric luminosity has weak correlations with the radio luminosity and 5 100 Å monochromatic luminosity, and for the RLQs, the bolometric luminosity has strong correlations with the radio luminosity and 5 100 Å monochromatic luminosity; (3) The RQQs and RLQs differ in the distributions of the black hole mass, emission line width and Eddington ratio. Based on these results, we suggest: the difference of emission line width between RQQs and RLQs is probably caused by the difference of black hole mass; the fundamental difference between RQQs and RLQs is caused by the difference of their intrinsic physical nature; the black hole mass, black hole spin, Eddington ratio, and host galaxy morphology are the important parameters to explain the origin of radio loudness and the double-peaked distribution; and the radio jet is closely related with the accretion rate of disk.     

A revised orbital ephemeris for HAT-P-9b

We present here three transit observations of HAT-P-9b taken on 14 February 2010 and 05 April 2010 UT from the University of Arizona’s 1.55 m Kuiper telescope on Mt. Bigelow. Our two light curves were obtained in the Arizona-I filter for all our observations, and underwent the same reduction process. Both transits occurred approximately 24 min earlier than expected from the ephemeris of Shporer et al. (2009). However, due to the large time span between our observed transits and those of Shporer et al. (2009), a 6.5 s shift downwards in orbital period from the value of Shporer et al. (2009) is sufficient to explain all available transit data. We find a new period of 3.922814 ± 0.000002 days for HAT-P-9b, an order of magnitude more precise than previous measurements, with no evidence for significant nonlinearities in the transit period.     

The size distribution of Jupiter's main ring from Galileo imaging and spectroscopy

Galileo's Solid State Imaging experiment (SSI) obtained 36 visible wavelength images of Jupiter's ring system during the nominal mission (Ockert-Bell et al., 1999, Icarus 138, 188-213) and another 21 during the extended mission. The Near Infrared Mapping Spectrometer (NIMS) recorded an observation of Jupiter's main ring during orbit C3 at wavelengths from 0.7 to 5.2 μm; a second observation was attempted during orbit E4. We analyze the high phase angle NIMS and SSI observations to constrain the size distribution of the main ring's micron-sized dust population. This portion of the population is best constrained at high phase angles, as the light scattering behavior of small dust grains dominates at these geometries and contributions from larger ring particles are negligible. High phase angle images of the main ring obtained by the Voyager spacecraft covered phase angles between 173.8° and 176.9° (Showalter et al., 1987, Icarus 69, 458-498). Galileo images extend this range up to 178.6°. We model the Galileo phase curve and the ring spectra from the C3 NIMS ring observation as the combination of two power law distributions. Our analysis of the main ring phase curve and the NIMS spectra suggests the size distribution of the smallest ring particles is a power law with an index of 2.0±0.3 below a size of ∼15 μm that transitions to a power law with an index of 5.0±1.5 at larger sizes. This combined power law distribution, or “broken power law” distribution, yields a better fit to the NIMS data than do the power law distributions that have previously been fit to the Voyager imaging data (Showalter et al., 1987, Icarus 69, 458-498). The broken power law distribution reconciles the results of Showalter et al. (1987, Icarus 69, 458-498) and McMuldroch et al. (2000, Icarus 146, 1-11), who also analyzed the NIMS data, and can be considered as an obvious extension of a simple power law. This more complex size distribution could indicate that ring particle production rates and/or lifetimes vary with size and may relate to the physical processes that control their evolution. The significant near arm/far arm asymmetry reported elsewhere (see Showalter et al., 1987, Icarus 69, 458-498; Ockert-Bell et al., 1999, Icarus 138, 188-213) persists in the data even after the main ring is isolated in the SSI images. However, the sense of the asymmetry seen in Galileo images differs from that seen in Voyager images. We interpret this asymmetry as a broad-scale, azimuthal brightness variation. No consistent association with the magnetic field of Jupiter has been observed. It is possible that these longitudinal variations may be similar to the random brightness fluctuations observed in Saturn's F ring by Voyager (Smith et al., 1982, Science 215, 504-537) and during the 1995 ring plane crossings (Nicholson et al., 1996, Science 272, 509-515; Bosh and Rivkin, 1996, Science 272, 518-521; Poulet et al., 2000, Icarus 144, 135-148). Stochastic events may thus play a significant role in the evolution of the jovian main ring.     

Polarization Observation Experiments for Radio Flares of Stars at the 6 cm Band

We have performed radio polarization observation experiments of the stars V772 Her and β Per with the Urumqi 25 m radio telescope at the 6 cm waveband, and obtained light curves of the stars after data processing and calibration. A radio ?are from the star V772 Her was detected on 2011 April 13. The degree of linear polarization of this ?are is about 30%, and the polarization angle is about 4°. We have detected also the slowly-varying component of the radio radiation from β Per, as well as a short ?are superposed on it, which has a very weak linear polarization.     

The reflectance spectrum of Titan's surface at the Huygens landing site determined by the descent imager/spectral radiometer

The descent imager/spectral radiometer aboard the Huygens probe successfully acquired images and spectra of the surface of Titan. To counter the effects of haze and atmospheric methane absorption it carried a surface science lamp to illuminate the surface just before landing. We reconstruct the reflectance spectrum of the landing site in the 500-1500 nm range from downward looking visual and infrared spectrometers data that show evidence of lamp light. Our reconstruction is a followup to the analysis by Tomasko et al. [2005. Rain, winds and haze during the Huygens probe's descent to Titan's surface. Nature 438, 765-778], who scaled their result to the ratio of the up- and down flux measured just before landing. Instead, we use the lamp flux from the calibration experiment, and find a significantly higher overall reflectance. We attribute this to a phase angle dependance, possibly representing the opposition surge commonly encountered on solar system bodies. The reconstruction in the visible wavelength range is greatly improved. Here, the reflectance spectrum features a red slope, consistent with the presence of organic material. We confirm the blue slope in the near-IR, featureless apart from a single shallow absorption feature at 1500 nm. We agree with Tomasko et al. that the evidence for water ice is inconclusive. By modeling of absorption bands we find a methane mixing ratio of 4.5±0.5% just above the surface. There is no evidence for the presence of liquid methane, but the data do not rule out a wet soil at a depth of several centimeters.     

In search of water vapor on Jupiter: Laboratory measurements of the microwave properties of water vapor under simulated jovian conditions

Detection and measurement of atmospheric water vapor in the deep jovian atmosphere using microwave radiometry has been discussed extensively by Janssen et al. (Janssen, M.A., Hofstadter, M.D., Gulkis, S., Ingersoll, A.P., Allison, M., Bolton, S.J., Levin, S.M., Kamp, L.W. [2005]. Icarus 173 (2), 447-453.) and de Pater et al. (de Pater, I., Deboer, D., Marley, M., Freedman, R., Young, R. [2005]. Icarus 173 (2), 425-447). The NASA Juno mission will include a six-channel microwave radiometer system (MWR) operating in the 1.3-50 cm wavelength range in order to retrieve water vapor abundances from the microwave signature of Jupiter (see, e.g., Matousek, S. [2005]. The Juno new frontiers mission. Tech. Rep. IAC-05-A3.2.A.04, California Institute of Technology). In order to accurately interpret data from such observations, nearly 2000 laboratory measurements of the microwave opacity of H₂O vapor in a H₂/He atmosphere have been conducted in the 5-21 cm wavelength range (1.4-6 GHz) at pressures from 30 mbars to 101 bars and at temperatures from 330 to 525 K. The mole fraction of H₂O (at maximum pressure) ranged from 0.19% to 3.6% with some additional measurements of pure H₂O. These results have enabled development of the first model for the opacity of gaseous H₂O in a H₂/He atmosphere under jovian conditions developed from actual laboratory data. The new model is based on a terrestrial model of Rosenkranz et al. (Rosenkranz, P.W. [1998]. Radio Science 33, 919-928), with substantial modifications to reflect the effects of jovian conditions. The new model for water vapor opacity dramatically outperforms previous models and will provide reliable results for temperatures from 300 to 525 K, at pressures up to 100 bars and at frequencies up to 6 GHz. These results will significantly reduce the uncertainties in the retrieval of jovian atmospheric water vapor abundances from the microwave radiometric measurements from the upcoming NASA Juno mission, as well as provide a clearer understanding of the role deep atmospheric water vapor may play in the decimeter-wavelength spectrum of Saturn.     

An Analysis of Light Periods of BL Lac Object S5 0716+714 with the MUSIC Algorithm

The multiple signal classi?cation (MUSIC) algorithm is introduced to the estimation of light periods of BL Lac objects. The principle of the MUSIC algorithm is given, together with a testing on its spectral resolution by using a simulative signal. From a lot of literature, we have collected a large number of effective observational data of the BL Lac object S5 0716+714 in the three optical wavebands V, R, and I from 1994 to 2008. The light periods of S5 0716+714 are obtained by means of the MUSIC algorithm and average periodogram algorithm, respectively. It is found that there exist two major periodic components, one is the period of (3.33±0.08) yr, another is the period of (1.24±0.01) yr. The comparison of the performances of periodicity analysis of two algorithms indicates that the MUSIC algorithm has a smaller requirement on the sample length, as well as a good spectral resolution and anti-noise ability, to improve the accuracy of periodicity analysis in the case of short sample length.     

AO 0235+164射电波段宽带谱指数周期性变化的研究

收集了AO 0235+164天体射电4.8 GHz和14.5 GHz波段的光变测量数据,并获得了长期的光变曲线,从光变曲线可以看出其活动是非常剧烈的。利用Jurkevich方法和自相关函数方法分别对AO 0235+164射电波段宽带谱指数进行周期性分析,并对流量和谱指数进行相关性分析,研究结果表明:(1)AO 0235+164天体射电波段4.8 GHz～14.5 GHz对应的宽带谱指数,可能存在5.30年的光变周期,与Liu等人用功率谱法在射电波段发现其流量密度可能存在5.59±0.47年的光变周期基本吻合;(2)宽带谱指数与流量密度之间存在相关性。     

Infrared imaging spectroscopy of Mars: H2O mapping and determination of CO2 isotopic ratios

High-resolution infrared imaging spectroscopy of Mars has been achieved at the NASA Infrared Telescope Facility (IRTF) on June 19-21, 2003, using the Texas Echelon Cross Echelle Spectrograph (TEXES). The areocentric longitude was 206°. Following the detection and mapping of hydrogen peroxide H₂O₂ [Encrenaz et al., 2004. Icarus 170, 424-429], we have derived, using the same data set, a map of the water vapor abundance. The results appear in good overall agreement with the TES results and with the predictions of the Global Circulation Model (GCM) developed at the Laboratory of Dynamical Meteorology (LMD), with a maximum abundance of water vapor of 3±1.5×¹⁰⁻⁴(17±9 pr-μm). We have searched for CH₄ over the martian disk, but were unable to detect it. Our upper limits are consistent with earlier reports on the methane abundance on Mars. Finally, we have obtained new measurements of CO₂ isotopic ratios in Mars. As compared to the terrestrial values, these values are: (¹⁸O/¹⁷O)[M/E] = 1.03 ± 0.09; (¹³C/¹²C)[M/E] = 1.00 ± 0.11. In conclusion, in contrast with the analysis of Krasnopolsky et al. [1996. Icarus 124, 553-568], we conclude that the derived martian isotopic ratios do not show evidence for a departure from their terrestrial values.     

The structure of Titan’s atmosphere from Cassini radio occultations

We present results from the two radio occultations of the Cassini spacecraft by Titan in 2006, which probed mid-southern latitudes. Three of the ingress and egress soundings occurred within a narrow latitude range, 31-34°S near the surface, and the fourth at 52.8°S. Temperature-altitude profiles for all four occultation soundings are presented, and compared with the results of the Voyager 1 radio occultation (Lindal, G.F., Wood, G.E., Hotz, H.B., Sweetnam, D.N., Eshleman, V.R., Tyler, G.L. [1983]. Icarus 53, 348-363), the HASI instrument on the Huygens descent probe (Fulchignoni, M. et al. [2005]. Nature 438, 785-791), and Cassini CIRS results (Flasar, F.M. et al. [2005]. Science 308, 975-978; Achterberg, R.K., Conrath, B.J., Gierasch, P.J., Flasar, F.M., Nixon, C.A. [2008b]. Icarus 194, 263-277). Sources of error in the retrieved temperature-altitude profiles are also discussed, and a major contribution is from spacecraft velocity errors in the reconstructed ephemeris. These can be reduced by using CIRS data at 300 km to make along-track adjustments of the spacecraft timing. The occultation soundings indicate that the temperatures just above the surface at 31-34°S are about 93 K, while that at 53°S is about 1 K colder. At the tropopause, the temperatures at the lower latitudes are all about 70 K, while the 53°S profile is again 1 K colder. The temperature lapse rate in the lowest 2 km for the two ingress (dawn) profiles at 31 and 33°S lie along a dry adiabat except within ∼200 m of the surface, where a small stable inversion occurs. This could be explained by turbulent mixing with low viscosity near the surface. The egress profile near 34°S shows a more complex structure in the lowest 2 km, while the egress profile at 53°S is more stable.     

AO0235＋164射电波段宽带谱指数周期性变化的研究

收集了AO0235＋164天体射电4．8GHz和14．5GHz波段的光变测量数据,并获得了长期的光变曲线,从光变曲线可以看出其活动是非常剧烈的。利用Jurkevieh方法和自相关函数方法分别对AO0235＋164射电波段宽带谱指数进行周期性分析,并对流量和谱指数进行相关性分析,研究结果表明：（1）AO0235＋164天体射电波段4．8GHz-14．5GHz对应的宽带谱指数,可能存在5．30年的光变周期,与Liu等人用功率谱法在射电波段发现其流量密度可能存在5．59±0．47年的光变周期基本吻合;（2）宽带谱指数与流量密度之间存在相关性。     

New insights on the binary Asteroid 121 Hermione

We report on the results of a 6-month photometric study of the main-belt binary C-type Asteroid 121 Hermione, performed during its 2007 opposition. We took advantage of the rare observational opportunity afforded by one of the annual equinoxes of Hermione occurring close to its opposition in June 2007. The equinox provides an edge-on aspect for an Earth-based observer, which is well suited to a thorough study of Hermione’s physical characteristics. The catalog of observations carried out with small telescopes is presented in this work, together with new adaptive optics (AO) imaging obtained between 2005 and 2008 with the Yepun 8-m VLT telescope and the 10-m Keck telescope. The most striking result is confirmation that Hermione is a bifurcated and elongated body, as suggested by Marchis, et al. [Marchis, F., Hestroffer, D., Descamps, P., Berthier, J., Laver, C., de Pater, I., 2005. Icarus 178, 450-464]. A new effective diameter of 187 ± 6 km was calculated from the combination of AO, photometric and thermal observations. The new diameter is some 10% smaller than the hitherto accepted radiometric diameter based on IRAS data. The reason for the discrepancy is that IRAS viewed the system almost pole-on. New thermal observations with the Spitzer Space Telescope agree with the diameter derived from AO and lightcurve observations. On the basis of the new AO astrometric observations of the small 32-km diameter satellite we have refined the orbit solution and derived a new value of the bulk density of Hermione of 1.4 + 0.5/−0.2 g cm⁻³. We infer a macroscopic porosity of ∼33 + 5/−20%.     

Deep Impact, Stardust-NExT and the behavior of Comet 9P/Tempel 1 from 1997 to 2010

We present observational data for Comet 9P/Tempel 1 taken from 1997 through 2010 in an international collaboration in support of the Deep Impact and Stardust-NExT missions. The data were obtained to characterize the nucleus prior to the Deep Impact 2005 encounter, and to enable us to understand the rotation state in order to make a time of arrival adjustment in February 2010 that would allow us to image at least 25% of the nucleus seen by the Deep Impact spacecraft to better than 80 m/pixel, and to image the crater made during the encounter, if possible. In total, ∼500 whole or partial nights were allocated to this project at 14 observatories worldwide, utilizing 25 telescopes. Seventy percent of these nights yielded useful data. The data were used to determine the linear phase coefficient for the comet in the R-band to be 0.045 ± 0.001 mag deg⁻¹ from 1° to 16°. Cometary activity was observed to begin inbound near r ∼ 4.0 AU and the activity ended near r ∼ 4.6 AU as seen from the heliocentric secular light curves, water-sublimation models and from dust dynamical modeling. The light curve exhibits a significant pre- and post-perihelion brightness and activity asymmetry. There was a secular decrease in activity between the 2000 and 2005 perihelion passages of ∼20%. The post-perihelion light curve cannot be easily explained by a simple decrease in solar insolation or observing geometry. CN emission was detected in the comet at 2.43 AU pre-perihelion, and by r = 2.24 AU emission from C₂ and C₃ were evident. In December 2004 the production rate of CN increased from 1.8 × 10²³ mol s⁻¹ to Q_CN = 2.75 × 10²³ mol s⁻¹ in early January 2005 and 9.3 × 10²⁴ mol s⁻¹ on June 6, 2005 at r = 1.53 AU.     

Near-nucleus photometry of comets using archived NEAT data

Though optimized to discover and track fast moving Near-Earth Objects (NEOs), the Near-Earth Asteroid Tracking (NEAT) survey dataset can be mined to obtain information on the comet population observed serendipitously during the asteroid survey. We have completed analysis of over 400 CCD images of comets obtained during the autonomous operations of two 1.2-m telescopes: the first on the summit of Haleakala on the Hawaiian island of Maui and the second on Palomar Mountain in southern California. Photometric calibrations of each frame were derived using background catalog stars and the near-nucleus comet photometry measured. We measured dust production and normalized magnitudes for the coma and nucleus in order to explore cometary activity and comet size-frequency distributions. Our data over an approximately two-year time frame (2001 August-2003 February) include 52 comets: 12 periodic, 19 numbered, and 21 non-periodic, obtained over a wide range of viewing geometries and helio/geocentric distances. Nuclear magnitudes were estimated for a subset of comets observed. We found that for low-activity comets (Afρ<100 cm) our model gave reasonable estimates for nuclear size and magnitude. The slope of the cumulative luminosity function of our sample of low-activity comets was 0.33 ± 0.04, consistent with the slope we measured for the Jupiter-family cometary nuclei collected by Fernández et al. [Fernández, J.A., Tancredi, G., Rickman, H., Licandro, J., 1999. Astron. Astrophys. 392, 327-340] of 0.38 ± 0.02. Our slopes of the cumulative size distribution α=1.50±0.08 agree well with the slopes measured by Whitman et al. [Whitman, K., Morbidelli, A., Jedicke, R., 2006. Icarus 183, 101-114], Meech et al. [Meech, K.J., Hainaut, O.R., Marsden, B.G., 2004. Icarus 170, 463-491], Lowry et al. [Lowry, S.C., Fitzsimmons, A., Collander-Brown, S., 2003. Astron. Astrophys. 397, 329-343], and Weissman and Lowry [Weissman, P.R., Lowry, S.C., 2003. Lunar Planet. Sci. 34. Abstract 34].     

Topographic modeling of Phoebe using Cassini images

High-resolution Cassini stereo images of Saturn's moon Phoebe have been used to derive a regional digital terrain model (DTM) and an orthoimage mosaic of the surface. For DTM-control a network of 130 points measured in 14 images (70-390 m/pixel resolution) was established which was simultaneously used to determine the orientation of the spin-axis. The J2000 spin-axis was found at Dec=78.0°±0.1° and RA=356.6°±0.3°, substantially different from the former Voyager solution. The control points yield a mean figure radius of 107.2 km with RMS residuals of 6.2 km demonstrating the irregular shape of this body. The DTM was computed from densely spaced conjugate image points determined by methods of digital image correlation. It has a horizontal resolution of 1-2 km and vertical accuracies in the range 50-100 m. It is limited in coverage, but higher in resolution than the previously derived global shape model of Phoebe [Porco et al., 2005. Cassini imaging science: initial results on Phoebe and Iapetus. Science 307, 1237-1242] and allows us to study the morphology of the surface in more detail. There is evidence for unconsolidated material from a steep and smooth slope at the rim of a 100 km impact feature. There are several conically shaped craters on Phoebe, which may hint at highly porous and low compaction material on the surface.