Jianghua Wu

Optical Monitoring of BL Lacertae Object S5 0716+714 with High Temporal Resolution

Optical monitoring of S5 0716+714 was performed with a 60/90 Schmidt telescope in 2003 November and December and 2004 January for studying the variability of the object on short timescales. Because of the high brightness of the source, we could carry out quasi-simultaneous measurements in three bands with a temporal resolution of about 20 minutes by using one single telescope. Intraday and intranight variations were observed, showing an overall change of ~0.9 mag during the whole campaign. Two outbursts were recorded on Julian Dates 2,453,005 and 2,453,009. Minimum timescales of a few hours were derived from the light curves of individual nights, but they were different from night to night. A bluer-when-brighter chromatism was present when the object was showing a fast flare but was absent when it was in a relatively quiescent state. Our results are basically consistent with the shock-in-jet model and demonstrate that geometric effects can sometimes play an important role in the variability of blazars.

The First Release of the CSTAR Point Source Catalog from Dome A, Antarctica

In 2008 January the twenty-fourth Chinese expedition team successfully deployed the Chinese Small Telescope ARray (CSTAR) to Dome A, the highest point on the Antarctic plateau. CSTAR consists of four 14.5 cm optical telescopes, each with a different filter (g, r, i, and open) and has a 4.5° × 4.5° field of view (FOV). It operates robotically as part of the Plateau Observatory, PLATO, with each telescope taking an image every ~30 s throughout the year whenever it is dark. During 2008, CSTAR 1 performed almost flawlessly, acquiring more than 0.3 million i-band images for a total integration time of 1728 hr during 158 days of observations. For each image taken under good sky conditions, more than 10,000 sources down to ~16th magnitude could be detected. We performed aperture photometry on all the sources in the field to create the catalog described herein. Since CSTAR has a fixed pointing centered on the south celestial pole (decl. = -90°), all the sources within the FOV of CSTAR were monitored continuously for several months. The photometric catalog can be used for studying any variability in these sources, and for the discovery of transient sources such as supernovae, gamma-ray bursts, and minor planets.

Sky Brightness and Transparency in the i-band at Dome A, Antarctica

The i-band observing conditions at Dome A on the Antarctic plateau have been investigated using data acquired during 2008 with the Chinese Small Telescope Array. The sky brightness, variations in atmospheric transparency, cloud cover, and the presence of aurorae are obtained from these images. The median sky brightness of moonless clear nights is 20.5 mag arcsec(-2) in the SDSS i band at the south celestial pole (which includes a contribution of about 0.06 mag from diffuse Galactic light). The median over all Moon phases in the Antarctic winter is about 19.8 mag arcsec(-2). There were no thick clouds in 2008. We model contributions of the Sun and the Moon to the sky background to obtain the relationship between the sky brightness and transparency. Aurorae are identified by comparing the observed sky brightness to the sky brightness expected from this model. About 2% of the images are affected by relatively strong aurorae.

The Whole Earth Blazar Telescope Campaign on the Intermediate BL Lac Object 3C 66A in 2007-2008

Prompted by a high optical state in September 2007, the Whole Earth Blazar Telescope (WEBT) consortium organized an intensive optical, near-IR (JHK) and radio observing campaign on the intermediate BL Lac object 3C 66A throughout the fall and winter of 2007 -- 2008. The source remained in a high optical state throughout the observing period and exhibited several bright flares on time scales of ~ 10 days. This included an exceptional outburst around September 15 - 20, 2007, reaching a peak brightness at R ~ 13.4. Our campaign revealed microvariability with flux changes up to |dR/dt| ~ 0.02 mag/hr. Our observations do not reveal evidence for systematic spectral variability or spectral lags. We infer a value of the magnetic field in the emission region of B ~ 19 e_B^{2/7} \tau_h^{-6/7} D_1^{13/7} G. From the lack of systematic spectral variability, we can derive an upper limit on the Doppler factor, D 50, required for a one-zone SSC interpretation of some high-frequency-peaked BL Lac objects detected at TeV gamma-ray energies.

Age Constraints for an M31 Globular Cluster from SEDs Fit

We have constrained the age of the globular cluster S312 in the Andromeda galaxy (M31) by comparing its multicolor photometry with theoretical stellar population synthesis models. This is both a check on the age of this globular cluster and a check on our methodology. Main-sequence photometry has been the most direct method for determining the age of a star cluster. S312 was observed as part of the Beijing-Arizona-Taiwan-Connecticut (BATC) Multicolor Sky Survey from 1995 February to 2003 December. The photometry of BATC images for S312 was taken with nine intermediate-band filters covering 5000-10000 A. Combined with photometry in the near-ultraviolet (NUV) of GALEX, broadband UBVR, and infrared JHKs of 2MASS, we obtained the accurate spectral energy distributions (SEDs) of S312 from 2267-20000 A. A quantitative comparison to simple stellar population models yields an age of 9.5 Gyr, which is in very good agreement with the previous determination by main-sequence photometry. S312 has a mass of 9.8 ± 1.85 × 105 M☉ and is a medium-mass globular cluster in M31. By analyzing the errors of ages determined based on the SED-fitting method of this paper, secure age constraints are derived with errors of ~10 Gyr. Therefore, our method does not accurately distinguish globular clusters (GCs) as old as the majority of the Galactic GCs. We emphasize that our results show that even with multiband photometry spanning from NUV to Ks, our age constraints from SED fitting are distressingly uncertain, which has implications for age derivations in extragalactic globular cluster systems.

Optical monitoring of PKS 1510¿089: a binary black hole system?

Three deep flux minima with nearly the same time-scales and intervals have been observed for the blazar PKS 1510−089 in the past few years. It has been proposed that there is a binary black hole system at the nucleus of this object, and a new minimum was predicted to occur in 2002 March. We monitored this source with a 60/90 cm Schmidt telescope from 2002 February to April. In combination with the data obtained by Xie et al. in the same period, for the 2002 minimum we present a nearly symmetric light curve, which would be required by an eclipsing model of a binary black hole system. We also constrain the time-scale of the minimum to be 35 min, which is more consistent with the time-scales (∼42 min) of the three previous minima than is the 89 min time-scale given by Xie et al. The wiggling milliarcsecond radio jet observed in this object is taken as further evidence for the binary black hole system. The ‘coupling’ of the periodicity in the light curve and the helicity in the radio jet is discussed for blazars in the framework of a binary black hole system.

Seven-year Multi-color Optical Monitoring of BL Lacertae Object S5 0716+714

We have monitored the BL Lacertae object S5 0716+714 in five intermediate optical wavebands from 2004 September to 2011 April. Here, we present data that include 8661 measurements representing one of the largest databases obtained for an object in the optical domain. A simple analysis of the data indicates that the object was active most of the time, and intraday variability was frequently observed. In total, the object varied by 2.614 mag in the i band. Strong bluer-when-brighter chromatism was observed on long, intermediate, and short timescales.

Testing and Data Reduction of the Chinese Small Telescope Array (CSTAR ) for Dome A, Antarctica

The Chinese Small Telescope Array (CSTAR) is the first Chinese astronomical instrument on the Antarctic ice cap. The low temperature and low pressure testing of the data acquisition system was carried out in a laboratory refrigerator and on the 4500 m Pamirs high plateau, respectively. The results from the final four nights of test observations demonstrated that CSTAR was ready for operation at Dome A, Antarctica. In this paper, we present a description of CSTAR and the performance derived from the test observations.

COLOR BEHAVIOR OF BL LACERTAE OBJECT OJ 287 DURING AN OPTICAL OUTBURST

This paper studies the color behavior of the BL Lac object OJ 287 during an optical outburst. Upon revisiting the data from the OJ 94 monitoring project, and from an analysis of the data obtained with the 60/90 cm Schmidt Telescope of NAOC, we found a bluer-when-brighter chromatism in this object. The amplitude of variation tends to decrease with frequency. These results are consistent with the shock-in-jet model. We ran some simulations and confirmed that both amplitude difference and time delay between variations at different wavelengths can result in the bluer-when-brighter phenomenon. Our observations confirmed that OJ 287 underwent a double-peaked outburst about 12 years after 1996, which provides further evidence for the binary black hole model in this object.

AGE AND MASS CONSTRAINTS FOR A YOUNG MASSIVE CLUSTER IN M31 BASED ON SPECTRAL ENERGY DISTRIBUTION FITTING

VDB0-B195D is a massive, blue star cluster in M31. It was observed as part of the Beijing-Arizona-Taiwan-Connecticut (BATC) Multicolor Sky Survey using 15 intermediate-band filters covering a wavelength range of 3000-10000 A. Based on aperture photometry, we obtain its spectral energy distribution (SED) as defined by the 15 BATC filters. We apply previously established relations between the BATC intermediate-band and the Johnson-Cousins UBVRI broadband systems to convert our BATC photometry to the standard system. A detailed comparison shows that our newly derived VRI magnitudes are fully consistent with previous results, while our new B magnitude agrees to within 2{sigma}. In addition, we determine the clusters age and mass by comparing its SED (from 3000 to 20000 A, comprising photometric data in the 15 BATC intermediate bands, optical broadband BVRI, and Two Micron All Sky Survey near-infrared JHK{sub s} data) with theoretical stellar population synthesis models, resulting in age and mass determinations of 60.0 {+-} 8.0 Myr and (1.1-1.6) x 10{sup 5} M{sub sun}, respectively. This age and mass confirms previous suggestions that VDB0-B195D is a young massive cluster in M31.