Yufeng Fan

The unpolarized macronova associated with the gravitational wave event GW 170817

The merger of two dense stellar remnants including at least one neutron star is predicted to produce gravitational waves (GWs) and short-duration gamma ray bursts1,2. In the process, neutron-rich material is ejected from the system and heavy elements are synthesized by r-process nucleosynthesis1,3. The radioactive decay of these heavy elements produces additional transient radiation termed kilonova or macronova4–10. We report the detection of linear optical polarization, P = (0.50 ± 0.07)%, 1.46 days after detection of the GWs from GW 170817—a double neutron star merger associated with an optical macronova counterpart and a short gamma ray burst11–14. The optical emission from a macronova is expected to be characterized by a blue, rapidly decaying component and a red, more slowly evolving component due to material rich in heavy elements—the lanthanides15. The polarization measurement was made when the macronova was still in its blue phase, during which there was an important contribution from a lanthanide-free outflow. The low degree of polarization is consistent with intrinsically unpolarized emission scattered by galactic dust, suggesting a symmetric geometry of the emitting region and low inclination of the merger system. Stringent upper limits to the polarization degree from 2.45–9.48 days post-burst are consistent with the lanthanides-rich macronova interpretation.A double neutron star merger gave rise to the gravitational-wave event GW 170817, with counterpart electromagnetic radiation in the optical and gamma-ray spectra. Polarization measurements of the optical emission reveal a lanthanide-rich macronova.

VERY HIGH ENERGY γ-RAY AFTERGLOW EMISSION OF NEARBY GAMMA-RAY BURSTS

The synchrotron self-Compton (SSC) emission from gamma-ray burst (GRB) forward shock can extend to the very high energy (VHE; E gamma > 100 GeV) range. Such high energy photons are rare and are attenuated by the cosmic infrared background before reaching us. In this work, we discuss the prospect to detect these VHE photons using the current ground-based Cherenkov detectors. Our calculated results are consistent with the upper limits obtained with several Cherenkov detectors for GRB 030329, GRB 050509B, and GRB 060505 during the afterglow phase. For five bursts in our nearby GRB sample (except for GRB 030329), current ground-based Cherenkov detectors would not be expected to detect the modeled VHE signal. Only for those very bright and nearby bursts like GRB 030329, detection of VHE photons is possible under favorable observing conditions and a delayed observation time of less than or similar to 10 hr.

Prediction on the very early afterglow of X-ray flashes

In the past two years, tremendous progress in understanding X-ray flashes has been made. Now it is widely believed that X-ray flashes and gamma-ray bursts are intrinsically the same, and that their very different peak energy and flux may be merely due to our different viewing angles to them. Here we analytically calculate the very early afterglow of X-ray flashes, i.e. the reverse shock emission powered by the outflows interacting with the interstellar medium. Assuming z similar to 0.3, we have shown that typically the R-band flux of reverse shock emission can be bright to similar to16-17th magnitude (the actual values are model-dependent and sensitive to the initial Lorentz factor of the viewed ejecta). That emission is bright enough to be detected by the telescope on work today such as Robotic Optical Transient Search Experiment (ROTSE-III) or the upcoming Ultraviolet and Optical Telescope (UVOT) carried by the Swift satellite, planned for launch in late 2004.

A Luminous Peculiar Type Ia Supernova SN 2011hr: More Like SN 1991T or SN 2007if?

Photometric and spectroscopic observations of a slowly declining, luminous Type Ia supernova (SN Ia) SN 2011hr in the starburst galaxy NGC 2691 are presented. SN. 2011hr is found to peak at MB = -19.84 +/- 0.40 mag, with a postmaximum decline rate Delta(m15)(B) = 0.92. +/- 0.03 mag. From the maximum-light bolometric luminosity, L = (2.30 +/- 0.90) x 10(43) erg s(-1), we estimate the mass of synthesized Ni-56 in SN 2011hr to be M(Ni-56) = 1.11 +/- 0.43M(circle dot). SN 2011hr appears more luminous than SN 1991T at around maximum light, and the absorption features from its intermediate-mass elements (IMEs) are noticeably weaker than those of the latter at similar phases. Spectral modeling suggests that SN 2011hr has IMEs of similar to 0.07 M-circle dot in the outer ejecta, which is much lower than the typical value of normal SNe Ia (i.e., 0.3-0.4 M-circle dot) and is also lower than the value of SN 1991T (i.e., similar to 0.18 M-circle dot). These results indicate that SN. 2011hr may arise from a Chandrasekhar-mass white dwarf progenitor that experienced a more efficient burning process in the explosion. Nevertheless, it is still possible that SN. 2011hr may serve as a transitional object connecting the SN 1991T-like SNe Ia with a superluminous subclass like SN 2007if given that the latter also shows very weak IMEs at all phases.

Ultraviolet/optical emission accompanying gamma-ray bursts

We discuss the possible simultaneously ultraviolet (UV)/optical emission accompanying gamma-ray bursts (GRBs). We show that as long as the intrinsic spectrum of GRBs can extend to similar to10 GeV or higher, there are large amounts of relativistic e(+/-) pairs generated due to the annihilation of soft gamma-rays with very energetic photons, which dominates over the electrons/positrons associated with the fireball, whether the fireball is highly magnetized or not (for the highly magnetized fireball, the magnetic field is ordered, high linear polarization of multiwavelength emission is expected). We find that these e pairs can power a UV flash with m similar or equal to 12-13 mag, and the corresponding optical emission can be up to m(R) similar or equal to 15-16 mag. Such bright UV emission will be able to be detected by the satellite Swift, planned for launch in 2004. The behaviour of the optical-UV spectrum (F(v) proportional to v(5/2)) differs significantly from that of the reverse shock emission (F(v) proportional to v(-beta/2), beta similar or equal to 2.2), which is a signature of the emission accompanying the GRB. Mild optical emission can be detected with the ROTSE-IIIa telescope system, if the response to the GRB alert is fast enough.

Rapid instrument exchanging system for the Cassegrain focus of the Lijiang 2.4-m Telescope

As a facility used for astronomical research, the Lijiang 2.4-m telescope of Yunnan Astronomical Observatories, requires the ability to change one auxiliary instrument with another in as short a time as possible. This arises from the need to quickly respond to scientific programs (e.g. transient observation, time domain studies) and changes in observation conditions (e.g. seeing and weather conditions). In this paper, we describe the design, construction and test of hardware and software in the rapid instrument exchange system (RIES) for the Cassegrain focal station of this telescope, which enables instruments to be quickly changed at night without much loss of observing time. Tests in the laboratory and at the telescope show that the image quality and pointing accuracy of RIES are satisfactory. With RIES, we observed the same Landolt standard stars almost at the same time with the Princeton Instruments VersArray 1300B Camera (PICCD) and the Yunnan Faint Object Spectrograph and Camera (YFOSC), while both were mounted at the Cassegrain focus. A quasi-simultaneous comparison shows that the image quality of the optical system inside the YFOSC is comparable with that provided by the PICCD.

Design and Implement of Astronomical Cloud Computing Environment In China-VO.

Astronomy cloud computing environment is a cyber-Infrastructure for Astronomy Research initiated by Chinese Virtual Observatory (China-VO) under funding support from NDRC (National Development and Reform commission) and CAS (Chinese Academy of Sciences). Based on virtualization technology, astronomy cloud computing environment was designed and implemented by China-VO team. It consists of five distributed nodes across the mainland of China. Astronomer can get compuitng and storage resource in this cloud computing environment. Through this environments, astronomer can easily search and analyze astronomical data collected by different telescopes and data centers , and avoid the large scale dataset transportation.

VHE γ‐ray Afterglow Emission from Nearby GRBs

Gamma‐ray Bursts (GRBs) are among the potential extragalactic sources of very‐high‐energy (VHE) γ‐rays. We discuss the prospects of detecting VHE γ‐rays with current ground‐based Cherenkov instruments during the afterglow phase. Using the fireball model, we calculate the synchrotron self‐Compton (SSC) emission from forward‐shock electrons. The modeled results are compared with the observational afterglow data taken with and/or the sensitivity level of ground‐based VHE instruments (e.g. STACEE, H.E.S.S., MAGIC, VERITAS, and Whipple). We find that modeled SSC emission from bright and nearby bursts such as GRB 030329 are detectable by these instruments even with a delayed observation time of ∼10 hours.