SHAO Astrophysics Colloquia
Title: The Interstellar Extinction Law & Dust Model in the Mid-Infrared
Speaker: Biwei Jiang (Beijing Normal University)
Time: Wednesday, 3:00pm, October 28th
Location: Lecture Hall, 3rd floor
Abstract:
The mid-infrared (MIR) extinction law has already been widely studied. The MIR extinction law is found to be much flatter than the classical dust model derived from the UV/optical extinction curve. On the other hand, some studies claim that the MIR extinction law varies with sight-lines, e.g. with spiral arms or the Galactic longitude or environment. As previous studies are mostly based on stellar photometry, the uncertainty is induced when assuming a constant intrinsic colour for the selected extinction-tracer sample stars and brings about the uncertainty in the results. With the availability of stellar parameters (mainly Teff, log g and Z) from the APOGEE spectroscopic survey, the stellar intrinsic colours can be calculated with significantly higher accuracy than from photometry only. Based on this spectroscopically derived intrinsic colours, the MIR extinction law is revisited. The extinctions relative to the KS band are derived in the 2-24μm range, covering the photometric bands involved in the WISE, Spitzer/IRAC, Spitzer/MIPS and AKARI surveys. The derived extinction law agrees well with the WD01 dust model given RV=5.5. Furthermore, a dust model is constructed to explain the observed flat extinction curve in the mid-infrared while consistent with the UV/optical extinction curve. We found that the extinction from the UV to the mid-IR could be closely reproduced by a mixture of submicrometer-sized amorphous silicate dust, submicrometer-sized graphitic dust, and an addition of micrometer-sized dust.
Title: Geometry of X-ray sources in accreting black-hole binaries
Speaker: Andrzej A. Zdziarski (NCAC, Poland)
Time: Friday, 3:00pm, Oct. 30th
Location: lecture Hall, 3rd floor
Abstract: The structure of the X-ray sources in the hard spectral state of accreting black-hole binaries has been a subject of intense debate. The paradigm dominant for many years postulated that the accretion disc in the hard state is truncated at some radius >> the innermost stable orbit (ISCO) whereas the disc reaches the ISCO in the soft state. This paradigm explains a large body of observed phenomena, including the spectral and variability differences between the states and outbursts of transient sources, proceeding from quiescence (where no disc is present) through the hard state to the peak flux in the soft state. On the other hand, there have been numerous claims in recent years that the disc extends to the ISCO in the hard state. Also, the primary X-ray source has been postulated to consist of a compact source on-axis of the rotating black hole (a 'lamppost'). Those claims are based on observations of broad Fe K lines and of soft X-ray components interpreted as blackbody-emitting accretion discs. I will review arguments for and against the disc truncation and the lamppost geometry. I will then present new results of a study of the X-ray spectra of GX 339-4 from XMM-Newton.
Seminar talk
Title: MHD simulations of GRB (gamma-ray burst) and blazar flares
Speaker: Wei Deng (University of Nevada, Las Vegas)
Time: Oct. 29, Thursday, 2pm
Location: the middle conference room
Abstract: The energy composition in the jet/outflow of astrophysical systems is an important and fundamental question. Recently, the collision-induced magnetic reconnection and dissipation models in the Poynting-flux-dominated (PFD) environment become more attractive to overcome some criticisms in the traditional matter-flux-dominated (MFD) models and interpret new observations. Here, we perform 3D relativistic MHD simulations to study the collisions between high-$\sigma$ (PFD) blobs which contain both poloidal and toroidal magnetic field components to mimic the above considerations. Our results strongly support these considerations from the energy dissipation efficiency ($\geq 35\%$) and the mini-jets generation point of view. We also find a possible relationship between the $\sigma$ values before and after the major EMF energy dissipation process. On the other hand, polarization observation is another important and independent information to constrain the energy environment and physical models. Here we study the polarization feature based on the magnetic field evolution from our above MHD simulation, and find that it can simultaneously interprets the representative polarization observations for both Gamma-Ray Bursts (GRBs) and Blazar flares quite well and concisely. This result also strongly supports the above PFD considerations. And more interestedly, it hints that, for some cases, the GRBs prompt emission and the Blazar flares may share similar physical process and energy environment.
Title: What if a black hole devours too much
Speaker: Jifeng Liu (NAOC)
Date & Time: Oct. 26, Monday, 2pm
Venue: Middle conference room
Abstract:
The formation of relativistic jets by an accreting compact object is one of the fundamental mysteries of astrophysics. While the theory is poorly understood, observations of relativistic jets from systems known as microquasars have led to a well-established phenomenology. Relativistic jets are not expected from sources with soft or supersoft X-ray spectra, although two such systems are known to produce relatively low-velocity bipolar outflows. Here we report optical spectra of an ultraluminous supersoft X-ray source (ULS) in the nearby galaxy M81 (M81 ULS-1) showing blueshifted broad Hαemission lines, characteristic of baryonic jets with relativistic speeds. The time variable jets have projected velocities ~17 per cent of the speed of light, and seem similar to those in the prototype microquasar SS 433. Such relativistic jets are not expected to be launched from white dwarfs, but an origin from a black hole or neutron star in M81 ULS-1 is hard to reconcile with its constant soft X-rays. The completely unexpected presence of relativistic jets in a ULS challenges the canonical theories for jet formation, but can be explained by a long speculated super-critically accreting black hole with optically thick outflows.
