14496803
LARGE-SCALE CAVITIES SURROUNDING MICROQUASARS INFERRED FROM EVOLUTION OF THEIR RELATIVISTIC JETS
The black hole (BH) X-ray transient XTE J1550-564 has undergone a strong outburst in 1998 and two relativistic X-ray jets have been detected years later with the Chandra X-ray observatory; the eastern jet was found previously to have decelerated after its first detection. Here, we report a full analysis of the evolution of the western jet; significant deceleration is also detected in the western side. Our analysis indicates that there is a cavity outside the central source and the jets first traveled with constant velocity and then were slowed down by the interactions between the jets and the interstellar medium. The best-fitted radius of the cavity is similar to 0.31 pc on the eastern side and similar to 0.44 pc on the western side, and the densities also show asymmetry, of similar to 0.034 cm(-3) on the east to similar to 0.12 cm(-3) on the west. The best-fitted magnetic fields on both sides are similar to 0.5 mG. Similar analysis is also applied to another microquasar system, H 1743-322, and a large-scale low-density region is also found. Based on these results and the comparison with other microquasar systems, we suggest a generic scenario for microquasar jets, classifying the observed jets into three main categories, with different jet morphologies (and sizes) corresponding to different scales of vacuous environments surrounding them. We also suggest that either continuous jets or accretion disk winds, or both may be responsible for creating these cavities. Therefore X-ray jets from microquasars provide us with a promising method of probing the environment of accreting BHs.
16218923
Evolution of relativistic jets from XTE J1550‐564 and the environment of microquasars
Two relativistic X‐ray jets have been detected with the Chandra X‐ray observatory in the black hole X‐ray transient XTE J1550‐564. We report a full analysis of the evolution of the two jets with a gamma‐ray burst external shock model. A plausible scenario suggests a cavity outside the central source and the jets first travelled with constant velocity and then are slowed down by the interactions between the jets and the interstellar medium (ISM). The best fitted radius of the cavity is ∼0.36 pc on the eastern side and ∼0.46 pc on the western side, and the densities also show asymmetry, of ∼0.015 cm−3 on the east to ∼0.21 cm−3 on the west. Large scale low density region is also found in another microquasar system, H 1743‐322. These results are consistent with previous suggestions that the environment of microquasars should be rather vacuous, compared to the normal Galactic environment. A generic scenario for microquasar jets is proposed, classifying the observed jets into three main categories, with differe...
17372476
ArborZ: Photometric Redshifts Using Boosted Decision Trees
Precision photometric redshifts will be essential for extracting cosmological parameters from the next generation of wide-area imaging surveys. In this paper, we introduce a photometric redshift algorithm, ArborZ, based on the machine-learning technique of boosted decision trees. We study the algorithm using galaxies from the Sloan Digital Sky Survey (SDSS) and from mock catalogs intended to simulate both the SDSS and the upcoming Dark Energy Survey. We show that it improves upon the performance of existing algorithms. Moreover, the method naturally leads to the reconstruction of a full probability density function (PDF) for the photometric redshift of each galaxy, not merely a single "best estimate" and error, and also provides a photo-z quality figure of merit for each galaxy that can be used to reject outliers. We show that the stacked PDFs yield a more accurate reconstruction of the redshift distribution N(z). We discuss limitations of the current algorithm and ideas for future work.
118891782
An Attractor solution of phantom field
In light of recent study on the dark energy models that manifest an equation of state $w<-1$, we investigate the cosmological evolution of phantom field in a specific potential, exponential potential in this paper. The phase plane analysis show that the there is a late time attractor solution in this model, which address the similar issues as that of fine tuning problems in conventional quintessence models. The equation of state $w$ is determined by the attractor solution which is dependent on the $\lambda$ parameter in the potential. We also show that this model is stable for our present observable universe.
17737177
Revisiting the parametrization of equation of state of dark energy via SNIa data
In this paper, we revisit the parametrizations of the equation of state of dark energy and point out that comparing merely the χ 2 of different fittings may not be optimal for choosing the ‘best’ parametrization. Another figure of merit for evaluating different parametrizations based on the area of the w(z) − z band is proposed. In light of the analysis of some two-parameter parametrizations and models based on available SNIa data, the area of w(z) − z band seems to be a good figure of merit, especially in the situation that the value of χ 2 for different parametrizations are very close. Therefore, we argue that both the area of the w(z) − z band and χ 2 should be synthetically considered for choosing a better parametrization of dark