An Ultra Low Mass and Small Radius Compact Object in 4U 1746-37?

Photospheric radius expansion (PRE) bursts have already been used to constrain the masses and radii of neutron stars. RXTE observed three PRE bursts in 4U 1746-37, all with low touchdown fluxes. We discuss here the possibility of low mass neutron star in 4U 1746-37 because the Eddington luminosity depends on stellar mass. With typical values of hydrogen mass fraction and color correction factor, a Monte-Carlo simulation was applied to constrain the mass and radius of neutron star in 4U 1746-37. 4U 1746-37 has a high inclination angle. Two geometric effects, the reflection of the far side accretion disc and the obscuration of the near side accretion disc have also been included in the mass and radius constraints of 4U 1746-37. If the reflection of the far side accretion disc is accounted, a low mass compact object (mass of $0.41\pm0.14~M_{\odot}$ and radius of $8.73\pm1.54~\rm km$ at 68% confidence) exists in 4U 1746-37. If another effect operated, 4U 1746-37 may contain an ultra low mass and small radius object ($M=0.21\pm0.06~M_{\odot},~R=6.26\pm0.99~\rm km$ at 68% confidence). Combined all possibilities, the mass of 4U 1746-37 is $0.41^{+0.70}_{-0.30}~M_\odot$ at 99.7% confidence. For such low mass NS, it could be reproduced by a self-bound compact star, i.e., quark star or quark-cluster star.Comment: accepted by Ap

The Warm-Hot Circumgalactic Medium and its Co-Evolution with the Galaxy Disk

In the past decade, multi-wavelength observations revealed that the galaxy disk is surrounded by a massive, low-density, and multi-phase gaseous medium -- the circumgalactic medium (CGM). This baryon reservoir feeds galaxy growth through the accretion of radiatively cooling gas, which fuels star formation in the disk. Stellar feedback in the disk ejects metals, momentum, and energy into and beyond the CGM. These feedback and accretion processes are the most uncertain pieces in our picture of galaxy evolution. These processes can be constrained by the properties of the CGM surrounding the galaxy, because the CGM is co-evolving with the disk. This dissertation focuses on observations of the warm-hot CGM at low redshift z<1 and its connection with galactic disks. The warm-hot CGM is observed in multi-wavelength bands including the ultraviolet (UV), the X-ray, and the Sunyaev-Zeldovich (SZ). Using archival UV spectra, I mainly studied the warm gas in the Milky Way (MW) (e.g., SiIV and OVI), where I developed a kinematical model. This model constrains both the density distribution and the bulk velocity field of the warm gas simultaneously. We applied this model to the line shape sample of SiIV absorption lines, and found that most observed column densities are close to the disk (d<20 kpc) rather than at large radii. This spatial distribution leads to a total warm gas mass of log M ~ 9 around the MW disk. Although the warm gas is less massive than the hot gas, it is more active with the disk because of its rapid cooling. Galactic fountain features are seen in the warm gas disk, which is co-rotating with the stellar disk. These phenomena suggest that the warm gas is dominated by feedback originating from the disk. I also developed a new extraction method for the large-scale SZ features aiming at the hot gas in the local universe (i.e., the MW and the Local Group). This method was applied to the archival Planck and WMAP data, which led to the discovery of a massive hot bridge connecting the MW and M31, which is also confirmed in X-ray emission. This method will be further developed in the future to detect the MW contribution to the SZ signal by suppressing dust contamination. To understand the connection between the warm-hot CGM and the disk, I developed a semi-analytic model, which assumes that the star formation is supplied by accretion from radiative cooling of the hot CGM. In this theoretical model, the star formation in the disk is balanced by the net accretion rate from the CGM, which is also modified by photoionization and stellar feedback. For the cooling flow, we consider a stable solution, where the mass cooling rate is a constant over all temperatures. The derived temperature distribution could match various observations of galaxy-absorber pairs at different redshifts. In this model, most high column density absorption systems are associated with the warm-hot CGM, by comparing the model prediction to the measured cosmic column density distribution for UV ions (e.g., OVI and NeVIII). However, the model prediction underestimates the detection rate of low column density absorption systems (e.g., log N < 13.5 for OVI), which suggests that these systems are hosted by unvirialized dark matter structures (e.g., cosmic filaments).PHDAstronomy and AstrophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/169839/1/quzhijie_1.pd