286 research outputs found
Observations of Stellar-Mass Black Holes in the Galaxy
No full textStellar-mass black holes (BHs), with masses comparable to stars, are a major
constituent of our Milky Way galaxy. This chapter describes the landscape of
challenging, and long-sought efforts to identify these objects in the Galaxy.
The first stellar-mass BHs were identified as persistent, but highly variable
cosmic X-ray sources. Later, transient BH candidates were detected, and now far
outnumber the persistent sources. Decades of effort have also yielded candidate
BHs via gravitational microlensing and their orbital effect on binary
companions. Populations of BH systems have begun to emerge from these detection
strategies, offering insight into the astrophysical context in which BHs exist
and driving questions about the formation, assembly, and ongoing evolution of
these enigmatic objects.Comment: This chapter is the pre-print of the version currently in production.
Please cite this chapter as the following: M. MacLeod and J. Grindlay
"Observations of Stellar-Mass Black Holes in the Galaxy," in The Encyclopedia
of Cosmology (Set 2): Black Holes, edited by Z. Haiman (World Scientific, New
Jersey, 2023). We welcome comments or feedbac
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Cataclysmic Variables Discovered in the Chandra Multi-Wavelength Plane Survey
No full textWe present 25 cataclysmic variables discovered in the Chandra Multi-wavelength Plane Survey (ChaMPlane: Grindlay et al. (2005); Hong et al. (2005); Zhao et al. (2005)), which is designed to investigate the nature of the serendipitous X-ray point sources discovered by the Chandra X-ray Observatory in the galactic plane (|b| < 12°), in order to constrain the populations of faint (L_x < 1033 erg/s) accretion-powered sources in the Galaxy.AstronomyVersion of Recor
A Search for Obscured Globular Clusters
No full textIt has been estimated that up to several tens of globular clusters in our Galaxy remain undiscovered, because they are hidden by the dust. Most of those clusters are expected to be in the bulge area, but some may be anywhere in the galactic plane. This search is based on the use of IRAS catalogs as candidate lists for the obscured globular clusters. It was found that the properties of detected known clusters are sufficiently distinct for a meaningful sifting through the PSC. A sequence of statistical “filters”, described below, was applied on the PSC, until a couple of hundred most promising candidates were found in the bulge
Surface Photometry of Globular Clusters
No full textMuch of what we know about the structure, dynamics, and evolution of globular clusters derives from their observed density profiles, and their interpretations. In this review, I will briefly describe the problems and techniques specific to the surface photometry of globular clusters, show some new results, and offer suggestions for future ground-based work
The Globular Cluster System of M87
No full textLong exposures with the 4-Shooter at the Cassegrain focus of the 200-inch telescope at Palomar Observatory have been obtained for M87 (and two other giant ellipticals in Virgo). Ellipse fitting with a code specially developed to reject point sources has been carried out to determine the surface brightness in various bandpasses of the underlying galaxy. The color gradients in the galaxy are quite small over the entire regime between 2 and 350 arc-sec from the nucleus of M87. Also I find that there is no difference between the ellipse parameters (position angle and eccentricity) derived in the various colors, i.e. the isochromes and the isophotes coincide. Details of the study of the halo of M87 are described in a paper submitted to the Astronomical Journal
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Tidal Disruption of Stars by Supermassive Black Holes
No full textThis thesis presents theoretical results on the tidal disruption of stars by supermassive black holes (SMBHs). The multiwavelength ares produced by tidal disruption events (TDEs) have supernova-like luminosities, and associated relativistic jets can be visible to cosmological distances. TDEs probe the demography of quiescent SMBHs, and are natural laboratories for jet launching mechanisms and super-Eddington accretion. The first chapter broadly surveys TDE physics. The second and third chapters estimate the TDE rate following gravitational wave (GW) recoil of a SMBH (after a SMBH binary merger). Immediately after GW recoil, the TDE rate increases, sometimes to TDEs per year. This "burst" of TDE flares can provide an electromagnetic counterpart to low frequency GW signals, localizing sources and measuring cosmological parameters. Millions of years later, recoiled SMBHs wandering through their host galaxies will produce spatially offset TDEs at a rate which is likely detectable with the LSST. In the fourth chapter, we show that standard estimates for , the energy spread of TDE debris, are wrong, sometimes by orders of magnitude. Correcting this error reduces the observability of many TDEs. We introduce a new analytic model for tidal disruption, calculate 's dependence on stellar spin, estimate general relativistic corrections to , and quantify the GW signal generated from tidal compression. The fifth chapter presents hydrodynamical simulations of TDE debris circularization, focusing on eccentric, rather than parabolic, orbits. General relativistic precession drives debris circularization, in contrast to past simulations using smaller black holes. In the sixth chapter, we show that TDE light curves can constrain or measure SMBH spins, as Lense-Thirring torques produce quasiperiodic variability in disk emission. Precession of a relativistic jet could also measure SMBH spin, and we apply our model to the relativistic Swift 1644+57 TDE. The seventh chapter considers the disruption of neutron stars (NSs) by stellar mass black holes (BHs) or other NSs. Jet precession in associated short-hard gamma ray bursts is uniquely possible for NS-BH (not NS-NS) mergers. We quantify typical precession amplitudes and periods, and calculate their time evolution. If disk viscosities are relatively low, electromagnetic observations alone could distinguish NS-BH from NS-NS mergers.Astronom
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Cataclysmic Variables and a Candidate Helium White Dwarf in the Globular Cluster NGC 6397
No full textWe have used the Hubble Space Telescope (HST) and the Faint Object Spectrograph (FOS) to study faint UV stars in the core of the nearby globular cluster NGC 6397. We confirm the presence of a fourth cataclysmic variable (CV) in NGC 6397 (hereafter CV 4), and we use the photometry of Cool et al. to present evidence that CVs 1-4 all have faint disks and probably low accretion rates. By combining these results with new UV spectra of CV 1 and the published spectra of Grindlay et al., we present new evidence that CVs 1-3 may be DQ Her systems, as originally suggested by Grindlay et al., and we show that CV 4 may either be a dwarf nova or another magnetic system. Another possibility is that the CVs could be old novae in hibernation between nova eruptions. We also present the first spectrum of a member of a new class of UV bright stars in NGC 6397. These faint, hot stars do not vary, unlike the CVs, and are thus denoted as "nonflickerers'" (NFs). Like the CVs, their spatial concentration is strongly concentrated toward the cluster center. Using detailed comparisons with stellar atmosphere models, we have determined log g = 6.25 +/- 1.0 and T(eff) = 17,500 +/- 5,000 K for this NF. Using these line parameters and the luminosity of the NF, we show that the NF spectrum is consistent with a helium WD having a mass of similar to 0.25 M(.) and an age between 0.1 and 0.5 Gyr (depending on the models used). The NF spectrum appears to be significantly Doppler shifted from the expected wavelength, suggesting the presence of a dark, massive companion, probably a carbon-oxygen (CO) WD.Accepted Manuscrip
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Constraints on Electromagnetic Parity Violation in the Cosmic Microwave Background using BICEP3
No full textConservation of parity in the electromagnetic (EM) force is a generally accepted assumption for the Standard Model of physics. This need not be true, however, and a number of frameworks have been invented which predict observables in the universe for cases of EM parity violations. For instance, a coupling of the EM Lagrangian with a Chern-Simons term manifests as a frequency-independent rotation of linearly polarized light as it travels through space and is called Cosmic Birefringence. The rotation of light in the presence of Cosmic Birefringence increases over the distance of travel and is thus expected to have the largest rotation from far away objects. As such, the Cosmic Microwave Background (CMB), which is the oldest and farthest source of light in the universe, is a natural candidate in the search for birefringent signatures. The CMB is naturally polarized and can be separated into parity-even and parity-odd (i.e., E-mode and B-mode) polarization states. Under parity violation, this results in a leakage between polarization states in a predictable way. However, this leakage signal as seen in experimental data is degenerate with an incorrect calibration of the polarization response angle of a CMB experiment. Thus, in order to be sensitive to celestial sources of polarization rotation, a CMB experiment must have a precise measurement of the polarization response.
In this thesis, we explore our potential to establish constraints on Cosmic Birefringence using the BICEP3 telescope which measures the linearly polarized light from the CMB at 95 GHz. We break the degeneracy between instrumental systematics and parity-violating physics by characterizing BICEP3's polarization response with respect to gravity using a Rotating Polarized Source (RPS). The RPS is a quasi-thermal and highly polarized source and its co-polar axis is registered with respect to gravity to within 0.09°. The RPS is capable of rotating 360° and we observe the source with BICEP3 at many different rotations of the RPS to derive the orientation of the co-polar axes of BICEP3's ~2000 linearly polarized detectors. We carry out a thorough assessment of the errors in the measured polarization angles, examining a variety of potential sources of systematic uncertainty. Most of the errors are well understood and controlled, with a minimum total statistical + systematic uncertainty of 0.086°. However, empirical benchtop characterization of the RPS's performance suggests there may be a large uncertainty that so far has been difficult to quantify accurately.
We construct an analysis to constrain Cosmic Birefringence by applying angle calibrations to BICEP3's full-year 2017 and 2018 CMB data. The uncertainties on this constraint due to instrument noise and CMB foregrounds are calculated through an extensive suite of end-to-end CMB simulations. In order to maintain a high degree of confidence in the analysis, we blind ourselves to real CMB data and only unblind to certain aspects of the real data when pre-established criteria are met. We are able to progress the analysis to the point where real data is used to complete a number of cross-checks that lend high confidence to calibration techniques. However, as a result of the uncertainty arising from benchtop measurements, we are unable to unblind fully to the birefringence angle. Instead, we establish a mock birefringence constraint for demonstration purposes by substituting a single realization from our suite of CMB simulations in lieu of real CMB data. The calibration techniques we have described in this thesis are shown to have the potential to measure birefringence angles with a precision of 0.13°, which would be the most powerful constraint supported by real telescope data. This potential will be realized in future work with continued advancements in the instrumental calibration, which were dominated entirely by the instrumental error from empirical benchtop measurements. Behind the instrumental systematic uncertainty, our next most limiting uncertainty comes from the statistical noise in our CMB maps. With the inclusion of more CMB data, the precision of future analyses will quickly be limited by extra-galactic gravitational lensing. Therefore, follow-up birefringence analyses will also need to consider CMB delensing techniques before significant improvement on birefringence constraints can be achieved
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