ABSTRACT Radio emission has been detected from tens of white dwarfs, in particular in accreting systems. Additionally, radio emission has been predicted as a possible outcome of a planetary system ...around a white dwarf. We searched for 3 GHz radio continuum emission in 846 000 candidate white dwarfs previously identified in Gaia using the Very Large Array Sky Survey (VLASS) Epoch 1 Quick Look Catalogue. We identified 13 candidate white dwarfs with a counterpart in VLASS within 2 arcsec. Five of those were found not to be white dwarfs in follow-up or archival spectroscopy, whereas seven others were found to be chance alignments with a background source in higher resolution optical or radio images. The remaining source, WDJ204259.71+152108.06, is found to be a white dwarf and M-dwarf binary with an orbital period of 4.1 d and long-term stochastic optical variability, as well as luminous radio and X-ray emission. For this binary, we find no direct evidence of a background contaminant, and a chance alignment probability of only ≈2 per cent. However, other evidence points to the possibility of an unfortunate chance alignment with a background radio and X-ray emitting quasar, including an unusually poor Gaia DR3 astrometric solution for this source. With at most one possible radio emitting white dwarf found, we conclude that strong (≳1–3 mJy) radio emission from white dwarfs in the 3 GHz band is virtually non-existent outside of interacting binaries.
This dissertation seeks to understand the political and economic relationships in the organization and use of neighborhood public space at the archaeological site of Angamuco in Michoacán, Mexico. ...Ethnohistoric sources describe multiple distinct social classes for the P’urépecha people at the time of European contact, but they are ambiguous about the exact political and economic relationships among them. There is some description of how these different interest groups articulated at the level of the city-state, but there is not much information on the internal dynamics of neighborhood or district-level subdivisions of the city-state. The discovery of the remains of a probable granary near a small temple in a plaza in 2014 by the LORE:LPB project suggests an avenue of research that could elaborate these relationships. The storage of grain in a public place provides a window through which to investigate the political and economic relationships operating within subdivisions of the city.Using a combination of collective action theory and heterodox Marxian political economy informed by ethnohistoric evidence, this dissertation proposes three competing hypotheses that could explain the organization of neighborhood public space and the resources, such as granaries, contained within these spaces. Seven different urban neighborhoods in Angamuco were selected for this study, including candidates associated with two different urban cores. Areas were chosen through visual inspection of lidar data and investigated through a combination of survey, geophysical prospection, and spatial analysis to assess visibility and accessibility of the spaces and resources associated with them. Based on the results of this first phase of research, five of these areas were targeted for excavation. A preliminary materials analysis was conducted to sort the various neighborhoods into a relative chronology. Each neighborhood was then examined in comparison to the three competing hypotheses on the configuration of the political economy in these spaces.The results of this study show two distinct patterns for neighborhoods: one for elites and one for commoners. Public space in commoner neighborhoods throughout the city seem to best conform with the model of a “club good,” which is managed collectively by a social group but with access restricted to those outside the group. Elite areas seem to construct similar space to be private, and the placement of granaries near small temples or shrines is associated with this latter pattern. In older areas near the second urban core, these patterns are distinct. In those neighborhoods which date to later phases of occupation near the southern core, the two patterns become superimposed onto the same spaces. This could reflect an elite colonization of public space. There is at least some evidence for conflict emerging as a result of this colonization, as the most recent of the neighborhoods in this study shows evidence for multiple competing interest groups each modifying the space for their own, conflicting agendas. This has implications for the formation of the Late Postclassic imperial state, which ideologically justified its existence as a way to hold local elites accountable for the benefit of the common people.
We present the discovery, from archival Chandra and XMM-Newton data, of X-ray eclipses in two ultraluminous X-ray sources (ULXs), located in the same region of the galaxy M 51: CXOM51 ...J132940.0\(+\)471237 (ULX-1, for simplicity) and CXOM51 J132939.5\(+\)471244 (ULX-2). Three eclipses were detected for ULX-1, two for ULX-2. The presence of eclipses puts strong constraints on the viewing angle, suggesting that both ULXs are seen almost edge-on and are certainly not beamed towards us. Despite the similar viewing angles and luminosities (\(L_{\rm X} \approx 2 \times 10^{39}\) erg s\(^{-1}\) in the \(0.3\)-\(8\) keV band for both sources), their X-ray properties are different. ULX-1 has a soft spectrum, well fitted by Comptonization emission from a medium with electron temperature \(kT_e \approx 1\) keV. ULX-2 is harder, well fitted by a slim disk with \(kT_{\rm in} \approx 1.5\)-\(1.8\) keV and normalization consistent with a \(\sim 10 M_{\odot}\) black hole. ULX-1 has a significant contribution from multi-temperature thermal plasma emission (\(L_{\rm X,mekal} \approx 2 \times 10^{38}\) erg s\(^{-1}\)); about \(10\)% of this emission remains visible during the eclipses, proving that the emitting gas comes from a region slightly more extended than the size of the donor star. From the sequence and duration of the Chandra observations in and out of eclipse, we constrain the binary period of ULX-1 to be either \(\approx 6.3\) days, or \(\approx 12.5-13\) days. If the donor star fills its Roche lobe (a plausible assumption for ULXs), both cases require an evolved donor; most likely a blue supergiant, given the young age of the stellar population in that galactic environment.
We have conducted long-slit spectroscopic observations and analyzed archival radio data for the ultraluminous X-ray source (ULX) NGC 4861 X-1. Our spectral line analysis unveils that NGC 4861 X-1 is ...the fourth ULX situated within an X-ray photoionized nebula, following three previous findings made approximately two decades ago. Remarkably, we discover NGC 4861 X-1 also possesses a radio nebula emitting optically thin synchrotron radiation, which contradicts its X-ray photoionization and raises doubts about the four ULXs being a mere coincidence. Instead of gradually accumulating from different bands bit by bit, our multi-band discovery is made all at once. Moreover, we tentatively perceive a faint continuum spectrum of the optical nebula. Further observations are needed to ascertain its radio structures and verify the optical continuum.
Ultraluminous supersoft sources (ULSs) are defined by a thermal spectrum with colour temperatures ~0.1 keV, bolometric luminosities ~ a few 10^39 erg/s, and almost no emission above 1 keV. It has ...never been clear how they fit into the general scheme of accreting compact objects. To address this problem, we studied a sample of seven ULSs with extensive Chandra and XMM-Newton coverage. We find an anticorrelation between fitted temperatures and radii of the thermal emitter, and no correlation between bolometric luminosity and radius or temperature. We compare the physical parameters of ULSs with those of classical supersoft sources, thought to be surface-nuclear-burning white dwarfs, and of ultraluminous X-ray sources (ULXs), thought to be super-Eddington stellar-mass black holes. We argue that ULSs are the sub-class of ULXs seen through the densest wind, perhaps an extension of the soft-ultraluminous regime. We suggest that in ULSs, the massive disk outflow becomes effectively optically thick and forms a large photosphere, shrouding the inner regions from our view. Our model predicts that when the photosphere expands to >10,000 km and the temperature decreases below approximately 50 eV, ULSs become brighter in the far-UV but undetectable in X-rays. Conversely, we find that harder emission components begin to appear in ULSs when the fitted size of the thermal emitter is smallest (interpreted as a shrinking of the photosphere). The observed short-term variability and absorption edges are also consistent with clumpy outflows. We suggest that the transition between ULXs (with a harder tail) and ULSs (with only a soft thermal component) occurs at blackbody temperatures of approximately 150 eV.
GRS 1747-312 is a bright Low-Mass X-ray Binary in the globular cluster Terzan 6, located at a distance of 9.5 kpc from the Earth. It exhibits regular outbursts approximately every 4.5 months, during ...which periodic eclipses are known to occur. These eclipses have only been observed in the outburst phase, and are not clearly seen when the source is quiescent. Recent Chandra observations of the source were performed in June 2019 and April, June, and August of 2021. Two of these observations captured the source during its outburst, and showed clear flux decreases at the expected time of eclipse. The other two observations occurred when the source was quiescent. We present the discovery of a dip that occurred during the quiescent state. The dip is of longer duration and its time of occurrence does not fit the ephemeris of the shorter eclipses. We study the physical characteristics of the dip and determine that it has all the properties of an eclipse by an object with a well defined surface. We find that there are several possibilities for the nature of the object causing the 5.3 ks eclipse. First, GRS 1747-312 may be an X-ray triple, with an LMXB orbited by an outer third object, which could be an M-dwarf, brown dwarf, or planet. Second, there could be two LMXBs in close proximity to each other, likely bound together. Whatever the true nature of the eclipser, its presence suggests that the GRS 1747-312 system is unique.
Some ultraluminous X-ray sources (ULXs) are surrounded by collisionally ionized bubbles, larger and more energetic than supernova remnants: they are evidence of the powerful outflows associated with ...super-Eddington X-ray sources. We illustrate the most recent addition to this class: a huge (350 pc x 220 pc in diameter) bubble around a ULX in NGC 5585. We modelled the X-ray properties of the ULX (a broadened-disc source with L_X ~ 2-4 x 10^{39} erg/s) from Chandra and XMM-Newton, and identified its likely optical counterpart in Hubble Space Telescope images. We used the Large Binocular Telescope to study the optical emission from the ionized bubble. We show that the line emission spectrum is indicative of collisional ionization. We refine the method for inferring the shock velocity from the width of the optical lines. We derive an average shock velocity ~125 km/s, which corresponds to a dynamical age of ~600,000 years for the bubble, and an average mechanical power P_w ~ 10^{40} erg/s; thus, the mechanical power is a few times higher than the current photon luminosity. With Very Large Array observations, we discovered and resolved a powerful radio bubble with the same size as the optical bubble, and a 1.4-GHz luminosity ~10^{35} erg/s, at the upper end of the luminosity range for this type of source. We explain why ULX bubbles tend to become more radio luminous as they expand while radio supernova remnants tend to fade.
We report the discovery of Pavo, a faint (\(M_V = -10.0\)), star-forming, irregular, and extremely isolated dwarf galaxy at \(D\approx2\) Mpc. Pavo was identified in Dark Energy Camera Legacy Survey ...imaging via a novel approach that combines low surface brightness galaxy search algorithms and machine learning candidate classifications. Follow-up imaging with the Inamori-Magellan Areal Camera & Spectrograph on the 6.5 m Magellan Baade telescope revealed a color--magnitude diagram (CMD) with an old stellar population, in addition to the young population that dominates the integrated light, and a tip-of-the-red-giant-branch distance estimate of \(1.99^{+0.20}_{-0.22}\) Mpc. The blue population of stars in the CMD is consistent with the youngest stars having formed no later than 150 Myr ago. We also detected no H\(\alpha\) emission with SOAR telescope imaging, suggesting we may be witnessing a temporary low in Pavo's star formation. We estimate the total stellar mass of Pavo to be \(\log M_\ast/\mathrm{M_\odot} = 5.6 \pm 0.2\) and measure an upper limit on its HI gas mass of \(1.0 \times 10^6\,\mathrm{M_\odot}\) based on the HIPASS survey. Given these properties, Pavo's closest analog is Leo P (\(D=1.6\) Mpc), previously the only known isolated, star-forming, Local Volume dwarf galaxy in this mass range. However, Pavo appears to be even more isolated, with no other known galaxy residing within over 600 kpc. As surveys and search techniques continue to improve, we anticipate an entire population of analogous objects being detected just outside the Local Group.
We have studied the unusual time variability of an ultraluminous X-ray source in M 101, 4XMM J140314.2\(+\)541806 (henceforth, J1403), using Chandra and XMM-Newton data. Over the last two decades, ...J1403 has shown short-duration outbursts with an X-ray luminosity \(\sim1-3 \times 10^{39}\) erg s\(^{-1}\), and longer intervals at luminosities \(\sim0.5-1 \times 10^{38}\) erg s\(^{-1}\). The bimodal behaviour and fast outburst evolution (sometimes only a few days) are more consistent with an accretor/propeller scenario for a neutron star than with the canonical outburst cycles of stellar-mass black holes. If this scenario is correct, the luminosities in the accretor and propeller states suggest a fast spin (\(P \approx\) 5 ms) and a low surface magnetic field (\(B \sim 10^{10}\) G), despite our identification of J1403 as a high-mass X-ray binary. The most striking property of J1403 is the presence of strong \(\sim\)600-s quasi periodic oscillations (QPOs), mostly around frequencies of \(\approx 1.3-1.8\) mHz, found at several epochs during the ultraluminous regime. We illustrate the properties of such QPOs, in particular their frequency and amplitude changes between and within observations, with a variety of techniques (Fast Fourier Transforms, Lomb-Scargle periodograms, weighted wavelet Z-transform analysis). The QPO frequency range \(<\)10 mHz is an almost unexplored regime in X-ray binaries and ultraluminous X-ray sources. We compare our findings with the (few) examples of very low frequency variability found in other accreting sources, and discuss possible explanations (Lense-Thirring precession of the inner flow or outflow; radiation pressure limit-cycle instability; marginally stable He burning on the neutron star surface).
This thesis uses the published historical literature to build a theoretical model of the political organization of P’urépecha cities. Ancient P’urépecha cities were the urban component of a larger ...polity known as an Ireta. These were territorial polities that were similar to the Aztec altepetl, and might be considered analogous to a “city-state.” Each Ireta could be divided into a series of nested territorial units. Larger units, the uapátzequecha, consisted of neighborhoods within cities and towns or villages in the countryside. Beneath these were smaller groupings of households that formed the basis of the ocámbecha tax system used by the Kingdom of Tzintzuntzan, the empire which dominated the region during the Late Postclassic Period (c. 1350 – 1530 AD). Small architectural complexes (complejos) at the archaeological site of Angamuco, Michoacan, Mexico approximately match the size of the unit that the ocámbecha administered. This study maps these units using Object-Based Image Analysis (OBIA). The results of this modeling produce a map of complejos that approximately matches hypothesized territorial divisions at the site. While more research is needed, the current evidence suggests that the territorial divisions which formed the basis of the ocámbecha tax system may predate the Late Postclassic empire. This could indicate that the empire simply co-opted existing territorial divisions for tax collection rather than creating new ones.
