Which Stars Form Black Holes and Neutron Stars?

I describe the current state of our knowledge of the mapping between the initial masses of stars and the compact objects -- particularly neutron stars and black holes -- that they produce. Most of that knowledge is theoretical in nature, and relies on uncertain assumptions about mass loss through winds, binary mass transfer, and the amount of mass ejected during a supernovae. Observational constraints on the initial masses of stars that produce neutron stars and black holes is scarce. They fall into three general categories: (1) models of the stars that produced the supernova remnants associated with known compact objects, (2) scenarios through with high mass X-ray binaries were produced, and (3) associations between compact objects and coeval clusters of stars for which the minimum masses of stars that have undergone supernovae are known. I focus on the last category as the most promising in the near term. I describe three highly-magnetized neutron stars that have been associated with progenitors that had initial masses of $>$30\msun, and evaluate the prospects of finding further associations between star clusters and compact objects.Comment: 8 pages, 3 figures. Uses aipproc.cls. To appear in the proceedings of the conference``The Multicoloured Landscape of Compact Objects and their Explosive Origins'', 2006 June 11--24, Cefalu, Sicily, to be published by AI

Millisecond Oscillations During Thermonuclear X-ray Bursts

I review the basic phenomenology and theory of the millisecond brightness oscillations observed during thermonuclear X-ray bursts from 13 of approximately 70 accreting neutron stars in low-mass X-ray binaries. Compelling observations indicate that the oscillations are produced by surface brightness patterns on the rapidly rotating neutron stars. However, it remains to be understood (1) why the brightness patterns producing them persist for up to 15 s during an X-ray burst, whereas the burning should cover the entire surface in less than 1 s, and (2) why the frequencies drift upward by about 5 Hz during the course of the burst. These peculiarities can probably be explained by taking into account the expansion of the surface layers caused by the burning, zonal flows that form due to pressure gradients between the equator and poles, and Rossby-Alfven modes that are excited in the surface ocean. Further progress toward understanding how burning progresses on the surface of the neutron star can be made with a next-generation X-ray timing mission, which would provide a larger sample of sources with oscillations, detect sideband signals produced by the spectrum of modes that should be excited in the neutron star ocean, and measure harmonic structure in the profiles of the oscillations. These observations would be crucial for measuring the distribution of the rotation rates of neutron stars, the progression of unstable nuclear burning in the accreted ocean, and the curvature of the space-time around the neutron star.Comment: Review article for "X-Ray Timing 2003: Rossi and Beyond", ed. P. Kaaret, F. K. Lamb, & J. H. Swank (Melville, NY: American Institute of Physics). 6 pages, including 5 figure

Radial Distribution of X-ray Point Sources near the Galactic Center

(Abridged) We present the LogN-LogS and spatial distributions of X-ray point sources in seven Galactic Bulge (GB) fields within 4 deg from the Galactic Center (GC). We compare the properties of 1159 X-ray point sources discovered in our deep (100 ks) Chandra observations of three low extinction Window fields near the GC with the X-ray sources in the other GB fields centered around Sgr B2, Sgr C, the Arches Cluster and Sgr A* using Chandra archival data. To reduce the systematic errors induced by the uncertain X-ray spectra of the sources coupled with field-and-distance dependent extinction, we classify the X-ray sources using quantile analysis and estimate their fluxes accordingly. The result indicates the GB X-ray population is highly concentrated at the center, more heavily than the stellar distribution models. We also compare the total X-ray and infrared surface brightness using the Chandra and Spitzer observations of the regions. The radial distribution of the total infrared surface brightness from the 3.6 band $\mu$m images appears to resemble the radial distribution of the X-ray point sources better than predicted by the stellar distribution models. Assuming a simple power law model for the X-ray spectra, the closer to the GC the intrinsically harder the X-ray spectra appear, but adding an iron emission line at 6.7 keV in the model allows the spectra of the GB X-ray sources to be largely consistent across the region. This implies that the majority of these GB X-ray sources can be of the same or similar type. Their X-ray luminosity and spectral properties support the idea that the most likely candidate is magnetic cataclysmic variables (CVs), primarily intermediate polars (IPs). Their observed number density is also consistent with the majority being IPs.Comment: 17 pages, 7 figures, 6 tables, accepted for publication in Astrophysical Journa

Mid-infrared emission from dust around quiescent low-mass X-ray binaries

We report the discovery of excess 4.5 and 8 μm emission from three quiescent black hole low-mass X-ray binaries, A0620−00, GS 2023+338, and XTE J1118+480, and the lack of similar excess emission from Cen X-4. The mid-infrared emission from GS 2023+338 probably originates in the accretion disk. However, the excess emission from A0620−00 and XTE J1118+480 is brighter and peaks at longer wavelengths, and thus most likely originates from circumbinary dust that is heated by the light of the secondary star. For these two sources, we find that the inner edges of the dust distributions lie near 1.7 times the binary separations, which are the minimum radii at which circumbinary disks would be stable against tidal disruption. The excesses are weak at 24 μm, which implies that the dust does not extend beyond about 3 times the binary separations. The total masses of circumbinary material are between 10^22 and 10^24 g. The material could be the remains of fallback disks produced in supernovae, or material from the companions injected into circumbinary orbits during mass transfer

Blackness is not a monolith: Deconstructing Black Identities in the Higher Education Setting

A Pew research study found that one in five Black people in America are immigrants or are children of immigrants. We see the rise of immigration, but the vast majority of studies still treat Black people as a monolith. Much research on the “Black experience” never specifies the ethnic background of those they are interviewing and when speaking about race it conflates the experiences without giving context to why those experiences are different. Much research uses Black people or African Americans hide behind the clear distinction of those they interview.For my project I am studying, How Black immigrants and Native Black Americans experience higher education differs and why. I highlight three main ideas that occurred regularly within the literature I researched. 1. These ways that American societies view these two groups and how it affects their experience in higher education. 2. The discussion around high admission and attainment rates of Black immigrants in higher education and the low rates of Native Black Americans in higher education. 3. The way both communities individually see themselves and the impact of the first two points on these communities. I review 20 sources on where research about these communities is currently at academically, how well the studies are conducted, and the value of the information that we learn from it. With that information, we can start the conversation and later systematic change, on how to give different groups of Black people the different support that is needed to be successful in higher education

Bright Supernovae from Magnetar Birth

Following an initial explosion that might be launched either by magnetic interactions or neutrinos, a rotating magnetar radiating according to the classic dipole formula could power a very luminous supernova. While some 56Ni might be produced in the initial explosion, the peak of the light curve in a Type I supernova would not be directly related to its mass. In fact, the peak luminosity would be most sensitive to the dipole field strength of the magnetar. The tail of the light curve could resemble radioactive decay for some time but, assuming complete trapping of the pulsar emission, would eventually be brighter. Depending on the initial explosion energy, both high and moderate velocities could accompany a very luminous light curve.Comment: submitted to Astrophysical Journal Letter

The behavior of subluminous X-ray transients near the Galactic center as observed using the X-ray telescope aboard Swift

In this paper we report on the spectral analysis of seven X-ray transients, which were found to be active during a monitoring campaign of the Galactic center carried out in 2006 and 2007 using the X-ray telescope aboard the Swift satellite. This campaign detected new outbursts of five known X-ray transients and discovered two new systems. Their 2-10 keV peak luminosities range from 1E34 to 6E36 erg/s. Two of the sources discussed in this paper are confirmed neutron star systems (AX J1745.6-2901 and GRS 1741-2853), while the five others have an unknown nature. We discuss the characteristics of the observed outbursts and the duty cycles of the various systems. Several of the detected transients seem to undergo enhanced X-ray activity with levels intermediate between quiescence and full outburst. We discuss the possibility that the subluminous appearance of the eclipsing X-ray burster AX J1745.6-2901 is due to line-of-sight effects. We detected two type-I X-ray bursts with a duration of 50-60 seconds from AX J1745.6-2901, which we discuss in view of the bursting behavior of low-luminosity X-ray transients. Assuming that we are dealing with accreting neutron stars and black holes, we estimate the time-average accretion rate, Mdot, of the transients, which is an important input parameter for binary evolution models that attempt to explain the nature of subluminous X-ray transients. Our estimates lie in the range of 3E-13 Msun/yr < Mdot < 1E-10 Msun/yr, if the systems are neutron star X-ray binaries and between 4E-14 Msun/yr < Mdot < 2E-11 Msun/yr for a scenario where the accreting object is a black hole. Some of the systems have such low estimated mass-accretion rates that they possibly pose a challenge for binary evolution models.Comment: Accepted for publication in A&A, some textual changes according to referee's report, 13 pages, 4 figure

Discovery of hot supergiant stars near the Galactic center

We report new results of a campaign to find Wolf-Rayet and O (WR/O) stars and high-mass X-ray binaries (HMXBs) in the Galactic center. We searched for candidates by cross-correlating the 2MASS catalog with a deep Chandra catalog of X-ray point sources in the Radio Arches region. Following up with K-band spectroscopy, we found two massive stellar counterparts to CXOGC J174555.3-285126 and CXOGC J174617.0-285131, which we classify as a broad-lined WR star of sub-type WN6b and an O Ia supergiant, respectively. Their X-ray properties are most consistent with those of known colliding-wind binaries in the Galaxy and the Large Magellanic Cloud, although a scenario involving low-rate accretion onto a compact object is also possible. The O Ia star lies 4.4 pc in projection from the Quintuplet cluster, and has a radial velocity consistent with that of the Quintuplet, suggesting that this star might have escaped from the cluster. We also present the discovery of a B2 Ia supergiant, which we identified as a candidate massive star using 8 micron Spitzer maps of the Galactic center in a region near the known massive X-ray-emitting star CXOGC J174516.1-290315. We discuss the origin of these stars in the context of evolving stellar clusters in the Galactic center.Comment: 21 pages, 5 figures, accepted for publication in the Astrophysical Journa

A four-year baseline Swift study of enigmatic X-ray transients located near the Galactic center

We report on continued monitoring observations of the Galactic center carried out by the X-ray telescope aboard the Swift satellite in 2008 and 2009. This campaign revealed activity of the five known X-ray transients AX J1745.6-2901, CXOGC J174535.5-290124, GRS 1741-2853, XMM J174457-2850.3 and CXOGC J174538.0-290022. All these sources are known to undergo very faint X-ray outbursts with 2-10 keV peak luminosities of Lx,peak~1E34-1E36 erg/s, although the two confirmed neutron star low-mass X-ray binaries AX J1745.6-2901 and GRS 1741-2853 can also become brighter (Lx,peak~1E36-1E37 erg/s). We discuss the observed long-term lightcurves and X-ray spectra of these five enigmatic transients. In 2008, AX J1745.6-2901 returned to quiescence following an unusually long accretion outburst of more than 1.5 years. GRS 1741-2853 was active in 2009 and displayed the brightest outburst ever recorded for this source, reaching up to a 2-10 keV luminosity of Lx~1E37 (D/7.2 kpc)^2 erg/s. This system appears to undergo recurrent accretion outbursts approximately every 2 years. Furthermore, we find that the unclassified transient XMM J174457-2850.3 becomes bright only during short episodes (days) and is often found active in between quiescence (Lx~1E32 erg/s) and its maximum outburst luminosity of Lx~1E36 erg/s. CXOGC J174535.5-290124 and CXOGC J174538.0-290022, as well as three other very-faint X-ray transients that were detected by Swift monitoring observations in 2006, have very low time-averaged mass-accretion rates of ~< 2E-12 Msun/yr. Despite having obtained two years of new data in 2008 and 2009, no new X-ray transients were detected.Comment: Minor textual revisions according to referee report, accepted for publication in A&

Discovery of a 552 Hz burst oscillation in the low-mass X-ray binary EXO 0748-676

We report the detection of pulsations at 552 Hz in the rising phase of two type-I (thermonuclear) X-ray bursts observed from the accreting neutron star EXO 0748-676 in 2007 January and December, by the Rossi X-ray Timing Explorer. The fractional amplitude was 15% (rms). The dynamic power density spectrum for each burst revealed an increase in frequency of approx. 1-2 Hz while the oscillation was present. The frequency drift, the high significance of the detections and the almost identical signal frequencies measured in two bursts separated by 11 months, confirms this signal as a burst oscillation similar to those found in 13 other sources to date. We thus conclude that the spin frequency in EXO 0748-676 is within a few Hz of 552 Hz, rather than 45 Hz as was suggested from an earlier signal detection by Villarreal & Strohmayer (2004). Consequently, Doppler broadening must significantly affect spectral features arising from the neutron star surface, so that the narrow absorption features previously reported from an XMM-Newton spectrum could not have arisen there. The origin of both the previously reported 45 Hz oscillation and the X-ray absorption lines is now uncertain.Comment: 5 pages, 2 figures, accepted for Astrophysical Journal Letters. Minor changes following referees repor