Haldan Cohn - Indiana University Department of Astronomy Bloomington

Haldan Cohn
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Haldan Cohn
Indiana University Department of Astronomy Bloomington
United States

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Astrophysics (7)
Astrophysics of Galaxies (3)
High Energy Astrophysical Phenomena (3)
Solar and Stellar Astrophysics (2)

Publications Authored By Haldan Cohn

We present near-simultaneous Chandra/HST observations of the very faint ($L_{x} < 10^{36}$ erg s$^{-1}$) X-ray transient source M15 X-3, as well as unpublished archival Chandra observations of M15 X-3. The Chandra observations constrain the luminosity of M15 X-3 to be $< 10^{34}$ erg s$^{-1}$ in all observed epochs. The X-ray spectrum shows evidence of curvature, and prefers a fit to a broken power-law with break energy $E_{\rm break} = 2. Read More

We report a sensitive X-ray search for the proposed intermediate mass black hole (IMBH) in the massive Galactic cluster, Omega Centauri (NGC 5139). Combining Chandra X-ray Observatory data from Cycles 1 and 13, we obtain a deep (~291 ks) exposure of the central regions of the cluster. We find no evidence for an X-ray point source near any of the cluster's proposed dynamical centers, and place an upper limit on the X-ray flux from a central source of f_X(0. Read More

We report the results of a joint \chandra-\hst study of the X-ray binary population in the massive, high-density globular cluster NGC 6388. NGC 6388 has one of the highest predicted X-ray binary production rate of any Galactic cluster. We detected a large population of 61 \chandra sources within the half-mass radius with L$_X > 5 \times 10^{30}$ \ergs. Read More

We have developed a set of dynamically evolving Fokker-Planck models for the collapsed-core globular star cluster M15, which directly address the issue of whether a central black hole is required to fit Hubble Space Telescope (HST) observations of the stellar spatial distribution and kinematics. As in our previous work reported by Dull et al., we find that a central black hole is not needed. Read More

We have searched for optical identifications for 79 Chandra X-ray sources that lie within the half-mass radius of the nearby, core-collapsed globular cluster NGC 6397, using deep Hubble Space Telescope Advanced Camera for Surveys Wide Field Channel imaging in H-alpha, R, and B. Photometry of these images allows us to classify candidate counterparts based on color-magnitude diagram location. In addition to recovering nine previously detected cataclysmic variables (CVs), we have identified six additional faint CV candidates, a total of 42 active binaries (ABs), two millisecond pulsars (MSPs), one candidate active galactic nucleus, and one candidate interacting galaxy pair. Read More

Affiliations: 1Indiana University Department of Astronomy Bloomington, 2Indiana University Department of Astronomy Bloomington, 3Indiana University Department of Astronomy Bloomington, 4Indiana University Department of Astronomy Bloomington, 5Space Telescope Science Institute, 6Department of Physics and Astronomy, San Francisco State University, 7Max Planck Institute for Astrophysics - Instituto de Ciencias del Espacio

We performed deep photometry of the central region of Galactic globular cluster M15 from archival Hubble Space Telescope data taken on the High Resolution Channel and Solar Blind Channel of the Advanced Camera for Surveys. Our data set consists of images in far-UV (FUV$_{140}$; F140LP), near-UV (NUV$_{220}$; F220W), and blue (B$_{435}$; F435W) filters. The addition of an optical filter complements previous UV work on M15 by providing an additional constraint on the UV-bright stellar populations. Read More

We present a deep Chandra X-ray Observatory study of the peculiar binary radio millisecond pulsar PSR J1740--5340 and candidate millisecond pulsars (MSPs) in the globular cluster NGC 6397. The X-rays from PSR J1740--5340 appear to be non-thermal and exhibit variability at the binary period. These properties suggest the presence of a relativistic intrabinary shock formed due to interaction of a relativistic rotation-powered pulsar wind and outflow from the unusual "red-straggler/sub-subgiant" companion. Read More

We have identified an X-ray transient (hereafter M15 X-3) in the globular cluster M15 from an archival Chandra grating observation. M15 X-3 appears at an X-ray luminosity of 6*10^{33} ergs/s with a spectrum consistent with an absorbed power law of photon index 1.51+-0. Read More

We report the detection of six discrete, low-luminosity (Lx < 10^33 erg/s) X-ray sources, located within 12 arcsec of the center of the collapsed-core globular cluster M30 (NGC 7099), and a total of 13 sources within the half-mass radius, from a 50 ksec Chandra ACIS-S exposure. Three sources lie within the very small upper limit of 1.9 arcsec on the core radius. Read More

The Chandra Multiwavength Plane (ChaMPlane) Survey of the galactic plane incorporates serendipitous sources from selected Chandra pointings in or near the galactic plane (b < 12deg; >20 ksec; lack of bright diffuse or point sources) to measure or constrain the luminosity function of low-luminosity accretion sources in the Galaxy. The primary goal is to detect and identify accreting white dwarfs (cataclysmic variables, with space density still uncertain by a factor of >10-100), neutron stars and black holes (quiescent low mass X-ray binaries) to constrain their space densities and thus origin and evolution. Secondary objectives are to identify Be stars in high mass X-ray binaries and constrain their space densities, and to survey the H-R diagram for stellar coronal sources. Read More

Affiliations: 1Harvard-Smithsonian Center for Astrophysics, 2Harvard-Smithsonian Center for Astrophysics, 3Harvard-Smithsonian Center for Astrophysics, 4Harvard-Smithsonian Center for Astrophysics, 5Harvard-Smithsonian Center for Astrophysics, 6Harvard-Smithsonian Center for Astrophysics, 7Indiana University, 8Indiana University
Category: Astrophysics

We provide a brief description of the optical survey being conducted under the NOAO Long Term Surveys program in support of the Chandra Multiwavelength Plane (ChaMPlane) Survey (see paper by Grindlay et al. in this Volume). A representative photometry result is shown, along with spectroscopic followup. Read More

We report results from HST-NICMOS imaging of the extremely dense core of the globular cluster Terzan 5. This highly obscured bulge cluster has been estimated to have one of the highest collision rates of any galactic globular cluster, making its core a particularly conducive environment for the production of interacting binary systems. We have reconstructed high-resolution images of the central 19"x19" region of Terzan 5 by application of the drizzle algorithm to dithered NIC2 images in the F110W, F187W, and F187N near-infrared filters. Read More

Affiliations: 1Harvard College Observatory, 2Harvard College Observatory, 3San Francisco State University, 4Indiana University, 5Indiana University, 6Yale University
Category: Astrophysics

We have used HST/FOS to study faint UV stars in the core of the nearby globular cluster NGC 6397. We confirm the presence of a 4th cataclysmic variable (CV) in NGC 6397 (CV 4), and we use the photometry of Cool et al. (1998) to present evidence that CVs 1--4 all have faint disks and probably low accretion rates. Read More


We present evidence that the globular cluster NGC 6397 contains two distinct classes of centrally-concentrated UV-bright stars. Color-magnitude diagrams constructed from U, B, V, and I data obtained with the HST/WFPC2 reveal seven UV-bright stars fainter than the main-sequence turnoff, three of which had previously been identified as cataclysmic variables (CVs). Lightcurves of these stars show the characteristic ``flicker'' of CVs, as well as longer-term variability. Read More