arXiv subjects

Frank Verbunt

Publications and source records attributed to Frank Verbunt.

At least 19 recordsLinked to original sources

arXiv2021

The star catalogue of Wilhelm IV, Landgraf von Hessen-Kassel

Near the end of the 16th century Wilhelm IV, Landgraf von Hessen-Kassel, set up an observatory with the main goal to increase the accuracy of stellar positions primarily for use in astrology and for calendar purposes. A new star catalogue was compiled from measurements of altitudes and angles between stars and a print ready version was prepared listing measurements as well as equatorial and ecliptic coordinates of stellar positions. Unfortunately, this catalogue appeared in print not before 1666, long after the dissemination of Brahe's catalogue. With the data given in the manuscript we are able to analyze the accuracy of measurements and computations. The measurements and the computations are very accurate, thanks to the instrument maker and mathematician Jost Bürgi. The star catalogue is more accurate by a factor two than the later catalogue of Tycho Brahe.

astro-ph.IM↗

arXiv2021

The star catalogue of Wilhelm IV, Landgraf von Hessen-Kassel: Accuracy of the catalogue and of the measurements

We analyse a manuscript star catalogue by Wilhem IV, Landgraf von Hessen-Kassel, from 1586. From measurements of altitudes and of angles between stars, given in the catalogue, we find that the measurement accuracy averages 26 arcsec for eight fundamental stars, compared to 49 arcsec of the measurements by Brahe. The computation in converting altitudes to declinations and angles between stars to celestial position is very accurate, with errors negligible with respect to the measurement errors. Due to an offset in the position of the vernal equinox the positional error of the catalogue is slightly worse than that of Brahe's catalogue, but when correction is made for the offset -- which was known to 17th century astronomers -- the catalogue is more accurate than that of Brahe by a factor two. We provide machine-readable Tables of the catalogue.

astro-ph.IM↗

arXiv2019

Why Halley did not discover proper motion and why Cassini did

In 1717 Halley compared contemporaneous measurements of the latitudes of four stars with earlier measurements by ancient Greek astronomers and by Brahe, and from the differences concluded that these four stars showed proper motion. An analysis with modern methods shows that the data used by Halley do not contain significant evidence for proper motion. What Halley found are the measurement errors of Ptolemaios and Brahe. Halley further argued that the occultation of Aldebaran by the Moon on 11 March 509 in Athens confirmed the change in latitude of Aldebaran. In fact, however, the relevant observation was almost certainly made in Alexandria where Aldebaran was not occulted. By carefully considering measurement errors Jacques Cassini showed that Halley's results from comparison with earlier astronomers were spurious, a conclusion partially confirmed by various later authors. Cassini's careful study of the measurements of the latitude of Arcturus provides the first significant evidence for proper motion.

physics.hist-ph↗

arXiv2017

A new look at distances and velocities of neutron stars

We take a fresh look at the determination of distances and velocities of neutron stars. The conversion of a parallax measurement into a distance, or distance probability distribution, has led to a debate quite similar to the one involving Cepheids, centering on the question whether priors can be used when discussing a single system. With the example of PSRJ0218+4232 we show that a prior is necessary to determine the probability distribution for the distance. The distance of this pulsar implies a gamma-ray luminosity larger than 10% of its spindown luminosity. For velocities the debate is whether a single Maxwellian describes the distribution for young pulsars. By limiting our discussion to accurate (VLBI) measurements we argue that a description with two Maxwellians, with distribution parameters sigma1=77 and sigma2=320 km/s, is significantly better. Corrections for galactic rotation, to derive velocities with respect to the local standards of rest, are insignificant.

astro-ph.HE↗

arXiv2017

The observed velocity distribution of young pulsars

We argue that comparison with observations of theoretical models for the velocity distribution of pulsars must be done directly with the observed quantities, i.e. parallax and the two components of proper motion. We develop a formalism to do so, and apply it to pulsars with accurate VLBI measurements. We find that a distribution with two maxwellians improves significantly on a single maxwellian. The `mixed' model takes into account that pulsars move away from their place of birth, a narrow region around the galactic plane. The best model has 42% of the pulsars in a maxwellian with average velocity sigma sqrt{8/pi}=120 km/s, and 58% in a maxwellian with average velocity 540 km/s. About 5% of the pulsars has a velocity at birth less than 60\,km/s. For the youngest pulsars (tau_c<10 Myr), these numbers are 32% with 130 km/s, 68% with 520 km/s, and 3%, with appreciable uncertainties.

astro-ph.HE↗

arXiv2017

The mass of the black hole in 1A 0620-00, revisiting the ellipsoidal light curve modeling

The mass distribution of stellar mass black holes can provide important clues to supernova modeling, but observationally it is still ill constrained. Therefore it is of importance to make black hole mass measurements as accurate as possible. The X-ray transient 1A 0620-00 is well studied, with a published black hole mass of $6.61\pm0.25\,$M$_{\odot}$, based on an orbital inclination $i$ of $51.0\pm0.9$ degrees. This was obtained by Cantrell et al. (2010), as an average of independent fits to $V$-, $I$- and $H$-band light curves. In this work we perform an independent check on the value of $i$ by re-analyzing existing YALO/SMARTS $V$-, $I$- and $H$-band photometry, using different modeling software and fitting strategy. Performing a fit to the three light curves simultaneously, we obtain a value for $i$ of $54.1\pm1.1$ degrees, resulting in a black hole mass of $5.86\pm0.24\,$M$_{\odot}$. Applying the same model to the light curves individually, we obtain $58.2\pm1.9$, $53.6\pm1.6$ and $50.5\pm2.2$ degrees for $V$-, $I$- and $H$-band, respectively, where the differences in best-fitting $i$ are caused by the contribution of the residual accretion disc light in the three different bands. We conclude that the mass determination of this black hole may still be subject to systematic effects exceeding the statistical uncertainty. Obtaining more accurate masses would be greatly helped by continuous phase-resolved spectroscopic observations simultaneous with photometry.

astro-ph.HE↗

arXiv2013

On the disruption of pulsar and X-ray binaries in globular clusters

The stellar encounter rate Gamma has been shown to be strongly correlated with the number of X-ray binaries in clusters and also to the number of radio pulsars. However, the pulsar populations in different clusters show remarkably different characteristics: in some GCs the population is dominated by binary systems, in others by single pulsars and exotic systems that result from exchange encounters. In this paper, we describe a second dynamical parameter for globular clusters, the encounter rate for a single binary, gamma. We find that this parameter provides a good characterization of the differences between the pulsar populations of different globular clusters. The higher gamma is for any particular globular cluster the more isolated pulsars and products of exchange interactions are observed. Furthermore, we also find that slow and "young" pulsars are found almost exclusively in clusters with a high gamma; this suggests that these kinds of objects are formed by the disruption of X-ray binaries, thus halting the recycling of a previously dead neutron star. We discuss the implications of this for the nature of young pulsars and for the formation of neutron stars in globular clusters.

astro-ph.SR↗

arXiv2013

A Chandra X-ray study of the interacting binaries in the old open cluster NGC6791

We present the first X-ray study of NGC6791, one of the oldest open clusters known (8 Gyr). Our Chandra observation is aimed at uncovering the population of close interacting binaries down to Lx ~ 1e30 erg/s (0.3-7 keV). We detect 86 sources within 8 arcmin of the cluster center, including 59 inside the half-mass radius. We identify twenty sources with proper-motion cluster members, which are a mix of cataclysmic variables (CVs), active binaries (ABs), and binaries containing sub-subgiants. With follow-up optical spectroscopy we confirm the nature of one CV. We discover one new, X-ray variable candidate CV with Balmer and HeII emission lines in its optical spectrum; this is the first X-ray--selected CV confirmed in an open cluster. The number of CVs per unit mass is consistent with the field, suggesting that the 3-4 CVs observed in NGC6791 are primordial. We compare the X-ray properties of NGC6791 with those of a few old open (NGC6819, M67) and globular clusters (47Tuc, NGC6397). It is puzzling that the number of ABs brighter than 1e30 erg/s normalized by cluster mass is lower in NGC6791 than in M67 by a factor ~3 to 7. CVs, ABs, and sub-subgiants brighter than 1e30 erg/s are under-represented per unit mass in the globular clusters compared to the oldest open clusters, and this accounts for the lower total X-ray luminosity per unit mass of the former. This indicates that the net effect of dynamical encounters may be the destruction of even some of the hardest (i.e. X-ray--emitting) binaries.

astro-ph.HE↗

arXiv2012

A phenomenological model for the X-ray spectrum of Nova V2491 Cygni

The X-ray flux of Nova V2491 Cyg reached a maximum some forty days after optical maximum. The X-ray spectrum at that time, obtained with the RGS of XMM-Newton, shows deep, blue-shifted absorption by ions of a wide range of ionization. We show that the deep absorption lines of the X-ray spectrum at maximum, and nine days later, are well described by the following phenomenological model with emission from a central blackbody and from a collisionally ionized plasma (CIE). The blackbody spectrum (BB) is absorbed by three main highly-ionized expanding shells; the CIE and BB are absorbed by cold circumstellar and interstellar matter that includes dust. The outflow density does not decrease monotonically with distance. The abundances of the shells indicate that they were ejected from an O-Ne white dwarf. We show that the variations on time scales of hours in the X-ray spectrum are caused by a combination of variation in the central source and in the column density of the ionized shells. Our phenomenological model gives the best description so far of the supersoft X-ray spectrum of nova V2491 Cyg, but underpredicts, by a large factor, the optical and ultraviolet flux. The X-ray part of the spectrum must originate from a very different layer in the expanding envelope, presumably much closer to the white dwarf than the layers responsible for the optical/ultraviolet spectrum. This is confirmed by absence of any correlation between the X-ray and UV/optical observed fluxes.

astro-ph.HE↗

arXiv2012

The star catalogues of Ptolemaios and Ulugh Beg: Machine-readable versions and comparison with the modern Hipparcos Catalogue

In late antiquity and throughout the middle ages, the positions of stars on the celestial sphere were obtained from the star catalogue of Ptolemaios. A catalogue based on new measurements appeared in 1437, with positions by Ulugh Beg, and magnitudes from the 10th-century astronomer al-Sufi. We provide machine-readable versions of these two star catalogues, based on the editions by Toomer (1998) and Knobel (1917), and determine their accuracies by comparison with the modern Hipparcos Catalogue. The magnitudes in the catalogues correlate well with modern visual magnitudes; the indication `faint' by Ptolemaios is found to correspond to his magnitudes 5 and 6. Gaussian fits to the error distributions in longitude / latitude give widths sigma ~ 27 arcmin / 23 arcmin in the range |Delta lambda, Delta beta|<50 arcmin for Ptolemaios and sigma ~ 22 arcmin /18 arcmin in Ulugh Beg. Fits to the range |Delta lambda, Delta beta|<100 arcmin gives 10-15 per cent larger widths, showing that the error distributions are broader than gaussians. The fraction of stars with positions wrong by more than 150 arcmin is about 2 per cent for Ptolemaios and 0.1 per cent in Ulugh Beg; the numbers of unidentified stars are 1 in Ptolemaios and 3 in Ulugh Beg. These numbers testify to the excellent quality of both star catalogues (as edited by Toomer and Knobel).

astro-ph.IM↗

arXiv2011

Early star catalogues of the southern sky: De Houtman, Kepler (Second and Third Classes), and Halley

De Houtman in 1603, Kepler in 1627 and Halley in 1679 published the earliest modern catalogues of the southern sky. We provide machine-readable versions of these catalogues, make some comparisons between them, and briefly discuss their accuracy on the basis of comparison with data from the modern Hipparcos Catalogue. We also compare our results for De Houtman with those by Knobel (1917) finding good overall agreement. About half of the about 200 new stars (with respect to Ptolemaios) added by De Houtman are in twelve new constellations, half in old constellations like Centaurus, Lupus and Argo. The right ascensions and declinations given by De Houtman have error distributions with widths of about 40 arcmin, the longitudes and latitudes given by Kepler have error distributions with widths of about 45 arcmin. Halley improves on this by more than an order of magnitude to widths of about 3 arcmin, and all entries in his catalogue can be identified. The measurement errors of Halley are due to a systematic deviation of his sextant (increasing with angle to 2 arcmin at 60 degrees) and random errors of 0.7 arcmin. The position errors in the catalogue of Halley are dominated by the position errors in the reference stars, which he took from Brahe.

astro-ph.IM↗

arXiv2010

Three editions of the Star Catalogue of Tycho Brahe

Tycho Brahe completed his catalogue with the positions and magnitudes of 1004 fixed stars in 1598. This catalogue circulated in manuscript form. Brahe edited a shorter version with 777 stars, printed in 1602, and Kepler edited the full catalogue of 1004 stars, printed in 1627. We provide machine-readable versions of the three versions of the catalogue, describe the differences between them and briefly discuss their accuracy on the basis of comparison with modern data from the Hipparcos Catalogue. We also compare our results with earlier analyses by Dreyer (1916) and Rawlins (1993), finding good overall agreement. The magnitudes given by Brahe correlate well with modern values, his longitudes and latitudes have error distributions with widths of about 2 arcmin, with excess numbers of stars with larger errors (as compared to Gaussian distributions), in particular for the faintest stars. Errors in positions larger than 10 arcmin, which comprise about 15 per cent of the entries, are likely due to computing or copying errors.

astro-ph.IM↗

arXiv2010

The Star Catalogue of Hevelius

The catalogue by Johannes Hevelius with the positions and magnitudes of 1564 entries was published by his wife Elisabeth Koopman in 1690. We provide a machine-readable version of the catalogue, and briefly discuss its accuracy on the basis of comparison with data from the modern Hipparcos Catalogue. We compare our results with an earlier analysis by Rybka (1984), finding good overall agreement. The magnitudes given by Hevelius correlate well with modern values. The accuracy of his position measurements is similar to that of Brahe, with sigma=2 arcmin for with more errors larger than 5 arcmin than expected for a Gaussian distribution. The position accuracy decreases slowly with magnitude. The fraction of stars with position errors larger than a degree is 1.5 per cent, rather smaller than the fraction of 5 per cent in the star catalogue of Brahe.

astro-ph.IM↗

arXiv2010

X-ray and Optical Study of Low Core Density Globular Clusters NGC6144 and E3

We report on the Chandra X-ray Observatory and Hubble Space Telescope observation of two low core density globular clusters, NGC6144 and E3. By comparing the number of X-ray sources inside the half-mass radius to those outside, we found 6 X-ray sources within the half-mass radius of NGC6144, among which 4 are expected to be background sources; 3 X-ray sources are also found within the half-mass radius of E3, of which 3 is expected to be background source. Therefore, we cannot exclude that all our sources are background sources. However, combining the results from X-ray and optical observations, we found that 1-2 sources in NGC6144 and 1 source in E3 are likely to be cataclysmic variables and that 1 source in NGC6144 is an active binary, based on the X-ray and optical properties. The number of faint X-ray sources in NGC6144 and E3 found with Chandra and HST is higher than a prediction based on collision frequency, but is closer to that based on mass. Our observations strongly suggest that the compact binary systems in NGC6144 and E3 are primordial in origin.

astro-ph.HE↗

arXiv2009

X-ray Sources and Their Optical Counterparts in the Galactic Globular Cluster M12 (NGC 6218)

We study a Chandra X-ray Observatory ACIS-S observation of the Galactic globular clusterM12. With a 26 ks exposure time, we detect 6 X-ray sources inside the half-mass radius (2.16 arcminutes) of which two are inside the core radius (0.72 arcminutes) of the cluster. If we assume these sources are all associated with globular cluster M12, the luminosity L_{X} among these sources between 0.3-7.0 keV varies roughly from 10^{30} to 10^{32} ergs s^{-1}. For identification, we also analyzed the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) and Wide Field and Planetary Camera 2 (WFPC2) data and identified the optical counterparts to five X-ray sources inside the HST ACS field of view. According to the X-ray and optical features, we found 2-5 candidate active binaries (ABs) or cataclysmic variables (CVs) and 0-3 background galaxies within the HST ACS field of view. Based on the assumption that the number of X-ray sources scales with the encounter rate and the mass of the globular cluster, we expect 2 X-ray source inside M12, and the expectation is consistent with our observational results. Therefore, the existence of identified X-ray sources (possible CVs or ABs) in M12 suggests the primordial origin of X-ray sources in globular clusters which is in agreement with previous studies.

astro-ph.HE↗

arXiv2008

When teaching: Out with magnitudes, in with monochromatic luminosities!

The goal of this document is to illustrate that teaching the concepts of magnitudes is a needless complication in introductory astronomy courses, and that use of monochromatic luminosities, rather than arbitrarily defined magnitudes, leads to a large gain in transparency. This illustration is done through three examples: the Hertzsprung-Russell diagram, the cosmic distance ladder, and interstellar reddening. I provide conversion equations from the magnitude-based to the luminosity-based system; a brief discussion; and a reference to sample lecture notes. I suggest that we, astronomers in the 21st century, abolish magnitudes and instead use (apparent) monochromatic luminosities in non-specialist teaching. Given the large gain in transparency I further propose that we seriously consider using (apparent) monochromatic luminosities also in research papers, bringing optical astronomy in line with astronomy at other wavelengths. Comments are welcome.

astro-ph↗

arXiv2007

Observational evidence for the origin of X-ray sources in globular clusters

Low-mass X-ray binaries, recycled pulsars, cataclysmic variables and magnetically active binaries are observed as X-ray sources in globular clusters. We discuss the classification of these systems, and find that some presumed active binaries are brighter than expected. We discuss a new statistical method to determine from observations how the formation of X-ray sources depends on the number of stellar encounters and/or on the cluster mass. We show that cluster mass is not a proxy for the encounter number, and that optical identifications are essential in proving the presence of primordial binaries among the low-luminosity X-ray sources.

astro-ph↗

arXiv2005

Globular Cluster X-ray Sources

After a brief historical overview we discuss the luminous X-ray sources in globular clusters of our Galaxy. This is followed by an overview of the very luminous X-ray sources studied in globular clusters of 14 other galaxies, and a discussion of their formation and the relation to X-ray sources outside globular clusters. We describe the discovery and classification of low-luminosity X-ray sources, and end the review with some remarks on the formation and evolution of X-ray sources in globular clusters. Observational results are summarized in three tables. Comments are very welcome. Please send them to F.W.M.Verbunt@astro.uu.nl and lewin@mit.edu.

astro-ph↗