59 research outputs found
Recommended from our members
Multiwavelength behaviour of Cygnus X-3 and related objects
I present a detailed study of the behaviour of the exotic X-ray binary Cygnus X-3 at radio, (sub)mm, infrared, red-optical and X-ray wavelengths. Further unusual properties of the system are unearthed and previously expounded models are refined by new observations. In order to address the broader picture, a multiwavelength comparison of Cygnus X-3 with other âradio-jetâ X-ray binaries is also undertaken.
Infrared observations of Cyg X-3 at high time resolution reveal many rapid flare events superimposed upon the 4.8 hr (presumed) orbital motion. Photometry simultaneously in the H & K-bands allows strong constraints to be placed upon T & NΔ for the flaring component. Dereddening of RI- J-H-K-L-Lâ photometry places limits on the likely extinction to Cyg X-3 of 4.5 †AJ †7.5 mag. Further infrared study, simultaneous with radio monitoring and observations with OSSE/GRO and the INT shed greater light on the source, including possible orbital colour changes and a longterm correlation between radio and infrared flux levels. Deep imaging of the field reveals many previously undiscovered infrared sources within a few arcsec of Cyg X-3.
Simultaneous millimetre and radio observations of Cyg X-3 reveal anomalously strong mm fluxes. Interpreting this in terms of significant absorption of the cm fluxes during the passage outwards of the radio-emitting plasmons, strengthens the case for a dense stellar wind in the Cyg X-3 system. Further radio and sub(mm) observations of Cyg X-3 during outburst confirm previously observed phenomena such as quenched radio emission prior to outburst, and establish the importance of radiation loss mechanisms in the decay of radio plasmons ejected from the source. A model describing the qualitative behaviour of the source during outburst is presented
Low Luminosity States of the Black Hole Candidate GX 339-4. I. ASCA and Simultaneous Radio/RXTE Observations
We discuss a series of observations of the black hole candidate GX 339-4 in
low luminosity, spectrally hard states. We present spectral analysis of three
separate archival Advanced Satellite for Cosmology and Astrophysics (ASCA) data
sets and eight separate Rossi X-ray Timing Explorer (RXTE) data sets. Three of
the RXTE observations were strictly simultaneous with 843 MHz and 8.3-9.1 GHz
radio observations. All of these observations have (3-9 keV) flux approximately
< 10^{-9} ergs s^{-1} cm^{-2}. The ASCA data show evidence for an 6.4 keV Fe
line with equivalent width 40 eV, as well as evidence for a soft excess that is
well-modeled by a power law plus a multicolor blackbody spectrum with peak
temperature 150-200 eV. The RXTE data sets also show evidence of an Fe line
with equivalent widths 20-140 eV. Reflection models show a hardening of the
RXTE spectra with decreasing X-ray flux; however, these models do not exhibit
evidence of a correlation between the photon index of the incident power law
flux and the solid angle subtended by the reflector. `Sphere+disk'
Comptonization models and Advection Dominated Accretion Flow (ADAF) models also
provide reasonable descriptions of the RXTE data. The former models yield
coronal temperatures in the range 20-50 keV and optical depths of \tau ~ 3. The
model fits to the X-ray data, however, do not simultaneously explain the
observed radio properties. The most likely source of the radio flux is
synchrotron emission from an extended outflow of size greater than O(10^7
GM/c^2).Comment: 18 pages in latex emulateapj.sty. Accepted for publication in the
Astrophysical Journa
Identification of a Likely Radio Counterpart of the Rapid Burster
We have identified a likely radio counterpart to the low-mass X-ray binary
MXB 1730-335 (the Rapid Burster). The counterpart has shown 8.4 GHz radio
on/off behavior correlated with the X-ray on/off behavior as observed by the
RXTE/ASM during six VLA observations. The probability of an unrelated, randomly
varying background source duplicating this behavior is 1-3% depending on the
correlation time scale. The location of the radio source is RA 17h 33m 24.61s;
Dec -33d 23' 19.8" (J2000), +/- 0.1". We do not detect 8.4 GHz radio emission
coincident with type II (accretion-driven) X-ray bursts. The ratio of radio to
X-ray emission during such bursts is constrained to be below the ratio observed
during X-ray persistent emission at the 2.9-sigma level. Synchrotron bubble
models of the radio emission can provide a reasonable fit to the full data set,
collected over several outbursts, assuming that the radio evolution is the same
from outburst to outburst, but given the physical constraints the emission is
more likely to be due to ~hour-long radio flares such as have been observed
from the X-ray binary GRS 1915+105.Comment: 28 pages, 4 figures; accepted for publication in ApJ (no changes
A One-sided, Highly Relativistic Jet from Cygnus X-3
Very Long Baseline Array images of the X-ray binary, Cygnus X-3, were
obtained 2, 4 and 7 days after the peak of a 10 Jy flare on 4 February 1997.
The first two images show a curved one-sided jet, the third a scatter-broadened
disc, presumably at the position of the core. The jet curvature changes from
the first to the second epoch, which strongly suggests a precessing jet. The
ratio of the flux density in the approaching to that in the (undetected)
receding jet is > 330; if this asymmetry is due to Doppler boosting, the
implied jet speed is > 0.81c. Precessing jet model fits, together with the
assumptions that the jet is intrinsically symmetric and was ejected during or
after the major flare, yield the following constraints: the jet inclination to
the line of sight must be < 14 degrees; the cone opening angle must be < 12
degrees; and the precession period must be > 60 days.Comment: 12 pages 7 figures, accepted by Ap
Lord of the Rings: A Kinematic Distance to Circinus X-1 from a Giant X-Ray Light Echo
Circinus X-1 exhibited a bright X-ray flare in late 2013. Follow-up
observations with Chandra and XMM-Newton from 40 to 80 days after the flare
reveal a bright X-ray light echo in the form of four well-defined rings with
radii from 5 to 13 arcminutes, growing in radius with time. The large fluence
of the flare and the large column density of interstellar dust towards Circinus
X-1 make this the largest and brightest set of rings from an X-ray light echo
observed to date. By deconvolving the radial intensity profile of the echo with
the MAXI X-ray lightcurve of the flare we reconstruct the dust distribution
towards Circinus X-1 into four distinct dust concentrations. By comparing the
peak in scattering intensity with the peak intensity in CO maps of molecular
clouds from the Mopra Southern Galactic Plane CO Survey we identify the two
innermost rings with clouds at radial velocity ~ -74 km/s and ~ -81 km/s,
respectively. We identify a prominent band of foreground photoelectric
absorption with a lane of CO gas at ~ -32 km/s. From the association of the
rings with individual CO clouds we determine the kinematic distance to Circinus
X-1 to be kpc. This distance rules out
earlier claims of a distance around 4 kpc, implies that Circinus X-1 is a
frequent super-Eddington source, and places a lower limit of on the Lorentz factor and an upper limit of on the jet viewing angle.Comment: 20 pages, 21 figures, Astrophysical Journal, in prin
Low Luminosity States of the Black Hole Candidate GX 339-4
We discuss a series of observations of the black hole candidate GX 339-4 in low luminosity, spectrally hard states. We present spectral analysis of three separate archival Advanced Satellite for Cosmology and Astrophysics (ASCA) data sets and eight separate Rossi X-ray Timing Explorer (RXTE) data sets. Three of the RXTE observations were strictly simultaneous with 843 Mega Hertz and 8.3-9.1 Giga Hertz radio observations. All of these observations have (3-9 keV) flux approximately less than 10(exp-9) ergs s(exp-1) CM(exp -2). The ASCA data show evidence for an approximately 6.4 keV Fe line with equivalent width approximately 40 eV, as well as evidence for a soft excess that is well-modeled by a power law plus a multicolor blackbody spectrum with peak temperature approximately equals 150-200 eV. The RXTE data sets also show evidence of an Fe line with equivalent widths approximately equal to 20-1OO eV. Reflection models show a hardening of the RXTE spectra with decreasing X-ray flux; however, these models do not exhibit evidence of a correlation between the photon index of the incident power law flux and the solid angle subtended by the reflector. 'Sphere+disk' Comptonization models and Advection Dominated Accretion Flow (ADAF) models also provide reasonable descriptions of the RXTE data. The former models yield coronal temperatures in the range 20-50 keV and optical depths of r approximately equal to 3. The model fits to the X-ray data, however, do not simultaneously explain the observed radio properties. The most likely source of the radio flux is synchrotron emission from an extended outflow of extent greater than O(10 (exp7) GM/c2)
SPIRITS 16tn in NGC 3556: A heavily obscured and low-luminosity supernova at 8.8 Mpc
We present the discovery by the SPitzer InfraRed Intensive Transients Survey
(SPIRITS) of a likely supernova (SN) in NGC 3556 at only 8.8 Mpc, which,
despite its proximity, was not detected by optical searches. A luminous
infrared (IR) transient at mag (Vega), SPIRITS 16tn is
coincident with a dust lane in the inclined, star-forming disk of the host.
Using IR, optical, and radio observations, we attempt to determine the nature
of this event. We estimate 8 - 9 mag of extinction, placing it
among the three most highly obscured IR-discovered SNe to date. The [4.5] light
curve declined at a rate of 0.013 mag day, and the color
grew redder from 0.7 to 1.0 mag by 184.7 days post discovery.
Optical/IR spectroscopy shows a red continuum, but no clearly discernible
features, preventing a definitive spectroscopic classification. Deep radio
observations constrain the radio luminosity of SPIRITS 16tn to erg s Hz between 3 - 15 GHz, excluding many
varieties of radio core-collapse SNe. A type Ia SN is ruled out by the observed
red IR color, and lack of features normally attributed to Fe-peak elements in
the optical and IR spectra. SPIRITS 16tn was fainter at [4.5] than typical
stripped-envelope SNe by 1 mag. Comparison of the spectral energy
distribution to SNe II suggests SPIRITS 16tn was both highly obscured, and
intrinsically dim, possibly akin to the low-luminosity SN 2005cs. We infer the
presence of an IR dust echo powered by a peak luminosity of the transient of erg s erg s,
consistent with the observed range for SNe II. This discovery illustrates the
power of IR surveys to overcome the compounding effects of visible extinction
and optically sub-luminous events in completing the inventory of nearby SNe.Comment: 25 pages, 10 figures, submitted to Ap
A very brief description of LOFAR - the Low Frequency Array
LOFAR (Low Frequency Array) is an innovative radio telescope optimized for
the frequency range 30-240 MHz. The telescope is realized as a phased aperture
array without any moving parts. Digital beam forming allows the telescope to
point to any part of the sky within a second. Transient buffering makes
retrospective imaging of explosive short-term events possible. The scientific
focus of LOFAR will initially be on four key science projects (KSPs): 1)
detection of the formation of the very first stars and galaxies in the universe
during the so-called epoch of reionization by measuring the power spectrum of
the neutral hydrogen 21-cm line (Shaver et al. 1999) on the ~5' scale; 2)
low-frequency surveys of the sky with of order expected new sources; 3)
all-sky monitoring and detection of transient radio sources such as gamma-ray
bursts, x-ray binaries, and exo-planets (Farrell et al. 2004); and 4) radio
detection of ultra-high energy cosmic rays and neutrinos (Falcke & Gorham 2003)
allowing for the first time access to particles beyond 10^21 eV (Scholten et
al. 2006). Apart from the KSPs open access for smaller projects is also
planned. Here we give a brief description of the telescope.Comment: 2 pages, IAU GA 2006, Highlights of Astronomy, Volume 14, K.A. van
der Hucht, e
Manager- und transaktionsspezifische Determinanten der Performance von Arbitrage CLOs
Der vorliegende Beitrag untersucht die Determinanten der Performance europĂ€ischer Arbitrage Collateralized Loan Obligations fĂŒr das Jahr 2009. Der Fokus liegt dabei auf der Bedeutung der performanceabhĂ€ngigen VergĂŒtung des CLO-Managers, den Eigenschaften des CLO-Managers und der Transaktionscharakteristika als mögliche Einflussfaktoren der Rating Performance. Es wird gezeigt, dass Transaktionen, bei denen dem CLO-Manager eine Incentive Management Fee gewĂ€hrt wird, mit einer höheren Wahrscheinlichkeit herabgestuft werden als Transaktionen ohne Incentive Fee. Dieser Befund bestĂ€tigt die Hypothese, dass durch die Incentive Fee Risikoanreize fĂŒr CLO-Manager geschaffen werden. Des Weiteren wird ein positiver Zusammenhang zwischen der Erfahrung bzw. der GröĂe eines CLO-Managers und der Rating Performance festgestellt. Der Einfluss des Managers auf die Performance einer CLO-Transaktion wird auch an den weiteren in der Studie herangezogenen managerspezifischen Charakteristika wie Typ und Unternehmenssitz bestĂ€tigt. FĂŒr die Transaktionscharakteristika wird hingegen im betrachteten Untersuchungszeitraum kein signifikanter Einfluss auf die Rating Performance nachgewiesen
Upper Limits on Central Black Hole Masses of Globular Clusters from Radio Emission and a Possible Black Hole Detection in the Ursa Minor Dwarf Galaxy
Intermediate mass black holes have been alternatively predicted to be quite
common in the centers of globular clusters or nearly impossible to form and
retain in the centers of globular clusters. As it has been recently shown that
radio observations are currently the most sensitive observational technique for
detecting such objects, we have obtained new deep radio observations of Omega
Cen, and have re-analyzed older observations of M~15 in hope of constraining
the masses of possible black holes in their centers. In both cases, upper
limits of about 100 Jy are found at GHz frequencies. We find that if the
Bondi-Hoyle accretion rate truly represents the spherical accretion rate onto a
black hole, then the masses of the black holes in the centers of these two
galaxies are severely constrained - with mass limits of less than about 100
solar masses in both cases. If more realistic assumptions are made based on
recent work showing the Bondi rate to be a severe overestimate, then the data
for Omega Cen are marginally consistent with a black hole of about 1/1000 the
cluster's mass (i.e. about 1000 ). The data for M~15 then are only
marginally consistent with previous reports of a solar mass black
hole, and we note that there is considerable hope for either detecting the
black hole or improving this upper limit with current instrumentation. Finally,
we discuss the possibility that the radio source near the core of the Ursa
Minor dwarf spheroidal galaxy is a black hole.Comment: 6 pages, no figures, accepted to MNRAS Letter
- âŠ