Search Publications

MOND plus classical neutrinos are not enough for cluster lensing

Natarajan, Priyamvada; Zhao, Hongsheng

Clusters of galaxies offer a robust test bed for probing the nature of dark matter that is insensitive to the assumption of the gravity theories. Both Modified Newtonian Dynamics (MOND) and General Relativity (GR) would require similar amounts of non-baryonic matter in clusters as MOND boosts the gravity only mildly on cluster scales. Gravitationa…

The nature of the hard state of Cygnus X-3

Larsson, S.; Zdziarski, A. A.; Vilhu, O. +4 more

The X-ray binary Cygnus X-3 (Cyg X-3) is a highly variable X-ray source that displays a wide range of observed spectral states. One of the main states is significantly harder than the others, peaking at ~20 keV, with only a weak low-energy component. Due to the enigmatic nature of this object, hidden inside the strong stellar wind of its Wolf-Raye…

Adaptive optics imaging and optical spectroscopy of a multiple merger in a luminous infrared galaxy

Östlin, G.; Farrah, D.; Kniazev, A. +16 more

We present near-infrared (NIR) adaptive optics imaging obtained with VLT/NACO and optical spectroscopy from the Southern African Large Telescope of a luminous IR galaxy (LIRG) IRAS 19115-2124. These data are combined with archival Hubble Space Telescope imaging and Spitzer imaging and spectroscopy, allowing us to study this disturbed interacting/m…

Chandra spectroscopy of the hot star βCrucis and the discovery of a pre-main-sequence companion

Miller, Nathan A.; Cohen, David H.; Jensen, Eric L. N. +2 more

In order to test the O star wind-shock scenario for X-ray production in less luminous stars with weaker winds, we made a pointed 74-ks observation of the nearby early B giant, βCrucis (βCru B0.5III), with the Chandra High Energy Transmission Grating Spectrometer. We find that the X-ray spectrum is quite soft, with a dominant thermal component near…

The fundamental parameters of the roAp star α Circini

Tuthill, P. G.; Davis, J.; Kurtz, D. W. +12 more

We have used the Sydney University Stellar Interferometer to measure the angular diameter of α Cir. This is the first detailed interferometric study of a rapidly oscillating A (roAp) star, α Cir being the brightest member of its class. We used the new and more accurate Hipparcos parallax to determine the radius to be 1.967 ± 0.066 R_⊙. W…

Weighing the black holes in ultraluminous X-ray sources through timing

Belloni, T.; Ponti, G.; Miniutti, G. +3 more

We describe a new method to estimate the mass of black holes in Ultraluminous X-ray Sources (ULXs). The method is based on the recently discovered `variability plane', populated by Galactic stellar-mass black-hole candidates (BHCs) and supermassive active galactic nuclei (AGNs), in the parameter space defined by the black-hole mass, accretion rate…

New Hubble Space Telescope imaging of the counterparts to six ultraluminous X-ray sources

Roberts, T. P.; Levan, A. J.; Goad, M. R.

We report the results of new Hubble Space Telescope imaging of the positions of six ultraluminous X-ray sources (ULXs). Using images in three Advanced Camera for Surveys (ACS) filters, we detect good candidate counterparts to four out of six ULXs, with one more possible detection, and observed magnitudes in the range m ~ 22-26 in the F606W filter.…

The defining characteristics of intermediate polars - the case of three-candidate systems

Ramsay, Gavin; Wheatley, Peter J.; Hakala, Pasi +2 more

Intermediate polars (IPs) are a group of cataclysmic variables (CVs) which are thought to contain white dwarfs which have a magnetic field strength in the range ~0.1-10MG. A significant fraction of the X-ray sources detected in recent deep surveys has been postulated to consist of IPs. Until now two of the defining characteristics of IPs have been…

Testing the blazar spectral sequence: X-ray-selected blazars

Maraschi, L.; Ghisellini, G.; Tavecchio, F. +2 more

We present simultaneous optical and X-ray data from Swift for a sample of radio-loud flat-spectrum quasars selected from the Einstein Medium Sensitivity Survey (EMSS). We present also a complete analysis of Swift and INTEGRAL data on four blazars recently discussed as possibly challenging the trends of the hypothesized `blazar spectral sequence'. …

The early-type galaxies NGC 1407 and NGC 1400 - II. Star formation and chemical evolutionary history

Forbes, Duncan A.; Proctor, Robert N.; Brough, Sarah +2 more

We present a possible star formation and chemical evolutionary history for two early-type galaxies NGC 1407 and NGC 1400. They are the two brightest galaxies of the NGC 1407 (or Eridanus-A) group, one of the 60 groups studied as part of the Group Evolution Multi-wavelength Study.

Our analysis is based on new high signal-to-noise ratio spatial…