We present UBV RIJHK photometry and optical spectroscopy of the socalled "peculiar" Type Ia supernova 1999by in NGC 2841. The observations began one week before visual maximum light which is well-defined by daily observations. The light curves and spectra are similar to those of the prototypical subluminous event SN 1991bg. We find that maximum light in B occurred on 1999 May 10.3 UT (JD 2,451,308.8±0.3) with B = 13.66 ± 0.02 and a color of B max − V max = 0.51 ± 0.03. The late-time color implies minimal dust extinction from the host galaxy. Our photometry, when combined with the recent Cepheid distance to NGC 2841 (Macri et al. 2001), gives a peak absolute magnitude

We analyse Chandra High Resolution Camera observations of the starburst galaxy M82, concentrating on the most luminous X‐ray source. We find a position for the source of (J2000) with a 1σ radial error of 0.7 arcsec. The accurate X‐ray position shows that the luminous source is neither at the dynamical centre of M82 nor coincident with any suggested radio AGN candidate. The source is highly variable between observations, which suggests that it is a compact object and not a supernova or remnant. There is no significant short‐term variability within the observations. Dynamical friction and the off‐centre position place an upper bound of 105–106 M⊙ on the mass of the object, depending on its age. The X‐ray luminosity suggests a compact object mass of at least 500 M⊙. Thus the luminous source in M82 may represent a new class of compact object with a mass intermediate between those of stellar‐mass black hole candidates and supermassive black holes.

We present a new technique called quantile analysis to classify spectral properties of X-ray sources with limited statistics. The quantile analysis is superior to the conventional approaches such as Xray hardness ratio or X-ray color analysis to study relatively faint sources or to investigate a certain phase or state of a source in detail, where poor statistics does not allow spectral fitting using a model. Instead of working with predetermined energy bands, we determine the energy values that divide the detected photons into predetermined fractions of the total counts such as median (50%), tercile (33% & 67%), and quartile (25% & 75%). We use these quantiles as an indicator of the X-ray hardness or color of the source. We show that the median is an improved substitute for the conventional X-ray hardness ratio. The median and other quantiles form a phase space, similar to the conventional Xray color-color diagrams. The quantile-based phase space is more evenly sensitive over various spectral shapes than the conventional color-color diagrams, and it is naturally arranged to properly represent the statistical similarity of various spectral shapes. We demonstrate the new technique in the 0.3-8 keV energy range using Chandra ACIS-S detector response function and a typical aperture photometry involving background subtraction. The technique can be applied in any energy band, provided the energy distribution of photons can be obtained. Subject headings:

The first extragalactic X-ray binary, LMC X-1, was discovered in 1969. In the 1980s, its compact primary was established as the fourth dynamical black hole candidate. Recently, we published accurate values for the mass of the black hole and the orbital inclination angle of the binary system. Building on these results, we have analyzed 53 X-ray spectra obtained by RXTE and, using a selected sample of 18 of these spectra, we have determined the dimensionless spin parameter of the black hole to be a * = 0.92 +0.05 −0.07 . This result takes into account all sources of observational and model-parameter uncertainties. The standard deviation around the mean value of a * for these 18 X-ray spectra, which were obtained over a span of several years, is only Δa * = 0.02. When we consider our complete sample of 53 RXTE spectra, we find a somewhat higher value of the spin parameter and a larger standard deviation. Finally, we show that our results based on RXTE data are confirmed by our analyses of selected X-ray spectra obtained by the XMM-Newton, BeppoSAX, and Ginga missions.

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