Gregory J. Sobczak

Ubvri light curves of 44 type ia supernovae

We present UBVRI photometry of 44 Type Ia supernovae (SNe Ia) observed from 1997 to 2001 as part of a continuing monitoring campaign at the Fred Lawrence Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics. The data set comprises 2190 observations and is the largest homogeneously observed and reduced sample of SNe Ia to date, nearly doubling the number of well-observed, nearby SNe Ia with published multicolor CCD light curves. The large sample of U-band photometry is a unique addition, with important connections to SNe Ia observed at high redshift. The decline rate of SN Ia U-band light curves correlates well with the decline rate in other bands, as does the U - B color at maximum light. However, the U-band peak magnitudes show an increased dispersion relative to other bands even after accounting for extinction and decline rate, amounting to an additional ~40% intrinsic scatter compared to the B band.

Complete RXTE Spectral Observations of the Black Hole X-ray Nova XTE J1550–564

We report on the X-ray spectral behavior of XTE J1550(564 during its 1998¨1999 outburst. XTE J1550(564 is an exceptionally bright X-ray nova and is also the third Galactic black hole candi- date known to exhibit quasi-periodic X-ray oscillations above 50 Hz. Our study is based on 209 pointed observations using the PCA and HEXTE instruments on board the Rossi X-Ray T iming Explorer (RXT E) spanning 250 days and covering the entire double-peaked eruption that occurred from 1998 September until 1999 May. The spectra are —tted to a model including multicolor blackbody disk and power-law components. The spectra from the —rst half of the outburst are dominated by the power-law component, whereas the spectra from the second half are dominated by the disk component. The source is observed in the very high and high/soft outburst states of black hole X-ray novae. During the very high state, when the power-law component dominated the spectrum, the inner disk radius is observed to vary by more than an order of magnitude; the radius decreased by a factor of 16 in one day during a 6.8 crab —are. If the larger of these observed radii is taken to be the last stable orbit, then the smaller observed radius would imply that the inner edge of the disk is inside the event horizon! However, we conclude that the apparent variations of the inner disk radius observed during periods of increased power-law emission are probably caused by the failure of the multicolor disk/power-law model; the actual physical radius of the inner disk may remain fairly constant. This interpretation is supported by the fact that the observed inner disk radius remains approximately constant over 120 days in the high state, when the power-law component is weak, even though the disk —ux and total —ux vary by an order of magnitude. The mass of the black hole inferred by equating the approximately constant inner disk radius observed in the high/soft state with the last stable orbit for a Schwarzschild black hole is M BH \ 7.4 kpc)(cos i)~1@2. M _ (D/6 Subject headings: black hole physicsstars: individual (XTE J1550(564) ¨ X-rays: stars On-line material: machine-readable tables

Characterizing the Quasi-periodic Oscillation Behavior of the X-Ray Nova XTE J1550–564

For all 209 RXTE observations of the X-ray nova XTE J1550–564 during its major outburst of 1998-1999, we have analyzed the X-ray power spectra, phase lags, and coherence functions. These observations constitute one of the richest and most complete data sets obtained for any black hole X-ray nova. The phase lags and coherence measures are used to distinguish three types of low-frequency QPOs (one more than those reported by Wijnands, Homan, & van der Klis 1999). For the most common type (“C”), the phase lag is correlated with both the QPO frequency and the amplitude. The physical significance of the QPO types is evident in the relationships between QPO properties and the apparent temperature and flux from the accretion disk. There is also a clear pattern in how the QPO types relate to the presence of high-frequency QPOs. In general, both the amplitude and the Q value (ν/FWHM) of low-frequency QPOs decrease as the high-frequency oscillations increase in frequency (100 to 284 Hz) and in Q value. We speculate that the antagonism between low-frequency and high-frequency QPOs arises from competing structures in a perturbed accretion disk. However, we find that the frequencies of slow (< 20 Hz) and fast (> 100 Hz) QPOs are not correlated. In addition, we encounter systematic problems in attempting to reliably compare the QPO frequencies with broad features in the power continuum, since there are a variable number of features or spectral breaks in the power spectra. These results cast some doubt on the reported global relationship between QPOs from neutrons stars and those from black hole systems. Subject headings: black hole physics — stars: individual (XTE J1550–564) — X-rays: stars Center for Space Research, MIT, Cambridge, MA 02139; rr@space.mit.edu, muno@space.mit.edu Harvard University, Astronomy Dept., 60 Garden St. MS-10, Cambridge, MA 02138 Harvard-Smithsonian Center for Astrophysics, 60 Garden St. MS-3, Cambridge, MA 02138; jem@cfa.harvard.eduFor all 209 Rossi X-Ray Timing Explorer observations of the X-ray nova XTE J1550-564 during its major outburst of 1998-1999, we have analyzed the X-ray power spectra, phase lags, and coherence functions. These observations constitute one of the richest and most complete data sets obtained for any black hole X-ray nova. The phase lags and coherence measures of low-frequency quasi-periodic oscillations (QPOs; ν 100 Hz). In general, the amplitude of low-frequency QPOs decreases when high-frequency oscillations appear, and when both low-frequency and high-frequency QPOs are present, their Q-values (ν/FWHM) are anticorrelated. We speculate that this opposition between low-frequency and high-frequency QPOs arises from competing structures in a perturbed accretion disk. However, we find that the frequencies of slow and fast QPOs are not correlated. In addition, we encounter systematic problems in attempting to reliably compare the QPO frequencies with broad features in the power continuum, since there are a variable number of features or spectral breaks in the power spectra. These results cast some doubt on the reported global relationship between QPOs from neutron stars and those from black hole systems.

CORRELATIONS BETWEEN LOW-FREQUENCY QUASI-PERIODIC OSCILLATIONS AND SPECTRAL PARAMETERS IN XTE J1550(564 AND GRO J1655(40

Utilizing observations obtained with the Rossi X-Ray Timing Explorer, we examine correlations between the properties of 0.08-22 Hz variable-frequency quasi-periodic oscillations (QPOs) and the X-ray spectral parameters for the black hole candidates XTE J1550-564 and GRO J1655-40. The spectra were fitted to a model including a multitemperature blackbody disk and a power-law component. We find that the QPO frequency and amplitude are well correlated with the spectral parameters, although the correlations found for XTE J1550-564 are generally opposite to those for GRO J1655-40. There is one exception: both sources exhibit a general increase in the QPO frequency as the disk flux increases (or as the rate of mass accretion through the disk increases). In addition, these QPOs are observed only when the power-law component contributes more than 20% of the 2-20 keV flux, which indicates that both the disk and the power-law components are linked to the QPO phenomenon.

X-ray Nova XTE J1550-564: RXTE spectral observation

Excellent coverage of the 1998 outburst of the X-ray nova XTE J15502564 was provided by the Rossi X-Ray Timing Explorer (RXTE). XTE J15502564 exhibited an intense (6.8 crab) flare on 1998 September 19 (UT), making it the brightest new X-ray source observed with RXTE. We present a spectral analysis utilizing 60 Proportional Counter Array spectra from 2.5‐20 keV spanning 71 days and a nearly continuous All Sky Monitor light curve. The spectra were fitted to a model including multicolor blackbody disk and power-law components. XTE J15502564 is observed in the very high, high/soft, and intermediate canonical outburst states of black hole X-ray novae. Subject headings: black hole physics — stars: individual (XTE J1550 2564) — X-rays: stars

X-Ray Nova XTE J1550–564: Discovery of a Quasi-periodic Oscillation near 185 Hz

We have investigated the X-ray timing properties of XTE J1550-564 during 60 Rossi X-Ray Timing Explorer Proportional Counter Array observations made between 1998 September 18 and November 28. We detect quasi-periodic oscillations (QPOs) near 185 Hz during four time intervals. The QPO widths (FWHM) are ~50 Hz, and the rms amplitudes are ~1% of the mean flux at 2-30 keV. This is the third Galactic black hole candidate known to exhibit a transient X-ray timing signature above 50 Hz, following the 67 Hz QPO in GRS 1915+105 and the 300 Hz QPO in GRO J1655-40. However, unlike the previous cases, which appear to show stationary frequencies, the QPO frequency in XTE J1550-564 must vary by at least ~10% to be consistent with observations. The occurrences and properties of the QPO were insensitive to large changes in the X-ray intensity (1.5-6.8 crab). However, the QPO appearance was accompanied by changes in the energy spectrum, namely, an increase in the temperature and a decrease in the normalization of the thermal component. The QPO is also closely related to the hard X-ray power-law component of the energy spectrum since the fractional amplitude of the QPO increases with photon energy. The fast QPOs in accreting black hole binaries are thought to be effects of general relativity; however, the relevance of the specific physical models that have been proposed remains largely uncertain. Low-frequency QPOs in the range 3-13 Hz were often observed. Occasionally, at high luminosity, the rms QPO amplitude was ~15% of the flux, a level previously reached only by GRS 1915+105. These extraordinary oscillations have a coherence parameter (ν/Δν) in the range 4-12 and are tied to the power-law component in the energy spectrum.