Paul J. De Groot

Dynamical Evidence for a Black Hole in the Microquasar XTE J1550-564

Optical spectroscopic observations of the companion star (type G8 IV to K4 III) in the microquasar system XTE J1550-564 reveal a radial velocity curve with a best-fitting spectroscopic period of Psp = 1.552 ± 0.010 days and a semiamplitude of K2 = 349 ± 12 km s-1. The optical mass function is f(M) = 6.86 ± 0.71 M☉ (1 σ). We tentatively measure the rotational velocity of the companion star to be Vrot sin i = 90 ± 10 km s-1, which when taken at face value implies a mass ratio of Q ≡ M1/M2 = 6.6 (1 σ), using the above value of K2. We derive constraints on the binary parameters from simultaneous modeling of the ellipsoidal light and radial velocity curves. We find 1 σ ranges for the photometric period (1.5430 days ≤ Pph ≤ 1.5440 days), K-velocity (350.2 ≤ K2 ≤ 368.6 km s-1), inclination (670 ≤ i ≤ 774), mass ratio (Q ≥ 12.0), and orbital separation (11.55 R☉ ≤ a ≤ 12.50 R☉). Given these geometrical constraints, we find that the most likely value of the mass of the compact object is 9.41 M☉ with a 1 σ range of 8.36 M☉ ≤ M1 ≤ 10.76 M☉. If we apply our tentative value of Vrot sin i = 90 ± 10 km s-1 as an additional constraint in the ellipsoidal modeling, we find 1 σ ranges of 1.5432 days ≤ Pph ≤ 1.5441 days for the photometric period, 352.2 ≤ K2 ≤ 370.1 km s-1 for the K-velocity, 708 ≤ i ≤ 754 for the inclination, 6.7 ≤ Q ≤ 11.0 for the mass ratio, and 12.35 R☉ ≤ a ≤ 13.22 R☉ for the orbital separation. These geometrical constraints imply the most likely value of the mass of the compact object of 10.56 M☉ with a 1 σ range of 9.68 M☉ ≤ M1 ≤ 11.58 M☉. In either case the mass of the compact object is well above the maximum mass of a stable neutron star, and we therefore conclude that XTE J1550-564 contains a black hole.

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 and simultaneous spectral observations of the black hole X-Ray nova XTE J1118+480

The X-ray nova XTE J1118+480 suffers minimal extinction (b = 62 degrees) and therefore represents an outstanding opportunity for multiwavelength studies. Hynes et al. (2000) conducted the first such study, which was centered on 2000 April 8 using UKIRT, EUVE, HST and RXTE. On 2000 April 18, the Chandra X-ray Observatory obtained data coincident with a second set of observations using all of these same observatories. A 30 ks grating observation using Chandra yielded a spectrum with high resolution and sensitivity covering the range 0.24-7 keV. Our near-simultaneous observations cover approximately 80% of the electromagnetic spectrum from the infrared to hard X-rays. The UV/X-ray spectrum of XTE J1118+480 consists of two principal components. The first of these is an approximately 24 eV thermal component which is due to an accretion disk with a large inner disk radius: > 35 Schwarzschild radii. The second is a quasi power-law component that was recorded with complete spectral coverage from 0.4-160 keV. A model for this two-component spectrum is presented in a companion paper by Esin et al. (2001).

THE STELLAR VELOCITY DISPERSION OF A COMPACT MASSIVE GALAXY AT z = 1.80 USING X-SHOOTER: CONFIRMATION OF THE EVOLUTION IN THE MASS-SIZE AND MASS-DISPERSION RELATIONS* , **

Recent photometric studies have shown that early-type galaxies at fixed stellar mass were smaller and denser at earlier times. In this Letter, we assess that finding by deriving the dynamical mass of such a compact quiescent galaxy at z = 1.8. We have obtained a high-quality spectrum with full UV-NIR wavelength coverage of galaxy NMBS-C7447 using X-Shooter on the Very Large Telescope. We determined a velocity dispersion of 294 ± 51 km s-1. Given this velocity dispersion and the effective radius of 1.64 ± 0.15 kpc (as determined from Hubble Space Telescope Wide Field Camera 3 F160W observations) we derive a dynamical mass of (1.7 ± 0.5) × 1011 M ☉. Comparison of the full spectrum with stellar population synthesis models indicates that NMBS-C774 has a relatively young stellar population (0.40 Gyr) with little or no star formation and a stellar mass of M ~ 1.5 × 1011 M ☉. The dynamical and photometric stellar masses are in good agreement. Thus, our study supports the conclusion that the mass densities of quiescent galaxies were indeed higher at earlier times, and this earlier result is not caused by systematic measurement errors. By combining available spectroscopic measurements at different redshifts, we find that the velocity dispersion at fixed dynamical mass was a factor of ~1.8 higher at z = 1.8 compared with z = 0. Finally, we show that the apparent discrepancies between the few available velocity dispersion measurements at z > 1.5 are consistent with the intrinsic scatter of the mass-size relation. Footnote* Based on X-Shooter-VLT observations collected at the European Southern Observatory, Paranal, Chile. Footnote** Based on observations with the NASA/ESA Hubble Space Telescope (HST), obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.

The Fading Optical Counterpart of GRB 970228, 6 Months and 1 Year Later

We report on observations of the fading optical counterpart of the gamma-ray burst GRB 970228, made with the Hubble Space Telescope (HST) and the Keck I telescope. The gamma-ray burst (GRB) was observed approximately 6 months after outburst, on 1997 September 4, using the HST/STIS CCD, and approximately 1 year after outburst, on 1998 February 24, using HST/NICMOS, and on 1998 April 4 using the NIRC on Keck. The unresolved counterpart is detected by STIS at V=28.0 ± 0.25, consistent with a continued power-law decline with exponent -1.10 ± 0.05. The counterpart is located within, but near the edge of, a faint extended source with diameter ~08 and integrated magnitude V=25.8 ± 0.25. A reanalysis of HST and New Technology Telescope observations performed shortly after the burst shows no evidence of proper motion of the point source or fading of the extended emission. Although the optical transient is not detected in the NICMOS images (H≥25.3), the extended source is visible and has a total magnitude H=23.3 ± 0.1. The Keck observations find K=22.8 ± 0.3. Comparison with observations obtained shortly after outburst suggests that the nebular luminosity has also been stable in the infrared. We find that several distinct and independent means of deriving the foreground extinction in the direction of GRB 970228 all agree with AV=0.75 ± 0.2. After adjusting for this Galactic extinction, we find that the size of the observed extended emission is consistent with that of galaxies of comparable magnitude found in the Hubble Deep Field (HDF) and other deep HST images. Only 2% of the sky is covered by galaxies of similar or greater surface brightness. We therefore conclude that the extended source observed about GRB 970228 is almost certainly its host galaxy. Additionally, we find that independent of assumed redshift, the host is significantly bluer than typical nearby blue dwarf irregulars. With the caveat that the presently available infrared observations of the HDF are only fully complete to a limit about one-half magnitude brighter than the host, we find that the extinction-corrected V-H and V-K colors of the host are as blue as any galaxy of comparable or brighter magnitude in the HDF. Taken in concert with recent observations of GRB 970508, GRB 971214, and GRB 980703 our work suggests that all four GRBs with spectroscopic identification or deep multicolor broadband imaging of the host lie in rapidly star-forming galaxies.

The Radio-to-X-Ray Spectrum of GRB 970508 on 1997 May 21.0 UT

We have reconstructed the spectrum of the afterglow of GRB 970508 on 1997 May 21.0 UT (12.1 days after the gamma-ray burst) on the basis of observations spanning the X-ray-to-radio range. The low-frequency power-law index of the spectrum, alpha =0.44+/-0.07 ( F_nu~ nu ^alpha ), is in agreement with the expected value alpha =1/3 for optically thin synchrotron radiation. The 1.4 GHz emission is self-absorbed. We infer constraints on the break frequencies nu c and nu m on 1997 May 21.0 UT from a spectral transition from F_nu~ nu ^{-0.6} to F_nu~ nu ^{-1.1} in the optical passband around 1.4 days. A model of an adiabatically expanding blast wave emitting synchrotron radiation, in which a significant fraction of the electrons cool rapidly, provides a successful and consistent description of the afterglow observations over nine decades in frequency, ranging in time from trigger until several months later.

The VST Photometric Hα Survey of the Southern Galactic Plane and Bulge (VPHAS

The VST Photometric HSurvey of the Southern Galactic Plane and Bulge (VPHAS+) is surveying the southern Milky Way in u,g,r,i and Hat �1 arcsec angular resolution. Its footprint spans the Galactic latitude range 5 o < b < +5 o at all longitudes south of the celestial equator. Extensions around the Galactic Centre to Galactic latitudes ±10 ◦ bring in much of the Galactic Bulge. This ESO public sur- vey, begun on 28th December 2011, reaches down to �20th magnitude (10�) and will provide single-epoch digital optical photometry for �300 million stars. The observing strategy and data pipelining is described, and an appraisal of the segmented narrow- band Hfilter in use is presented. Using model atmospheres and library spectra, we compute main-sequence (u g), (g r), (r i) and (r H�) stellar colours in the Vega system. We report on a preliminary validation of the photometry using test data obtained from two pointings overlapping the Sloan Digital Sky Survey. An example of the (u g,g r) and (r H�,r i) diagrams for a full VPHAS+ survey field is given. Attention is drawn to the opportunities for studies of compact nebulae and nebular morphologies that arise from the image quality being achieved. The value of the u band as the means to identify planetary-nebula central stars is demonstrated by the discovery of the central star of NGC 2899 in survey data. Thanks to its excellent imaging performance, the VST/OmegaCam combination used by this survey is a per- fect vehicle for automated searches for reddened early-type stars, and will allow the discovery and analysis of compact binaries, white dwarfs and transient sources.

A search for optical afterglow from GRB 970828

We report on the results of R-band observations of the error box of the gamma -ray burst of 1997 August 28 made between 4 hr and 8 days after this burst occurred. No counterpart was found varying by more than 0.2 mag down to R=23.8 . We discuss the consequences of this nondetection for relativistic blast wave models of gamma -ray bursts and the possible effect of redshift on the relation between optical absorption and the low-energy cutoff in the X-ray afterglow spectrum.

Optical Follow-Up of GRB 970508

We report on the results of optical follow-up observations of the counterpart of the gamma-ray burst GRB 970508, starting 7 hr after the event. Multicolor U-, B-, V-, Rc-, and Ic-band observations were obtained during the first three consecutive nights. The counterpart was monitored regularly in Rc until ~4 months after the burst. The light curve after the maximum follows a decline that can be fitted with a power law with exponent α = -1.141 ± 0.014. Deviations from a smooth power-law decay are moderate (rms = 0.15 mag). We find no flattening of the light curve at late times. The optical afterglow fluence is a significant fraction, ~5%, of the GRB fluence. The optical energy distribution can be well represented by a power law, the slope of which changed at the time of the maximum (the spectrum became redder).

The X-Ray, optical, and infrared counterpart to GRB 980703

We report on X-ray, optical, and infrared follow-up observations of GRB 980703. We detect a previously unknown X-ray source in the GRB error box; assuming a power-law decline, we find for its decay index α 1.3 × 1017 Hz. For this epoch we obtain an extinction of AV = 1.50 ± 0.11. From the X-ray data we estimate the optical extinction to be AV = 20.2+12.3-7.3, inconsistent with the former value. Our optical spectra confirm the redshift of z = 0.966. We compare the afterglow of GRB 980703 with that of GRB 970508 and find that the fraction of the energy in the magnetic field, B < 6 × 10-5, is much lower in the case of GRB 980703, as a consequence of the high frequency of the cooling break.