arXiv subjects

K. C. Sahu

Publications and source records attributed to K. C. Sahu.

65 records · Page 4Linked to original sources

arXiv1999

Limits on Stellar and Planetary Companions in Microlensing Event OGLE-1998-BUL-14

We present the PLANET photometric data set for \ob14, a high magnification ($A_{\rm max}\sim 16$) event alerted by the OGLE collaboration toward the Galactic bulge in 1998. The PLANET data set consists a total of 461 I-band and 139 $V-$band points, the majority of which was taken over a three month period. The median sampling interval during this period is about 1 hour, and the $1σ$ scatter over the peak of the event is 1.5%. The excellent data quality and high maximum magnification of this event make it a prime candidate to search for the short duration, low amplitude perturbations that are signatures of a planetary companion orbiting the primary lens. The observed light curve for \ob14 is consistent with a single lens (no companion) within photometric uncertainties. We calculate the detection efficiency of the light curve to lensing companions as a function of the mass ratio and angular separation of the two components. We find that companions of mass ratio $\ge 0.01$ are ruled out at the 95% confidence level for projected separations between $0.4-2.4 \re$, where $\re$ is the Einstein ring radius of the primary lens. Assuming that the primary is a G-dwarf with $\re\sim3 {\rm AU}$ our detection efficiency for this event is $\sim 60%$ for a companion with the mass and separation of Jupiter and $\sim5%$ for a companion with the mass and separation of Saturn. Our efficiencies for planets like those around Upsilon And and 14 Her are > 75%.

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arXiv1999

The Hubble Deep Field South - STIS Imaging

We present the imaging observations made with the Space Telescope Imaging Spectrograph of the Hubble Deep Field - South. The field was imaged in 4 bandpasses: a clear CCD bandpass for 156 ksec, a long-pass filter for 22-25 ksec per pixel typical exposure, a near-UV bandpass for 23 ksec, and a far-UV bandpass for 52 ksec. The clear visible image is the deepest observation ever made in the UV-optical wavelength region, reaching a 10 sigma AB magnitude of 29.4 for an object of area 0.2 square arcseconds. The field contains QSO J2233-606, the target of the STIS spectroscopy, and extends 50"x50" for the visible images, and 25"x25" for the ultraviolet images. We present the images, catalog of objects, and galaxy counts obtained in the field.

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arXiv1999

The Fading Optical Counterpart of GRB~970228, Six Months and One Year Later

We report on observations of the fading optical counterpart of the gamma-ray burst GRB 970228, made with the Hubble Space Telescope STIS CCD approximately six months after outburst and with the HST/NICMOS and Keck/NIRC approximately one year after outburst. The unresolved counterpart is detected by STIS at V=28.0 +/- 0.25, consistent with a continued power-law decline with exponent -1.14 +/- 0.05. The counterpart is located within, but near the edge of, a faint extended source with diameter ~0."8 and integrated magnitude V=25.8 +/- 0.25. A reanalysis of HST and NTT observations performed shortly after the burst shows no evidence of proper motion of the point source or fading of the extended emission. Only the extended source is visible in the NICMOS images with a magnitude of H=23.3 +/- 0.1. The Keck observations find K = 22.8 +/- 0.3. Several distinct and independent means of deriving the foreground extinction in the direction of GRB 970228 all agree with A_V = 0.75 +/- 0.2. After adjusting for 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; therefore the extended source is almost certainly the host galaxy. Additionally, 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 broad-band imaging of the host lie in rapidly star-forming galaxies.

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arXiv1999

Limb-Darkening of a K Giant in the Galactic Bulge: PLANET Photometry of MACHO 97-BLG-28

We present the PLANET photometric dataset for the binary-lens microlensing event MACHO 97-BLG-28 consisting of 696 I and V-band measurements, and analyze it to determine the radial surface brightness profile of the Galactic bulge source star. The microlensed source, demonstrated to be a K giant by our independent spectroscopy, crossed the central isolated cusp of the lensing binary, generating a sharp peak in the light curve that was well-resolved by dense (3 - 30 minute) and continuous monitoring from PLANET sites in Chile, South Africa, and Australia. Our modeling of these data has produced stellar profiles for the source star in the I and V bands that are in excellent agreement with those predicted by stellar atmospheric models for K giants. The limb-darkening coefficients presented here are the first derived from microlensing, among the first for normal giants by any technique, and the first for any star as distant as the Galactic bulge. Modeling indicates that the lensing binary has a mass ratio q = 0.23 and an (instantaneous) separation in units of the angular Einstein ring radius of d = 0.69 . For a lens in the Galactic bulge, this corresponds to a typical stellar binary with a projected separation between 1 and 2 AU. If the lens lies closer, the separation is smaller, and one or both of the lens objects is in the brown dwarf regime. Assuming that the source is a bulge K2 giant at 8 kpc, the relative lens-source proper motion is mu = 19.4 +/- 2.6 km/s /kpc, consistent with a disk or bulge lens. If the non-lensed blended light is due to a single star, it is likely to be a young white dwarf in the bulge, consistent with the blended light coming from the lens itself.

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arXiv1999

A Complete Set of Solutions For Caustic-Crossing Binary Microlensing Events

We present a method to analyze binary-lens microlensing light curves with one well-sampled fold caustic crossing. In general, the surface of chi^2 shows extremely complicated behavior over the 9-parameter space that characterizes binary lenses. This makes it difficult to systematically search the space and verify that a given local minimum is a global minimum. We show that for events with well-monitored caustics, the caustic-crossing region can be isolated from the rest of the light curve and easily fit to a 5-parameter function. Four of these caustic-crossing parameters can then be used to constrain the search in the larger 9-parameter space. This allows a systematic search for all solutions and thus identification of all local minima. We illustrate this technique using the PLANET data for MACHO 98-SMC-1, an excellent and publicly available caustic-crossing data set. We show that a very broad range of parameter combinations are compatible with the PLANET data set, demonstrating that observations of binary-lens lightcurves with sampling of only one caustic crossing do not yield unique solutions. The corollary to this is that the time of the second caustic crossing cannot be reliably predicted on the basis of early data including the first caustic crossing alone. We investigate the requirements for determination of a unique solution and find that occasional observations of the first caustic crossing may be sufficient to derive a complete solution.

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arXiv1999

The Lyman-alpha forest of the QSO in the Hubble Deep Field South

The quasar in the Hubble Deep Field South (HDFS), J2233-606 (z=2.23) has been exhaustively observed by ground based telescopes and by the STIS spectrograph on board the Hubble Space Telescope at low, medium and high resolution in the spectral interval from 1120 A to 10000 A. This very large base-line represents a unique opportunity to study in detail the distribution of clouds associated with emitting structures in the field of the quasar and in nearby fields already observed as part of the HDFS campaign. Here we report the main properties of the Lyman-alpha clouds in the intermediate redshift range 1.20-2.20, where our present knowledge has been complicated by the difficulty in producing good data. The number density is shown to be higher than what is expected by extrapolating the results from both lower and higher redshifts: 63\pm8 lines with log N_{HI}\geq14.0 are found (including metal systems) at =1.7, to be compared with ~40 lines predicted by extrapolating from previous studies. The redshift distribution of the Lyman-alpha clouds shows a region spanning z=1.383-1.460 (comoving size of 94 h^{-1}_{65} Mpc, Omega_o=1) with a low density of absorption lines; we detect 5 lines in this region, compared with the 16 expected from an average density along the line of sight. The two point correlation function shows a positive signal up to scales of about 3 h^{-1}_{65} Mpc and an amplitude that is larger for larger HI column densities. The average Doppler parameter is about 27 km/s, comparable to the mean value found at z > 3, thus casting doubts on the temperature evolution of the Lyman-alpha clouds.

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arXiv1998

The Relative Lens-Source Proper Motion in MACHO 98-SMC-1

We present photometric and spectroscopic data for the second microlensing event seen toward the Small Magellanic Cloud (SMC), MACHO-98-SMC-1. The lens is a binary. We resolve the caustic crossing and find that the source took 2 Delta t = 8.5 hours to transit the caustic. We measure the source temperature T_eff=8000 K both spectroscopically and from the color (V-I)_0~0.22. We find two acceptable binary-lens models. In the first, the source crosses the caustic at phi=43.2 deg and the unmagnified source magnitude is I_s=22.15. The angle implies that the lens crosses the source radius in time t_* = Delta t sin phi = 2.92 hours. The magnitude (together with the temperature) implies that the angular radius of the source is theta_* = 0.089 micro-arsec. Hence, the proper motion is mu=theta_*/t_*=1.26 km/s/kpc. For the second solution, the corresponding parameters are phi=30.6 deg, I_s=21.81, t_*=2.15 hours, theta_* = 0.104 micro-asrsec, mu=2.00 km/s/kpc. Both proper-motion estimates are slower than 99.5% of the proper motions expected for halo lenses. Both are consistent with an ordinary binary lens moving at ~ 75-120 km/s within the SMC itself. We conclude that the lens is most likely in the SMC proper.

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arXiv1998

Lensing and the luminosity of Gamma-ray bursts and their hosts

The quoted intrinsic luminosity of objects has a dependency on the assumed cosmology, and the (less often considered) assumed gravitational lensing due to the matter content of the viewing beam; the so called `filled' or `empty' beam cases. We consider the implications of filled vs empty beam assumptions for the derived total luminosities of recent gamma-ray bursts (GRBs) in which counterparts at cosmological distances have been confirmed. Conversion factors between filled and empty beam bolometric luminosities are presented graphically for a range of cosmologies and redshifts. The tendency for sources to be most probably demagnified further supports the need for non-isotropic GRB emission in neutron star merger models, or the need for at least one massive star in other models. In the most extreme case the true energy of GRB971214 at z=3.418 could be as high as ~ 8 x 10^53 ergs, a factor ~ 2 more than previously estimated. Similarly, the effect may account for some of the observed bias towards fainter host magnitudes.

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arXiv1998

The 1995 Pilot Campaign of PLANET: Searching for Microlensing Anomalies through Precise, Rapid, Round-the-Clock Monitoring

PLANET (the Probing Lensing Anomalies NETwork) is a worldwide collaboration of astronomers whose primary goal is to monitor microlensing events densely and precisely in order to detect and study anomalies that contain information about Galactic lenses and sources that would otherwise be unobtainable. The results of PLANET's highly successful first year of operation are presented here. Details of the observational setup, observing procedures, and data reduction procedures used to track the progress in real time at the three participating observing sites in 1995 are discussed. The ability to follow several events simultaneously with a median sampling interval of 1.6 hours and a photometric precision of better than 0.10 mag even at I=19 has been clearly demonstrated. During PLANET's 1995 pilot campaign, 10 microlensing events were monitored; the binary nature of one of these, MACHO 95-BLG-12 was recognized by PLANET on the mountain. Another event, OGLE 95-BLG-04, displayed chromaticity that may betray the presence of blending with unresolved stars projected onto the same resolution element. Although lasting only about a month, the campaign may allow constraints to be placed on the number of planets with mass ratios to the parent star of 0.01 or greater.

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arXiv1998

Birth and early evolution of a planetary nebula

The final expulsion of gas by a star as it forms a planetary nebula --- the ionized shell of gas often observed surrounding a young white dwarf --- is one of the most poorly understood stages of stellar evolution. Such nebulae form extremely rapidly (about 100 years for the ionization) and so the formation process is inherently difficult to observe. Particularly puzzling is how a spherical star can produce a highly asymmetric nebula with collimated outflows. Here we report optical observations of the Stingray Nebula which has become an ionized planetary nebula within the past few decades. We find that the collimated outflows are already evident, and we have identified the nebular structure that focuses the outflows. We have also found a companion star, reinforcing previous suspicions that binary companions play an important role in shaping planetary nebulae and changing the direction of successive outflows.

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arXiv1996

The Planet Collaboration: Probing Lensing Anomalies

The Probing Lensing Anomalies NETwork (PLANET) is a worldwide collaboration of astronomers using semi-dedicated European, South African, and Australian telescopes to perform continuous, rapid and precise multi-band CCD photometric monitoring of on-going Galactic microlensing events. As well as providing important additional information on the nature, distribution and kinematics of Galactic microlenses, PLANET photometry is optimized for the detection of Jovian-mass planets orbiting several AU from Galactic lenses. The final PLANET database is expected to contain hundreds of variable stars sampled at hourly time scales with 1-5% precision.

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