arXiv subjects

Kailash C. Sahu

Publications and source records attributed to Kailash C. Sahu.

48 records · Page 3Linked to original sources

arXiv1998

Spectroscopy of MACHO 97-SMC-1: self-lensing within the SMC

More than a dozen microlensing events have been detected so far towards the LMC and 2 towards the SMC. If all the lenses are in the Galactic halo, both the LMC and the SMC events are expected to have similar time scales. However, the first event towards the SMC, MACHO 97-SMC-1, had a time scale of 123 days which is much larger than the typical time scale for the LMC events. From optical depth estimates, we first show that the stars within the SMC play a dominant role as gravitational lenses and can fully account for the observed microlensing events, mainly due to its large physical depth. We also show that if the lenses are within the Magellanic Clouds, then the SMC events should be longer in duration than the LMC events, a fact that is consistent with the observations. The time scale of the event implies that the mass of the lens is >2 solar masses if it is in the Milky Way disk or halo, in which case the lens is expected to be bright and should reveal itself in the spectrum. Here, we present an optical spectrum of MACHO 97-SMC-1 which shows that the lens is not in the Milky Way disk or halo, but is a low-mass star within the SMC. It is worth noting here that MACHO SMC-98-1 is the only OTHER observed event towards the SMC. This was a binary lens event where the caustic crossing time-scale as observed by PLANET, MACHO, EROS and OGLE collaborations, suggests that the lens is within the SMC. Furthermore, the only LMC event where the location of the lens is known is the binary lens event MACHO LMC-9, for which the lens is also within the LMC. Thus, ALL the 3 microlensing events towards the Magellanic Clouds for which we have independent knowledge of the location of the lenses are due to self-lensing within the Magellanic Clouds.

astro-ph↗

arXiv1998

HST/STIS Observations of the Optical Counterpart to GRB 970228

We report on observations of the fading optical counterpart of the gamma-ray burst GRB 970228, made on 4~September~1997 using the STIS CCD on the Hubble Space Telescope. The unresolved counterpart is detected at V=28 +/- 0.25, consistent with a continued power-law decline with exponent -1.14 +/- 0.05. No proper motion is detected, in contradiction of an earlier claim. The counterpart is located within, but near the edge of, a faint extended source with diameter ~0."8 and integrated magnitude 25.7 +/- 0.25. Comparison with WFPC2 data taken one month after the initial burst and NTT data taken on March 13 shows no evidence for fading of the extended emission. After adjusting for the probable Galactic extinction in the direction of GRB 970228 of A_v=0.7, we find that the observed nebula is consistent with the sizes of galaxies of comparable magnitude found in the Hubble Deep Field and other deep HST images, and that 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 most likely a galaxy at moderate redshift, and is almost certainly the host of the gamma-ray burst.

astro-ph↗

arXiv1998

Imaging and spectroscopy of arcs around the most luminous X-ray cluster RX J1347.5-1145

The cluster RX J1347.5-1145, the most luminous cluster in the X-ray wavelengths, was imaged with the newly installed Space Telescope Imaging Spectrograph (STIS) on-board HST. Its relatively high redshift (0.451) and luminosity indicate that this is one of the most massive of all known clusters. The STIS images unambiguously show several arcs in the cluster. The largest two arcs (> 5 arcsec in length) are symmetrically situated on opposite sides of the cluster, at a distance of ~ 35 arcsec from the central galaxy. The STIS images also show approximately 100 faint galaxies within the radius of the arcs whose combined luminosity is ~ 4 x 10^11 Lsun. We also present ground-based spectroscopic observations of the northern arc which show one clear emission line at 6730 A, which is consistent with an identification as [OII] 3727 A, implying a redshift of 0.81 for this arc. The southern arc shows a faint continuum but no emission features. The surface mass within the radius of the arcs (240 kpc), as derived from the gravitational lensing, is 6.3 x 10^14 Msun. The resultant mass-to-light ratio of ~1200 is higher than what is seen in many clusters but smaller than the value recently derived for some `dark' X-ray clusters (Hattori et al. 1997). The total surface mass derived from the X-ray flux within the radius of the arcs is ~2.1 - 6.8 x 10^14 Msun, which implies that the ratio of the gravitational to the X-ray mass is ~1 to 3. The surface GAS mass within this radius is ~3.5 x 10^13 Msun, which implies that at least 6% of the total mass within this region is baryonic.

astro-ph↗

arXiv1997

STIS Near Ultraviolet Time-Tagged Spectra of the Crab Pulsar

We present the spectrum and pulse profile of the Crab Pulsar in the near ultraviolet (1600-3200 Angstroms) observed with the Space Telescope Imaging Spectrograph (STIS) during the Hubble Space Telescope (HST) second Servicing Mission Orbital Verification (SMOV) period. The two-dimensional Near-Ultraviolet Multi-Anode Microchannel Array (NUV MAMA) was used in time-tag mode with a 2 arcsec by 2 arcsec aperture and the low dispersion grating, G230L, to obtain a cube with axes of slit position, wavelength, and time. The observation-derived pulse profile is consistent with radio measurements, and the pulse profile agrees well with previous NUV broadband measurements by the High Speed Photometer. The pulsar spectrum includes the 2200 A dust absorption feature, plus several interstellar absorption lines. Dereddening the spectrum using the Savage-Mathis model with E(B-V)=0.55+- 0.05 leads to a good fit to a power law with slope of -0.3+-0.2. Spectra of the main pulse, the interpulse, and the individual rising and falling edges are similar to the total spectrum within the limits of photon statistics. The pulse profile is stable across the NUV spectral range. Histogram analysis reveals no evidence for the superpulses seen at radio wavelengths. The interstellar absorption line equivalent widths of Mg I, Mg II and FeII are lower than expected based upon the implied HI column density from E(B-V)=0.5. While several explanations are possible, additional studies will be necessary to narrow the options.

astro-ph↗

arXiv1997

The decay of optical emission from the Gamma-Ray Burst GRB 970228

We present the R_c band light curve of the optical transient (OT) associated with GRB970228, based on re-evaluation of existing photometry. Data obtained until April 1997 suggested a slowing down of the decay of the optical brightness. However, the HST observations in September 1997 show that the light curve of the point source is well represented by a single power law, with a ``dip'', about a week after the burst occured. The exponent of the power law decay is $α$ = --1.10 $\pm$ 0.04. As the point source weakened it also became redder.

astro-ph↗

arXiv1997

The Gamma Ray Bursts GRB970228 and GRB970508: What Have We Learnt?

We examine what we regard as key observational results on GRB 970228 and GRB 970508 and show that the accumulated evidence strongly suggests that gamma-ray bursts (GRBs) are cosmological fireballs. We further show that the observations suggest that GRBs are not associated with the nuclear activity of active galactic nuclei, and that late-type galaxies are more prolific producers of GRBs. We suggest that GRBs can be used to trace the cosmic history of the star-formation rate. Finally, we show that the GRB locations with respect to the star-forming regions in their host galaxies and the total burst energies can be used to distinguish between different theoretical models for GRBs.

astro-ph↗

arXiv1997

Search for Extra-Solar Planets Through Monitoring Microlensing Events from Antarctica

During the months when the galactic bulge is visible from the southern hemisphere, there are typically about 8 to 10 on-going microlensing events at any given time. If the lensing stars have planets around them, then the signature of the planets can be seen as sharp, extra peaks on the microlensing light curves. And if every lensing star has a Jupiter around it, then the probability of detecting an extra spike is of the order of 10%. Thus continuous and frequent monitoring of the on-going microlensing events, with a sampling interval of a few hours, provides a powerful new method to search for planets around lensing stars. Such monitoring programs are now being carried out using a network of 1-meter class telescopes situated at appropriately spaced longitudes in the southern hemisphere (for example, by PLANET collaboration). However, the galactic bulge is visible from the south pole throughout this period, and hence a single automated telescope at the south-pole can provide an efficient means of carrying out the monitoring program. Up to about 20 events can be monitored during a single 3-month season with a 1-meter telescope, potentially leading to the detection of two planetary signals. The telescope can also be used for several other research projects involving microlensing and variability of stars.

astro-ph↗

arXiv1997

Observations of GRB 970228 and GRB 970508, and the Neutron-Star Merger Model

We present the discovery observations for the optical counterpart of the $γ$-ray burster GRB 970508 and discuss its light curve in the context of the fireball model. We analyze the HST data for this object, and conclude that any underlying galaxy must be fainter than $R$ = 25.5. We also present a detailed analysis of the HST images of GRB 970228 claimed to show a proper motion of the optical counterpart and conclude that, within the uncertainties, there is no proper motion. We examine several aspects of the neutron-star merger model for $γ$-ray bursts. In particular, we use this model to predict the redshift distribution of $γ$-ray bursters, and adopting a recent determination of the cosmic star-formation history, we show that the predicted distribution of $\log N - \log P$ relation is consistent with that observed for GRBs.

astro-ph↗

arXiv1997

The optical counterpart to gamma-ray burst GRB970228 observed using the Hubble Space Telescope

Although more than 2,000 astronomical gamma-ray bursts (GRBs) have been detected, and numerous models proposed to explain their occurrence, they have remained enigmatic owing to the lack of an obvious counterpart at other wavelengths. The recent ground-based detection of a transient source in the vicinity of GRB 970228 may therefore have provided a breakthrough. The optical counterpart appears to be embedded in an extended source which, if a galaxy as has been suggested, would lend weight to those models that place GRBs at cosmological distances. Here we report the observations using the Hubble Space Telescope of the transient counterpart and extended source 26 and 39 days after the initial gamma-ray outburst. We find that the counterpart has faded since the initial detection (and continues to fade), but the extended source exhibits no significant change in brightness between the two dates of observations reported here. The size and apparent constancy between the two epochs of HST observations imply that it is extragalactic, but its faintness makes a definitive statement about its nature difficult. Nevertheless, the decay profile of the transient source is consistent with a popular impulsive-fireball model, which assumes a merger between two neutron stars in a distant galaxy.

astro-ph↗

arXiv1997

Detecting Planets Through Microlensing

More than 100 microlensing events have been detected during the last ~4 years, most of them towards the Galactic Bulge. Since the line of sight towards the Bulge passes through the disk and the Bulge itself, the known stars towards the Bulge play a dominant role as gravitational lenses. If these stars have planets around them, then the signature of the planets can be seen as sharp, extra peaks on the microlensing light curves. Frequent, continuous monitoring of the on-going microlensing events thus provides a powerful new method to search for planets around lensing stars. Here I first review the background on stars acting as gravitational lenses. I then review the theoretical work on possible observational features due to planets, and the probability of detecting the planets through microlensing. I then discuss the status/strategy/results of the observational programs currently active in this field.

astro-ph↗

arXiv1994

Microlensing Events of the LMC are Better Explained by Stars within the LMC Than by Machos

The recently reported microlensing events of the LMC have caused much excitement, and have been interpreted as due to `dark objects' (MACHOs) in the halo of our Galaxy. It is shown here that the stars within the LMC play a dominant role as gravitational lenses and can indeed account for the observed events. The MACHO event and one of the EROS events lie within the bar for which the probability of microlensing is consistent with being caused by an object within the LMC. If the stars within the LMC play a dominant role as lenses, the events should be concentrated towards the center of the LMC. If MACHOs play a dominant role as lenses then, for a given number of monitored stars, the events should be uniformly distributed over the whole of the LMC. Thus the galactic and the LMC lenses can be statistically distinguished in most cases. It is further shown that, under certain conditions, the light curve of an event caused by a star within the LMC would be different from the one caused by a MACHO. This can also be a distinguishing signature, and seems to have been observed in case of the MACHO event. The fit discrepancy near the peak which the authors say ``is not yet understood" would be a natural consequence if the event is caused by an object within the LMC, which further indicates that the lensing is due to a low mass star within the LMC itself.

astro-ph↗

arXiv1994

Stars within the Large Magellanic Cloud as Potential Lenses for Observed Microlensing Events

Massive Compact objects in the halo, known as MACHOs, have been postulated as the origin of a substantial fraction of `dark matter' known to exist in the haloes of galaxies$^{1,2}$. Paczyński$^3$ has suggested that it might possible to detect these low-luminosity objects by their potential to act as gravitational lenses, causing a characteristic brightening when they cross the path of light from a star in a nearby galaxy. very recently, two groups reported possible detections of microlensing of stars in the Large Magellanic Cloud (LMC)$^{4,5}$. Here I show that microlensing by stars within the LMC itself can account for the observed events. It is further shown that if stars within the LMC are the lenses, the observed light curve can differ from the light curve due to a galactic lens even at relatively low magnifications. This provides a possibility of distinguishing between the galactic lenses and the LMC lenses. For a given number of monitored stars, the LMC induced events should be strongly concentrated towards the central region of the LMC, while the galactic events should be uniformly spread over the whole area of the LMC, so the two can also be distinguished statistically.

astro-ph↗