arXiv subjects

S. R. Kulkarni

Publications and source records attributed to S. R. Kulkarni.

At least 307 records · Page 17Linked to original sources

arXiv2006

An X-ray Search for Compact Central Sources in Supernova Remnants II: Six Large Diameter SNRs

We present the second in a series of results in which we have searched for undiscovered neutron stars in supernova remnants (SNRs). This paper deals with the largest six SNRs in our sample, too large for Chandra or XMM-Newton to cover in a single pointing. These SNRs are nearby, with typical distances of <1 kpc. We therefore used the ROSAT Bright Source Catalog and past observations in the literature to identify X-ray point sources in and near the SNRs. Out of 54 sources, we were immediately able to identify optical/IR counterparts to 41 from existing data. We obtained Chandra snap-shot images of the remaining 13 sources. Of these, 10 were point sources with readily identified counterparts, two were extended, and one was not detected in the Chandra observation but is likely a flare star. One of the extended sources may be a pulsar wind nebula, but if so it is probably not associated with the nearby SNR. We are then left with no identified neutron stars in these six SNRs down to luminosity limits of \~1e32 ergs/s. These limits are generally less than the luminosities of typical neutron stars of the same ages, but are compatible with some lower-luminosity sources such as the neutron stars in the SNRs CTA 1 and IC 443.

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arXiv2006

Evidence of Orbital Motion in the Binary Brown Dwarf Kelu-1AB

We have resolved Kelu-1 into a binary system with a separation of ~290 mas using the Laser Guide Star Adaptive Optics system on the Keck II telescope. We have also re-analyzed a 1998 HST observation of Kelu-1 and find that the observed PSF is best fit by a binary object separated by 45 mas. Observations on multiple epochs confirm the two objects share a common proper motion and clearly demonstrate the first evidence of orbital motion. Kelu-1B is fainter than Kelu-1A by 0.39+/-0.01 magnitudes in the K' filter and 0.50+/-0.01 magnitudes in the H filter. We derive spectral types of L2+/-1 and L3.5+/-1 for Kelu-1A and B, respectively. The separation of flux into the two components rectifies Kelu-1's over-luminosity problem that has been known for quite some time. Given the available data we are able to constrain the inclination of the system to >81 degrees and the orbital period to >~40 years.

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arXiv2006

The ultra-cool white dwarf companion of PSR J0751+1807

We present optical and near-infrared observations with Keck of the binary millisecond pulsar PSR J0751+1807. We detect a faint, red object - with R=25.08+-0.07, B-R=2.5+-0.3, and R-I=0.90+-0.10 - at the celestial position of the pulsar and argue that it is the white dwarf companion of the pulsar. The colours are the reddest among all known white dwarfs, and indicate a very low temperature, Teff~4000 K. This implies that the white dwarf cannot have the relatively thick hydrogen envelope that is expected on evolutionary grounds. Our observations pose two puzzles. First, while the atmosphere was expected to be pure hydrogen, the colours are inconsistent with this composition. Second, given the low temperature, irradiation by the pulsar should be important, but we see no evidence for it. We discuss possible solutions to these puzzles.

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arXiv2005

Spectroscopy of GRB 051111 at z=1.54948: Kinematics and Elemental Abundances of the GRB Environment and Host Galaxy

We present a high-resolution, high signal-to-noise optical spectrum of the afterglow of GRB 051111 obtained with the HIRES spectrograph on the Keck I 10-m telescope. The spectrum exhibits three redshifted absorption systems with the highest, at z=1.54948, arising in the GRB host galaxy. While the Ly-alpha feature is outside the range of our spectrum, the high column density of weakly-depleted Zn suggests that the host is a damped Lyman-alpha system with N(HI)>10^21(Z/Z_sun)^-1. The bulk of the gas (>80%) is confined to a narrow velocity range of |v|<30 km/s exhibiting strong dust depletion of refractory elements such as Fe and Cr. The depletion pattern is similar to that observed in warm disk clouds of the Milky Way. We also detect absorption from all ground-level fine-structure states of FeII, the first such example in a QSO-DLA or GRB-absorption spectrum, which indicate conditions that are consistent with the "warm disk" depletion pattern. The absorption profiles of FeII and MgII extend over several hundred km/s, with a depletion pattern that more closely resembles that of QSO-DLAs, suggesting that the sight line to GRB 051111 probes the halo of the host galaxy in addition to the dense disk. Thus, detailed diagnostics of the interstellar medium of GRB host galaxies continue to provide insight into regions which are generally missed in quasar surveys.

astro-ph↗

arXiv2005

Fine-Structure FeII and SiII Absorption in the Spectrum of GRB 051111: Implications for the Burst Environment

We present an analysis of fine-structure transitions of FeII and SiII detected in a high-resolution optical spectrum of the afterglow of GRB 051111 (z=1.54948). The fine-structure absorption features arising from FeII* to FeII****, as well as SiII*, are confined to a narrow velocity structure extending over +/-30 km/s, which we interpret as the burst local environment, most likely a star forming region. We investigate two scenarios for the excitation of the fine-structure levels by collisions with electrons and radiative pumping by an infra-red or ultra-violet radiation field produced by intense star formation in the GRB environment, or by the GRB afterglow itself. We find that the conditions required for collisional excitation of FeII fine-structure states cannot be easily reconciled with the relatively weak SiII* absorption. Radiative pumping by either IR or UV emission requires >10^3 massive hot OB stars within a compact star-forming region a few pc in size, and in the case of IR pumping a large dust content. On the other hand, it is possible that the GRB itself provides the source of IR and/or UV radiation, in which case we estimate that the excitation takes place at a distance of ~10-20 pc from the burst. Detailed radiative transfer calculations are required in order to verify that excitation of the low-ionization fine-structure states is possible given the intense UV flux from the burst. Still, it is clear that GRB absorption spectroscopy can provide direct information on the mode and conditions of star formation at high redshift.

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arXiv2005

A Redshift Determination for XRF 020903: First Spectroscopic Observations of an X-Ray Flash

We report the discovery of optical and radio afterglow emission from the extremely soft X-ray flash, XRF 020903. Our spectroscopic observations provide the first redshift for an X-ray flash, thereby setting the distance scale for these events. At $z=0.251$, XRF 020903 is one of the nearest cosmic explosions ever detected, second only to the recent GRB 030329 and the unusual GRB 980425/SN 1998bw. Moreover, XRF 020903 is the first X-ray flash for which we detect an optical afterglow. The luminosity of the radio afterglow of XRF 020903 is 1000 times greater than that of Ibc supernovae but similar to those of GRB afterglows. From broadband afterglow modeling we show that the explosion energy of XRF 020903 is not dissimilar from values inferred for typical gamma-ray bursts, suggesting that these cosmological explosions may derive from a similar mechanism.

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arXiv2005

An HST Study of the Supernovae Accompanying GRB 040924 and GRB 041006

We present the results from a {\it Hubble Space Telescope/ACS} study of the supernovae associated with gamma-ray bursts 040924 ($z=0.86$) and 041006 ($z=0.71$). We find evidence that both GRBs were associated with a SN 1998bw-like supernova dimmed by $\sim 1.5$ and $\sim 0.3$ magnitudes, respectively, making GRB 040924 the faintest GRB-associated SN ever detected. We study the luminosity dispersion in GRB/XRF-associated SNe and compare to local Type Ibc supernovae from the literature. We find significant overlap between the two samples, suggesting that GRB/XRF-associated SNe are not necessarily more luminous nor produce more $^{56}$Ni than local SNe. Based on the current (limited) datasets, we find that the two samples may share a similar $^{56}$Ni production mechanism.

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arXiv2005

Late-time Radio Observations of 68 Type Ibc Supernovae: Strong Constraints on Off-Axis Gamma-ray Bursts

We present late-time radio observations of 68 local Type Ibc supernovae, including six events with broad optical absorption lines (``hypernovae''). None of these objects exhibit radio emission attributable to off-axis gamma-ray burst jets spreading into our line-of-sight. Comparison with our afterglow models reveals the following conclusions: (1) Less than $\sim 10%$ of Type Ibc supernovae are associated with typical gamma-ray bursts initially directed away from our line-of-sight; this places an empirical constraint on the GRB beaming factor of $ \lesssim 10^4$ corresponding to an average jet opening angle, $θ_j \gtrsim 0.8$ degrees. (2) This holds in particular for the broad-lined supernovae (SNe 1997dq, 1997ef, 1998ey, 2002ap, 2002bl and 2003jd) which have been argued to host GRB jets. Our observations reveal no evidence for typical (or even sub-energetic) GRBs and rule out the scenario in which every broad-lined SN harbors a GRB at the 84% confidence level. Their large photospheric velocities and asymmetric ejecta (inferred from spectropolarimetry and nebular spectroscopy) appear to be characteristic of the non-relativistic supernova explosion and do not necessarily imply the existence of associated GRB jets.

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arXiv2005

The afterglow and elliptical host galaxy of the short gamma-ray burst GRB 050724

Despite a rich phenomenology, gamma-ray bursts (GRBs) are divided into two classes based on their duration and spectral hardness -- the long-soft and the short-hard bursts. The discovery of afterglow emission from long GRBs was a watershed event, pinpointing their origin to star forming galaxies, and hence the death of massive stars, and indicating an energy release of about 10^51 erg. While theoretical arguments suggest that short GRBs are produced in the coalescence of binary compact objects (neutron stars or black holes), the progenitors, energetics, and environments of these events remain elusive despite recent localizations. Here we report the discovery of the first radio afterglow from a short burst, GRB 050724, which unambiguously associates it with an elliptical galaxy at a redshift, z=0.257. We show that the burst is powered by the same relativistic fireball mechanism as long GRBs, with the ejecta possibly collimated in jets, but that the total energy release is 10-1000 times smaller. More importantly, the nature of the host galaxy demonstrates that short GRBs arise from an old (>1 Gyr) stellar population, strengthening earlier suggestions, and providing support for coalescing compact object binaries as the progenitors.

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arXiv2005

Spectroscopy of GRB 050505 at z=4.275: A logN(HI)=22.1 DLA Host Galaxy and the Nature of the Progenitor

We present the discovery of the optical afterglow of GRB 050505 and an optical absorption spectrum obtained with the Keck I 10-m telescope. The spectrum exhibits three redshifted absorption systems with the highest, at z=4.2748, arising in the GRB host galaxy. The host absorption system is marked by a damped Ly-alpha (DLA) feature with a neutral hydrogen column density of logN(HI)=22.05+/-0.10, higher than that of any QSO-DLA detected to date, but similar to several other recent measurements from GRB spectra. In addition, we detect absorption lines from both low- and high-ionization species from which we deduce a metallicity, Z~0.06 Z_solar, with a depletion pattern that is roughly similar to that of the Galactic warm halo, warm disk, or disk+halo. More importantly, we detect strong absorption from SiII* indicating a dense environment, n_H>10^2 cm^-3, in the vicinity of the burst, with a size of about 4 pc. In addition, the CIV absorption system spans a velocity range of about 1000 km/s, which is not detected in any other absorption feature. We show that the most likely interpretation for this wide velocity range is absorption in the wind from the progenitor star. In this context, the lack of corresponding SiIV absorption indicates that the progenitor had a mass of <25 M_solar and a metallicity <0.1 Z_solar, and therefore required a binary companion to eject its hydrogen envelope prior to the GRB explosion. Finally, by extending the GRB-DLA sample to z~4.3 we show that these objects appear to follow a similar metallicity-redshift relation as in QSO-DLAs, but with systematically higher metallicities. It remains to be seen whether this trend is simply due to the higher neutral hydrogen columns in GRB-DLAs, or if it is a manifestation of different star formation properties in GRB-DLAs. [abridged]

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arXiv2005

The Progenitors of Short-Hard Gamma-Ray Bursts from an Extended Sample of Events

The detection of the afterglow emission and host galaxies of short-hard gamma-ray bursts (SHBs) is one of the most exciting recent astronomical discoveries. Indications that SHB progenitors belong to old stellar populations, in contrast to those of the long-soft GRBs, provide a strong clue about their physical nature. Definitive conclusions however are limited by the small number of SHBs with known hosts. Here, we present our investigation of SHBs previously localized by the interplanetary network (IPN) using new and archival optical and X-ray observations. We show that we can likely identify the host galaxies/clusters for additional two bursts, significantly increasing the sample of SHBs with known hosts and/or distances. In particular, we determine that the bright SHB 790613 occurred within the rich galaxy cluster Abell 1892, making it probably the nearest SHB currently known. We show that the brightest galaxy within the error box of SHB 000607, at z=0.14, is most likely the host galaxy of this event. Additionally, we rule out the existence of galaxy overdensities (down to ~21mag) near the locations of two other SHBs, and set a lower limit on their probable redshift. We combine our SHB sample with events discovered recently by the Swift and HETE-2 missions, and investigate the properties of the extended sample. Comparison to SNe Ia shows that the progenitors of SHBs are typically older, implying a typical life time of several Gy. We also show that it is unlikely that there is a significant population of progenitors with life time shorter than 1Gy. This result disfavors the popular model of NS-NS mergers as the progenitors of SHBs.

astro-ph↗

arXiv2005

Modeling Supernova-like Explosions Associated with Gamma-ray Bursts with Short Durations

There is now good evidence linking short-hard GRBs with both elliptical and spiral galaxies at relatively low redshifts, redshift of about 0.2. This contrasts with the average redshift of about 2 of long-duration events, which also occur only in star-forming galaxies. The diversity of hosts is reminiscent of type Ia supernovae, which are widely (but not universally) believed to originate from the coalescence of white dwarfs. By analogy, it has been postulated that short-hard bursts originate from neutron star mergers. Mergers, as well as stellar core-collapse events (type II SNe and long-duration GRBs) are accompanied by long-lived sub-relativistic components powered by radioactive decay of unstable elements produced in the explosion. It is therefore interesting to explore whether short duration events also have ejecta powered by radioactivity (i.e. that are supernova-like). Observations already inform us that any supernova like component in the first few well studied short hard bursts must be fainter than those typical of type Ia or core-collapse supernovae. Rather than refer to weaker supernova-like component as ``mini-super nova'', an etymologically indefensible term, I use the term {\it macronova}. I investigate the observability of macronovae powered by neutron decay and by radioactive Nickel. Separately, I note that a macronova will reprocess energetic emission arising from a long lived central source. I find that surprisingly interesting limits on the basic parameters of macronovae can be obtained provided observations are obtained with current 10-m class telescopes over a range of one hour to one day following the burst.

astro-ph↗

arXiv2005

The afterglow of GRB050709 and the nature of the short-hard gamma-ray bursts

The final chapter in the long-standing mystery of the gamma-ray bursts (GRBs) centres on the origin of the short-hard class, suspected on theoretical grounds to result from the coalescence of neutron star or black hole binary systems. Numerous searches for the afterglows of short-hard bursts have been made, galvanized by the revolution in our understanding of long-duration GRBs that followed the discovery in 1997 of their broadband (X-ray, optical, and radio) afterglow emission. Here we present the discovery of the X-ray afterglow of a short-hard burst whose accurate position allows us to unambiguously associate it with a star-forming galaxy at redshift z=0.160, and whose optical lightcurve definitively excludes a supernova association. Together with results from three other recent short-hard bursts, this suggests that short-hard bursts release much less energy than the long-duration GRBs. Models requiring young stellar populations, such as magnetars and collapsars, are ruled out, while coalescing degenerate binaries remain the most promising progenitor candidates.

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arXiv2005

PHASES Differential Astrometry and the Mutual Inclination of the V819 Herculis Triple Star System

V819 Herculis is a well-studied triple star system consisting of a ``wide'' pair with 5.5 year period, one component of which is a 2.2-day period eclipsing single-line spectroscopic binary. Differential astrometry measurements from the Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES) are presented and used to determine a relative inclination between the short- and long-period orbits of 23.6 +- 4.9 degrees. This represents only the sixth unambiguous determination of the mutual inclination of orbits in a hierarchical triple system. This result is combined with those for the other five systems for analysis of the observed distribution of mutual inclinations in nearby triple systems. It is found that this distribution is different than that which one would expect from random orientations with statistical significance at the 94% level; implications for studying the spatial distribution of angular momentum in star forming regions is discussed.

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arXiv2005

PHASES Differential Astrometry and Iodine Cell Radial Velocities of the kappa Pegasi Triple Star System

kappa Pegasi is a well-known, nearby triple star system. It consists of a ``wide'' pair with semi-major axis 235 milli-arcseconds, one component of which is a single-line spectroscopic binary (semi-major axis 2.5 milli-arcseconds). Using high-precision differential astrometry and radial velocity observations, the masses for all three components are determined and the relative inclinations between the wide and narrow pairs' orbits is found to be 43.8 +/- 3.0 degrees, just over the threshold for the three body Kozai resonance. The system distance is determined to 34.60 +/- 0.21 parsec, and is consistent with trigonometric parallax measurements.

astro-ph↗

arXiv2005

Density Evolution for Asymmetric Memoryless Channels

Density evolution is one of the most powerful analytical tools for low-density parity-check (LDPC) codes and graph codes with message passing decoding algorithms. With channel symmetry as one of its fundamental assumptions, density evolution (DE) has been widely and successfully applied to different channels, including binary erasure channels, binary symmetric channels, binary additive white Gaussian noise channels, etc. This paper generalizes density evolution for non-symmetric memoryless channels, which in turn broadens the applications to general memoryless channels, e.g. z-channels, composite white Gaussian noise channels, etc. The central theorem underpinning this generalization is the convergence to perfect projection for any fixed size supporting tree. A new iterative formula of the same complexity is then presented and the necessary theorems for the performance concentration theorems are developed. Several properties of the new density evolution method are explored, including stability results for general asymmetric memoryless channels. Simulations, code optimizations, and possible new applications suggested by this new density evolution method are also provided. This result is also used to prove the typicality of linear LDPC codes among the coset code ensemble when the minimum check node degree is sufficiently large. It is shown that the convergence to perfect projection is essential to the belief propagation algorithm even when only symmetric channels are considered. Hence the proof of the convergence to perfect projection serves also as a completion of the theory of classical density evolution for symmetric memoryless channels.

cs.IT↗

arXiv2005

A high angular-resolution search for the progenitor of the type Ic Supernova 2004gt

We report the results of a high-spatial-resoltion search for the progenitor of type Ic supernova SN 2004gt, using the newly commissioned Keck laser-guide star adaptive optics system (LGSAO) along with archival Hubble Space Telescope data. This is the deepest search yet performed for the progenitor of any type Ib/c event in a wide wavelength range stretching from the far UV to the near IR. We determine that the progenitor of SN 2004gt was most likely less luminous than M_V=-5.5 and M_B=-6.5 magnitudes. The massive stars exploding as hydrogen-deficient core-collapse supernovae (SNe) should have lost their outer hydrogen envelopes prior to their explosion, either through winds -- such stars are identified within our Galaxy as Wolf-Rayet (W-R) stars -- or to a binary companion. The luminosity limits we set rule out more than half of the known galactic W-R stars as possible progenitors of this event. In particular, they imply that a W-R progenitor should have been among the more-evolved (highly stripped, less luminous) of these stars, a concrete constraint on its evolutionary state just prior to core collapse. The possibility of a less luminous, lower-mass binary progenitor cannot be constrained. This study demonstrates the power of LGS observations in furthering our understanding of core collapse, and the physics powering supernovae, GRBs and XRFs.

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arXiv2005

PHASES High Precision Differential Astrometry of delta Equulei

delta Equulei is among the most well-studied nearby binary star systems. Results of its observation have been applied to a wide range of fundamental studies of binary systems and stellar astrophysics. It is widely used to calibrate and constrain theoretical models of the physics of stars. We report 27 high precision differential astrometry measurements of delta Equulei from the Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES). The median size of the minor axes of the uncertainty ellipses for these measurements is 26 micro-arcseconds. These data are combined with previously published radial velocity data and other previously published differential astrometry measurements using other techniques to produce a combined model for the system orbit. The distance to the system is determined to within a twentieth of a parsec and the component masses are determined at the level of a percent. The constraints on masses and distance are limited by the precisions of the radial velocity data; we outline plans improve this deficiency and discuss the outlook for further study of this binary.

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