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Kulkarni, S. R.

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

At least 37 records · Page 2
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NASA NTRS2013

X-ray Emission from Supernovae in Dense Circumstellar Matter Environments: a Search for Collisionless Shocks

The optical light curve of some supernovae (SNe) may be powered by the outward diffusion of the energy deposited by the explosion shock (the so-called shock breakout) in optically thick (Tau approx > 30) circumstellar matter (CSM). Recently, it was shown that the radiation-mediated and radiation-dominated shock in an optically thick wind must transform into a collisionless shock and can produce hard X-rays. The X-rays are expected to peak at late times, relative to maximum visible light. Here we report on a search, using Swift/XRT and Chandra, for X-ray emission from 28 SNe that belong to classes whose progenitors are suspected to be embedded in dense CSM. Our sample includes 19 Type IIn SNe, one Type Ibn SN, and eight hydrogen-poor superluminous SNe (SLSN-I such as SN 2005ap). Two SNe (SN 2006jc and SN 2010jl) have X-ray properties that are roughly consistent with the expectation for X-rays from a collisionless shock in optically thick CSM. However, the X-ray emission from SN 2006jc can also be explained as originating in an optically thin region. Thus, we propose that the optical light curve of SN 2010jl is powered by shock breakout in CSM. We suggest that two other events (SN 2010al and SN 2011ht) were too X-ray bright during the SN maximum optical light to be explained by the shock-breakout model.We conclude that the light curves of some, but not all, SNe IIn/Ibn are powered by shock breakout in CSM. For the rest of the SNe in our sample, including all of the SLSN-I events, our X-ray limits are not deep enough and were typically obtained too early (i.e., near the SN maximum light) for definitive conclusions about their nature. Late-time X-ray observations are required in order to further test whether these SNe are indeed embedded in dense CSM. We review the conditions required for a shock breakout in a wind profile. We argue that the timescale, relative to maximum light, for the SN to peak in X-rays is a probe of the column density and the density profile above the shock region. In SNe whose X-ray emission slowly rises, and peaks at late times, the optical light curve is likely powered by the diffusion of shock energy in a dense CSM. We note that if the CSM density profile falls faster than a constant-rate wind-density profile, then X-rays may escape at earlier times than estimated for the wind-profile case. Furthermore, if the CSM has a region in which the density profile is very steep relative to a steady wind-density profile, or if the CSM is neutral, then the radio free-free absorption may be sufficiently low for radio emission to be detected.

matter↗
NASA NTRS2012

Early Radio and X-Ray Observations of the Youngest Nearby Type Ia Supernova PTF 11kly (SN 2011fe)

On August 24 (UT) the Palomar Transient Factory (PTF) discovered PTF11kly (SN 2011fe), the youngest and most nearby type Ia supernova (SN Ia) in decades. We followed this event up in the radio (centimeter and millimeter bands) and X-ray bands, starting about a day after the estimated explosion time. We present our analysis of the radio and X-ray observations, yielding the tightest constraints yet placed on the pre-explosion mass-loss rate from the progenitor system of this supernova. We find a robust limit of M(raised dot) less than or equal to 10(exp -8) (w /100 kilometers per second ) solar mass yr(exp -1) from sensitive X-ray non-detections, as well as a similar limit from radio data, which depends, however, on assumptions about microphysical parameters. We discuss our results in the context of single-degenerate models for SNe Ia and find that our observations modestly disfavor symbiotic progenitor models involving a red giant donor, but cannot constrain systems accreting from main sequence or sub-giant stars, including the popular supersoft channel. In view of the proximity of PTF11kly and the sensitivity of our prompt observations we would have to wait for a long time (decade or longer) in order to more meaningfully probe the circumstellar matter of Ia supernovae.

Horesh, Assaf↗
NASA NTRS2012

A VLA (Very Large Array) Search for 5 GHz Radio Transients and Variables at Low Galactic Latitudes

We present the results of a 5GHz survey with the Very Large Array (VLA) and the expanded VLA, designed to search for short-lived (approx < 1 day) transients and to characterize the variability of radio sources at milli-Jansky levels. A total sky area of 2.66 sq. deg, spread over 141 fields at low Galactic latitudes (b approx equals 6 - 8 deg) was observed 16 times with a cadence that was chosen to sample timescales of days, months and years. Most of the data were reduced, analyzed and searched for transients in near real time. Interesting candidates were followed up using visible light telescopes (typical delays of 1 - 2 hr) and the X-Ray Telescope on board the Swift satellite. The final processing of the data revealed a single possible transient with a flux density of f(sub v) approx equals 2.4mJy. This implies a transients sky surface density of kappa(f(sub v) > 1.8mJy) = 0.039(exp +0.13,+0.18) (sub .0.032,.0.038) / sq. deg (1, 2 sigma confidence errors). This areal density is consistent with the sky surface density of transients from the Bower et al. survey extrapolated to 1.8mJy. Our observed transient areal density is consistent with a Neutron Stars (NSs) origin for these events. Furthermore, we use the data to measure the sources variability on days to years time scales, and we present the variability structure function of 5GHz sources. The mean structure function shows a fast increase on approximately 1 day time scale, followed by a slower increase on time scales of up to 10 days. On time scales between 10 - 60 days the structure function is roughly constant. We find that approx > 30% of the unresolved sources brighter than 1.8mJy are variable at the > 4-sigma confidence level, presumably due mainly to refractive scintillation.

Ofek, E. O.↗
NASA NTRS2012

X-Ray Emission from Supernovae in Dense Circumstellar Matter Environments: A Search for Collisionless Shock

The optical light curve of some supernovae (SNe) may be powered by the outward diffusion of the energy deposited by the explosion shock (so-called shock breakout) in optically thick (tau approx > 30) circumstellar matter (CSM). Recently, it was shown that the radiation-mediated and -dominated shock in an optically thick wind must transform into 8. collisionless shock and can produce hard X-rays. The X-rays are expected to peak at late times, relative to maximum visible light. Here we report on a search, using Swift-XRT and Chandra, for X-ray emission from 28 SNe that belong to classes whose progenitors are suspected to be embedded in dense CSM. Our sample includes 19 type-IIn SNe, one type-Ibn SN and ei~ht hydrogen-poor super-luminous SNe (SLSN-I; SN 2005ap like). Two SNe (SN 2006jc and SN 2010jl) have X-ray properties that are roughly consistent with the expectation for X-rays from a collisionless shock in optically thick CSl\l. Therefore, we suggest that their optical light curves are powered by shock breakout in CSM. We show that two other events (SN 2010al and SN 2011ht) were too X-ray bright during the SN maximum optical light to be explained by the shock breakout model. We conclude that the light curves of some, but not all, type-IIn/Ibn SNe are powered by shock breakout in CSM. For the rest of the SNe in our sample, including all the SLSN-I events, our X-ray limits are not deep enough and were typically obtained at too early times (i.e., near the SN maximum light) to conclude about their nature. Late time X-ray observations are required in order to further test if these SNe are indeed embedded in dense CSM. We review the conditions required for a shock breakOut in a wind profile. We argue that the time scale, relative to maximum light, for the SN to peak in X-rays is a probe of the column density and the density profile above the shock region. The optical light curves of SNe, for which the X-ray emission peaks at late times, are likely powered by the diffusion of shock energy from a dense CSM. We note that if the CSM density profile falls faster than a constant-rate wind density profile, then X-rays may escape at earlier times than estimated for the wind profile case. Furthermore, if the CSM have a region in which the density profile is very steep, relative to a steady wind density profile, or the CSM is neutral, then the radio free-free absorption may be low enough, and radio emission may be detected.

Ofek, E.O↗
NASA NTRS2011

Swift J2058.4+0516: Discovery of a Possible Second Relativistic Tidal Disruption Flare

We report the discovery by the Swift hard X-ray monitor of the transient source Swift J2058.4+0516 (Sw J2058+05). Our multi-wavelength follow-up campaign uncovered a long-lived (duration approximately greater than months), luminous X-ray (L(sub x.iso) approximates 3 X 10(exp47) erg/s) and radio (vL(sub v.iso) approximates 10(exp 42) erg/s) counterpart. The associated optical emission, however, from which we measure a redshift of 1.1853, is relatively faint, and this is not due to a large amount of dust extinction in the host galaxy. Based on numerous similarities with the recently discovered GRB 110328A / Swift 1164449.3+573451 (Sw 11644+57), we suggest that Sw J2058+05 may be the second member of a new class of relativistic outbursts resulting from the tidal disruption of a star by a supermassive black hole. If so, the relative rarity of these sources implies that either these outflows are extremely narrowly collimated (theta < 1 deg), or only a small fraction of tidal disruptions generate relativistic ejecta. Analogous to the case of long duration gamma-ray bursts and core-collapse supernovae, we speculate that the spin of the black hole may be a necessary condition to generate the relativistic component. Alternatively, if powered by gas accretion (i.e., an active galactic nucleus), this would imply that some galaxies can transition from apparent quiescence to a radiatively efficient state of accretion on quite short time scales.

Cenko, S. Bradely↗
NASA NTRS2007

A Spectacular Radio Flare from XRF 050416a at 40 Days and Implications for the Nature of X-Ray Flashes

We present detailed optical, near-infrared, and radio observations of the X-ray flash 050416a obtained with Palomar and Siding Springs Observatories as well as HST and the VLA, placing this event among the best-studied X-ray flashes to date. In addition, we present an optical spectrum from Keck LRIS from which we measure the redshift of the burst, Z=0.6528. At this redshift the isotropic-equivalent prompt energy release was about 10(exp 51) erg, and using a standard afterglow synchrotron model we find that the blastwave kinetic energy is a factor of 10 larger, E-K,iso approximately equals 10 (exp 52) erg. The lack of an observed jet break to t - 20 days indicates that the opening angle is larger than 7 deg and the total beaming-corrected relativistic energy is larger than 10 exp (50) erg. We further show that the burst produced a strong radio flare at t is similar to 40 days accompanied by an observed flattening in the X-ray band which we attribute to an abrupt circumburst density jump or an episode of energy injection (either from a refreshed shock or off-axis ejecta). Late-time observations with HST show evidence for an associated supernova with peak optical luminosity roughly comparable to that of SN 1998bw. Next, we show that the host galaxy of XRF 050416a is actively forming stars at a rate of at least 2 M-solar per year with a luminosity of L-B is similar to 0.5L* and metallicity of Z is similar to 0.2-0.8 Z-solar. Finally, we discuss the nature of XRF 050416a in the context of short-hard gamma-ray bursts and under the framework of off-axis and dirty fireball models for X-ray flashes.

Soderberg, A. M.↗
NASA NTRS2004

A distance of 133-137 parsecs to the Pleiades star cluster

Nearby open clusters of stars (those that are not gravitationally bound)have played a crucial role in the development of steller astronomy because, as a consequence of the stars having a common age, they provide excellent natural laboratories to test theoretical stellar models.

Theoretical↗
NASA NTRS2003

Gamma-ray Burst Energetics an the Gamma-ray Burst Hubble Diagram: Promises and Limitations

We present a complete sample of 29 gamma-ray bursts (GRBs) for which it has been possible to determine temporal breaks (or limits) from their afterglow light curves. We interpret these breaks within the framework of the uniform conical jet model, incorporating realistic estimates of the ambient density and propagating error estimates on the measured quantities. In agreement with our previous analysis of a smaller sample, the derived jet opening angles of those 16 bursts with redshifts result in a narrow clustering of geometrically corrected gamma-ray energies about 1.33 x 10(exp 51) ergs; the burst-to-burst variance about this value is 0.35 dex, a factor of 2.2. Despite this rather small scatter, we demonstrate in a series of GRB Hubble diagrams that the current sample cannot place meaningful constraints upon the fundamental parameters of the universe. Indeed, for GRBs to ever be useful in cosmographic measurements, we argue the necessity of two directions. First, GRB Hubble diagrams should be based upon fundamental physical quantities such as energy, rather than empirically derived and physically ill-understood distance indicators (such as those based upon prompt burst time-profiles and spectra). Second, a more homogeneous set should be constructed by culling subclasses from the larger sample. These subclasses, although now first recognizable by deviant energies, ultimately must be identifiable by properties other than those directly related to energy. We identify a new subclass of GRBs (" f-GRBs ") that appear both underluminous by factors of at least 10 and exhibit a rapid fading (f(sub nu is proportional to t(sup -2) at early times (t < or = 0.5 day). About 10%-20% of observed long-duration bursts appear to be f-GRBs.

Bloom, J. S.↗
NASA NTRS2001

A Faint Near-Infrared Counterpart to the AXP 1E 2259+58.6

We present near-infrared and optical observations of the field of the Anomalous X-ray Pulsar 1E 2259+58.6 taken with the Keck telescope. We derive a subarcsecond Chandra position and tie it to our optical reference frame using other stars in the field. We find a very faint source, K(s) = 21.7 +/- 0.2 mag, with a position coincident with the Chandra position. We argue that this is the counterpart. In the J, I, and R bands, we derive (two sigma) limits of 23.8, 25.6 and 26.4mag, respectively. As with 4U 0142+61, for which a similarly faint counterpart was found, our results are inconsistent with models in which the source is powered by accretion from a disk. The only model that is not inconsistent, appears to be that in which 1E 2259+58.6 is a magnetar.

Hulleman, F.↗
NASA NTRS1998

The Discovery and Broadband Follow-Up of the Transient Afterglow of GRB 980703

We report on the discovery of the radio, infrared, and optical transient coincident with an X-ray transient proposed to be the afterglow of GRB 980703. At later times when the transient has faded below detection, we see an underlying galaxy with R = 22.6; this galaxy is the brightest host galaxy (by nearly 2 mag) of any cosmological gamma-ray burst (GRB) thus far. In keeping with an established trend, the GRB is not significantly offset from the host galaxy. Interpreting the multiwavelength data in the framework of the popular fireball model requires that the synchrotron cooling break was between the optical and X-ray bands on 1998 July 8.5 UT and that the intrinsic extinction of the transient is A(sub v) = 0.9. This is somewhat higher than the extinction for the galaxy as a whole, as estimated from spectroscopy.

Bloom, J. S.↗
NASA NTRS1998

An Interferometric Search for Bright Companions to 51 Pegasi

We report on a near-infrared, long-baseline interferometric search for luminous companions to the star 51 Pegasi conducted with the Palomar Testbed Interferometer. Our data is completely consistent with a single-star hypothesis.

binaries spectroscopy planetary systems stars 51 P↗
NASA NTRS1996

Spectral Energy Distribution and Bolometric Luminosity of the Cool Brown Dwarf Gliese 229B

Infrared broadband photometry of the cool brown dwarf Gliese 229B extending in wavelength from 0.8 to 10.5 micron is reported. These results are derived from both new data and reanalyzed, previously published data. Existing spectral data reported have been rereduced and recalibrated. The close proximity of the bright Gliese 229A to the dim Gliese 229B required the use of special techniques for the observations and also for the data analysis. We describe these procedures in detail. The observed luminosity between 0.8 and 10.5 micron is (4.9 +/- 0.6) x 10(exp -6) solar luminosity. The observed spectral energy distribution is in overall agreement with a dust-free model spectrum by Tsuji et al. for T(eff) approx. equal to 900 K. If this model is used to derive the bolometric correction, the best estimate of the bolometric luminosity is 6.4 x 10(exp -6) solar luminosity and 50% of this luminosity ties between 1 and 2.5 microns. Our best estimate of the effective temperature is 900 K. From the observed near-infrared spectrum and the spectral energy distribution, the brightness temperatures (T(sub B) are estimated. The highest, T(sub B) = 1640 K, is seen at the peak of the J band spectrum, while the lowest, T(sub B) is less than or equal to 600 K, is at 3.4 microns, which corresponds to the location of the fundamental methane band.

Matthews, K.↗
NASA NTRS1995

Detection of a very low mass companion to the astrometric binary Gliese 105A

Optical coronagraph images of the high proper motion astrometric binary Gl 105A reveal a very red companion, Gl 105C, located 3.27 arcsecs from Gl 105A at P.A. 287 deg. At this location, it is not clear whether Gl 105C can fully account for the astrometric perturbation of Gl 105A. Aperture photometry gives I(sub C) = 12.6 and R(sub C) - I(sub C) = 3.7 for Gl 105C, indicating that it is a very low mass M dwarf. Using the observed I(sub C), an empirical M(sub I) versus I-K relation, and an assumed distance of 8.2 pc to Gl 105A, M(sub K) = 9.7 is derived for Gl 105C. An empirical mass-M(sub K) relation for low-mass stars suggest a mass of 0.084 solar mass for Gl 105C, which is just above the minimum mass for stable hydrogen burning. Gl 105C was not detected in previous K-band searches; its detection demonstrates the usefulness of optical coronagraphy for identifying very low mass objects.

Golimowski, D. A.↗
NASA NTRS1995

A luminous companion to SGR 1806-20

We have obtained infrared spectra of the star suggested to be the counterpart of the soft gamma-ray repeater (SGR) 1806-20. We found strong emission lines similar to those seen in the spectra of the rare luminous blue variables and B(e) stars. A He I absorption line is also seen, from which we infer a spectral type O9-B2. This classification, in combination with the minimum distance of greater than or approximately = 6 kpc inferred from its extinction, makes the star one of the most luminous in the Galaxy. We infer that it is a companion to SGR 1806-20 and suggest that the presence of a companion is somehow related to the SGR phenomenon.

Van Kerkwijk, M. H.↗
NASA NTRS1995

Optical and infrared observations of SGR 1806-20

The soft gamma-ray repeater (SGR) 1806-20 is associated with the center-brightened nonthermal nebula G10.0-0.3, thought to be a plerion. As in other plerions, a steady X-ray source, AX 1805.7-2025, has been detected coincident with the peak of the nebular radio emission. Vasisht et al. have shown that the radio peak has a core-jet appearance and argue that the core marks the true position of the SGR. At optical wavelengths, we detect three objects in the vicinity of the radio core. Only for the star closest to the core, barely visible in the optical but bright in the infrared (K-8.4 mag), the reddening is consistent with the high extinction (A(sub V) is approximately equal to 30 mag) that has been inferred for AX 1805.7 - 2025. From the absence of CO band absorption, we infer that the spectral type of this star is earlier than late-G/early-K. The large extinction probably arises in a molecular cloud located at a distance of 6 kpc, which means that the star, just like AX 1805.7-2025, is in or behind this cloud. This implies that the star is a supergiant. Since supergiants are rare, a chance coincidence with the compact radio core is very unlikely. To our knowledge, there are only three other examples of luminous stars embedded in nonthermal radio nebulae: SS 433, Cir X-1, and G70.7+1.2. Given this and the low coincidence probability, we suggest that the bright star is physically associated with SGR 1806-20, making it the first stellar indentification of a high-energy transient.

Kulkarni, S. R.↗
NASA NTRS1995

G359.23-0.92, the Mouse, a pulsar powered bow shock?

We have discovered a faint X-ray object coincident with the head of the 'Mouse' (G359.23-0.82), a member of the rare class of axially symmetric radio nebula. Such objects are believed to be excited by a fast moving central source which supplies relativistic particles that emit the observed non-thermal radiation. The origin of most such objects and the Mouse in particular is obscure. We argue that the Mouse is the wake left by a young pulsar rather than an accreting binary, a popular model for this class. On phenomenological grounds we suggest that the Mouse is similar in some ways to the bow shock of the moderately young pulsar 1951+32 embedded in the supernova remnant CTB80. The rarity of Mouse-like nebulae is puzzling and can only be explained by assuming that most pulsars are in low density media.

Predehl, P.↗