S. R. Kulkarni

Precision Timing of PSR J0437?4715: An Accurate Pulsar Distance, a High Pulsar Mass, and a Limit on the Variation of Newton's Gravitational Constant

Analysis of 10 years of high-precision timing data on the millisecond pulsar PSR J0437–4715 has resulted in a model-independent kinematic distance based on an apparent orbital period derivative, P_b, determined at the 1.5% level of precision (D_k = 157.0 ± 2.4 pc), making it one of the most accurate stellar distance estimates published to date. The discrepancy between this measurement and a previously published parallax distance estimate is attributed to errors in the DE200 solar system ephemerides. The precise measurement of P_b allows a limit on the variation of Newtons gravitational constant, |G/G| ≤ 23 × 10^−12 yr^−1. We also constrain any anomalous acceleration along the line of sight to the pulsar to |a⊙/c| ≤ 1.5 × 10^−18 s^−1 at 95% confidence, and derive a pulsar mass, m_(psr) = 1.76 ± 0.20 M⊙, one of the highest estimates so far obtained.

Implications of the Radio Afterglow from the Gamma-Ray Burst of 1997 May 8

Radio observations of the afterglow of the γ-ray burst GRB 970508 provide unique new constraints on afterglow models. The quenching of diffractive scintillation at ~4 weeks delay provides the first direct estimate of source size and expansion rate. It implies an apparent size R ~ 1017 cm and an expansion at a speed comparable to that of light at t ~ 4 weeks, in agreement with the fireball model prediction, R = 1017(t/week)5/8 cm. The radio flux and its dependence on time and frequency at 1-5 weeks delay are in agreement with the model and imply a fireball energy (assuming spherical symmetry) ~1052 ergs, consistent with the value inferred from observations at shorter delay. The observed radio behavior deviates from model predictions at delays greater than 5 weeks. This is expected, since at this delay the fireball is in transition from highly relativistic to subrelativistic expansion, with Lorentz factor γ ≤ 2. Deviation may result from a change in the physical processes associated with the shock wave as it becomes subrelativistic (e.g., a decrease in the fraction of energy carried by the magnetic field) or from the fireball being a cone of opening angle ~ 1/γ ~ . We predict the future behavior of the radio flux assuming that the latter interpretation is valid. These predictions may be tested by radio observations in the frequency range 0.1-10 GHz on a timescale of months.

The sub-energetic gamma-ray burst GRB 031203 as a cosmic analogue to the nearby GRB 980425.

Over the six years since the discovery of the γ-ray burst GRB 980425, which was associated with the nearby (distance ∼40u2009Mpc) supernova 1998bw, astronomers have debated fiercely the nature of this event. Relative to bursts located at cosmological distance (redshift z ≈ 1), GRB 980425 was under-luminous in γ-rays by three orders of magnitude. Radio calorimetry showed that the explosion was sub-energetic by a factor of 10. Here we report observations of the radio and X-ray afterglow of the recent GRB 031203 (refs 5–7), which has a redshift of z = 0.105. We demonstrate that it too is sub-energetic which, when taken together with the low γ-ray luminosity, suggests that GRB 031203 is the first cosmic analogue to GRB 980425. We find no evidence that this event was a highly collimated explosion viewed off-axis. Like GRB 980425, GRB 031203 appears to be an intrinsically sub-energetic γ-ray burst. Such sub-energetic events have faint afterglows. We expect intensive follow-up of faint bursts with smooth γ-ray light curves (common to both GRB 031203 and 980425) to reveal a large population of such events.

A Comprehensive Study of GRB 070125, A Most Energetic Gamma-Ray Burst

We present a comprehensive multiwavelength analysis of the bright, long-duration gamma-ray burst GRB 070125, comprised of observations in gamma-ray, X-ray, optical, millimeter, and centimeter wave bands. Simultaneous fits to the optical and X-ray light curves favor a break on day 3.78, which we interpret as the jet break from a collimated outflow. Independent fits to optical and X-ray bands give similar results in the optical bands but shift the jet break to around day 10 in the X-ray light curve. We show that for the physical parameters derived for GRB 070125, inverse Compton scattering effects are important throughout the afterglow evolution. While inverse Compton scattering does not affect radio and optical bands, it may be a promising candidate to delay the jet break in the X-ray band. Radio light curves show rapid flux variations, which are interpreted as due to interstellar scintillation and used to derive an upper limit of 2.4 × 10^(17) cm on the radius of the fireball in the lateral expansion phase of the jet. Radio light curves and spectra suggest a high synchrotron self-absorption frequency indicative of the afterglow shock wave moving in a dense medium. Our broadband modeling favors a constant density profile for the circumburst medium over a windlike profile (R^(−2)). However, keeping in mind the uncertainty of the parameters, it is difficult to unambiguously distinguish between the two density profiles. Our broadband fits suggest that GRB 070125 is a burst with high radiative efficiency (>60%).

Detection of Magnetospheric X-Ray Pulsation from the Millisecond Pulsar B1821–24

We report the first clear detection of X-ray pulsation of magnetospheric origin from a millisecond pulsar, PSR B1821-24, with the Advanced Satellite for Cosmology and Astrophysics. The photon arrival time shows a periodicity of 3.05 ms period, as observed in the radio band. The observed X-ray pulse is double peaked. The pulses are characterized by a sharp temporal profile and hard power-law spectrum with a phase-averaged photon index of ~1.9. These two features are quite similar to the X-ray/γ-ray pulses from the Crab pulsar and characteristic of the nonthermal emission from the magnetosphere of the neutron star, but contradictory to the thermal emission from its surface. Since this pulsar has physical parameters completely different from those of young pulsars, we expect these observations to provide significant constraints on models of pulsar magnetospheric emission, which have been developed mostly based on the observations of young pulsars.

MASSES OF ASTROMETRICALLY DISCOVERED AND IMAGED BINARIES: G78-28AB AND GJ 231.1BC

The Stellar Planet Survey (STEPS) is an ongoing astrometric search for giant planets and brown dwarfs around a sample of ~30 M dwarfs. We have discovered several low-mass companions by measuring the motion of our target stars relative to their reference frames. The highest mass discovery thus far is G78-28B, a companion to the M dwarf G78-28A. The orbital period is 4.18 ± 0.03 yr, the system mass is 0.565 ± 0.055 M_☉, and the semimajor axis is 2.19 ± 0.10 AU. Imaging observations with the Keck laser guide star adaptive optics (LGSAO) and the Palomar AO instruments resolved the system and also yielded JHK-band delta magnitudes. We use the orbital solution, light ratios, and mass-luminosity relationships to derive component masses of M_A = 0.370 ± 0.034 M_☉ and M_B = 0.195 ± 0.021 M_☉. G78-28B is of type M4 V based on its colors and mass. We also discovered GJ 231.1C, a companion to GJ 231.1B, with STEPS and imaged the companion with LGSAO and Palomar AO, but the orbital period is longer than our observing baseline; thus, the system parameters are less constrained. In GJ 231.1BC the masses are M_B = 0.25 ± 0.06 M_☉ and M_C = 0.12 ± 0.02 M_☉. The inferred spectral type of GJ 231.1C is M5 V. We demonstrate the results of the current state of mass estimation techniques with our data.

The Visual Orbit of Pegasi

We have determined the visual orbit for the spectroscopic binary ι Pegasi with interferometric visibility data obtained by the Palomar Testbed Interferometer in 1997. ι Peg is a double-lined binary system whose minimum masses and spectral typing suggests the possibility of eclipses. Our orbital and component diameter determinations do not favor the eclipse hypothesis: the limb-to-limb separation of the two components is 0.151±0.069 mas at conjunction. Our conclusion that the ι Peg system does not eclipse is supported by high-precision photometric observations. The physical parameters implied by our visual orbit and the spectroscopic orbit of Fekel & Tomkin are in good agreement with those inferred by other means. In particular, the orbital parallax of the system is determined to be 86.9±1.0 mas, and masses of the two components are determined to be 1.326±0.016 and 0.819±0.009 M, respectively.

PTF 10fqs: A LUMINOUS RED NOVA IN THE SPIRAL GALAXY MESSIER 99

The Palomar Transient Factory (PTF) is systematically charting the optical transient and variable sky. A primary science driver of PTF is building a complete inventory of transients in the local Universe (distance less than 200 Mpc). Here, we report the discovery of PTF10fqs, a transient in the luminosity gap between novae and supernovae. Located on a spiral arm of Messier 99, PTF 10fqs has a peak luminosity of Mr = -12.3, red color (g-r = 1.0) and is slowly evolving (decayed by 1 mag in 68 days). It has a spectrum dominated by intermediate-width H (930 km/s) and narrow calcium emission lines. The explosion signature (the light curve and spectra) is overall similar to thatof M85OT2006-1, SN2008S, and NGC300OT. The origin of these events is shrouded in mystery and controversy (and in some cases, in dust). PTF10fqs shows some evidence of a broad feature (around 8600A) that may suggest very large velocities (10,000 km/s) in this explosion. Ongoing surveys can be expected to find a few such events per year. Sensitive spectroscopy, infrared monitoring and statistics (e.g. disk versus bulge) will eventually make it possible for astronomers to unravel the nature of these mysterious explosions.

An ancient nova shell around the dwarf nova Z Camelopardalis

Cataclysmic variables (classical novae and dwarf novae) are binary star systems in which a red dwarf transfers hydrogen-rich matter, by way of an accretion disk, to its white dwarf companion. In dwarf novae, an instability is believed to episodically dump much of the accretion disk onto the white dwarf. The liberation of gravitational potential energy then brightens these systems by up to 100-fold every few weeks or months. Thermonuclear-powered eruptions thousands of times more luminous occur in classical novae, accompanied by significant mass ejection and formation of clearly visible shells from the ejected material. Theory predicts that the white dwarfs in all dwarf novae must eventually accrete enough mass to undergo classical nova eruptions. Here we report a shell, an order of magnitude more extended than those detected around many classical novae, surrounding the prototypical dwarf nova Z Camelopardalis. The derived shell mass matches that of classical novae, and is inconsistent with the mass expected from a dwarf nova wind or a planetary nebula. The shell observationally links the prototypical dwarf nova Z Camelopardalis with an ancient nova eruption and the classical nova process.

DISCOVERY AND REDSHIFT OF AN OPTICAL AFTERGLOW IN 71 deg2: iPTF13bxl AND GRB 130702A

We report the discovery of the optical afterglow of the γ-ray burst (GRB) 130702A, identified upon searching 71 deg^2 surrounding the Fermi Gamma-ray Burst Monitor (GBM) localization. Discovered and characterized by the intermediate Palomar Transient Factory, iPTF13bxl is the first afterglow discovered solely based on a GBM localization. Real-time image subtraction, machine learning, human vetting, and rapid response multi-wavelength follow-up enabled us to quickly narrow a list of 27,004 optical transient candidates to a single afterglow-like source. Detection of a new, fading X-ray source by Swift and a radio counterpart by CARMA and the Very Large Array confirmed the association between iPTF13bxl and GRB 130702A. Spectroscopy with the Magellan and Palomar 200 inch telescopes showed the afterglow to be at a redshift of z = 0.145, placing GRB 130702A among the lowest redshift GRBs detected to date. The prompt γ-ray energy release and afterglow luminosity are intermediate between typical cosmological GRBs and nearby sub-luminous events such as GRB 980425 and GRB 060218. The bright afterglow and emerging supernova offer an opportunity for extensive panchromatic follow-up. Our discovery of iPTF13bxl demonstrates the first observational proof-of-principle for ~10 Fermi-iPTF localizations annually. Furthermore, it represents an important step toward overcoming the challenges inherent in uncovering faint optical counterparts to comparably localized gravitational wave events in the Advanced LIGO and Virgo era.