Detection of polarized quasi-periodic microstructure emission in millisecond pulsars

Microstructure emission, involving short time scale, often quasi-periodic, intensity fluctuations in subpulse emission, is well known in normal period pulsars. In this letter, we present the first detections of quasi-periodic microstructure emission from millisecond pulsars (MSPs), from Giant Metrewave Radio Telescope (GMRT) observations of two MSPs at 325 and 610 MHz. Similar to the characteristics of microstructure observed in normal period pulsars, we find that these features are often highly polarized, and exhibit quasi-periodic behavior on top of broader subpulse emission, with periods of the order of a few $\mu$s. By measuring their widths and periodicities from single pulse intensity profiles and their autocorrelation functions, we extend the microstructure timescale - rotation period relationship by more than an order of magnitude down to rotation periods $\sim$ 5 ms, and find it to be consistent with the relationship derived earlier for normal pulsars. The similarity of behavior is remarkable, given the significantly different physical properties of MSPs and normal period pulsars, and rules out several previous speculations about the possible different characteristics of microstructure in MSP radio emission. We discuss the possible reasons for the non-detection of these features in previous high time resolution MSP studies along with the physical implications of our results, both in terms of a geometric beam sweeping model and temporal modulation model for micropulse production.Comment: 6 pages, 4 figures, 1 table. Accepted for publication in ApJ Letter

Dusty, Self-obscured Transients from Stellar Coalescence

We discuss the central role that dust condensation plays in shaping the observational appearance of outflows from coalescing binary systems. As binaries begin to coalesce, they shock-heat and expel material into their surroundings. Depending on the properties of the merging system, this material can expand to the point where molecules and dust form, dramatically increasing the gas opacity. We use the existing population of Luminous Red Novae (LRNe) to constrain the thermodynamics of these ejecta, then apply our findings to the progressive obscuration of merging systems in the lead in to their coalescence. Compact progenitor stars near the main sequence or in the Hertzsprung gap along with massive progenitor stars have sufficiently hot circumstellar material to remain unobscured by dust. By contrast, more extended, low-mass giants should become completely optically obscured by dust formation in the circumbinary environment. We predict that 30--50\% of stellar coalescence transients for solar-mass stars will be dusty, infrared-luminous sources. Of these, the optical transients may selectively trace complete merger outcomes while the infrared transients trace common envelope ejection outcomes.Comment: v2, published in AAS Journal

Single pulse polarization study of pulsars B0950+08 and B1642-03: micropulse properties and mixing of orthogonal modes

We present the results of a high-time resolution polarization study of single pulses from pulsars B0950+08 and B1642-03. Single pulses from pulsar B0950+08 sometimes show isolated micropulses without any significant associated subpulse emission. Assuming that the properties of such micropulses represent the intrinsic nature of micropulse emission, we characterize the width and polarization properties of these `intrinsic' microstructures. Most of the `intrinsic' micropulses (~90%) follow common characteristic polarization properties, while the average width of these micropulses is consistent with the general micropulse population from this pulsar. Single pulses from these pulsars show a diverse range of polarization properties, including depolarization and mixing of two orthogonal modes resulting in polarization position angle jumps. We present a superposition model of the two orthogonal modes which can explain depolarization, the observed position angle jumps, and associated changes in other polarization parameters.Comment: 12 pages, 18 figures, Accepted for publication in the Monthly Notices of the Royal Astronomical Society (MNRAS

Infrared spectroscopy of SWIFT J0850.8-4219: Identification of the second red supergiant X-ray binary in the Milky Way

High mass X-ray binaries hosting red supergiant (RSG) donors are a rare but crucial phase in massive stellar evolution, with only one source previously known in the Milky Way. In this letter, we present the identification of the second Galactic RSG X-ray binary SWIFT J0850.8-4219. We identify the source 2MASS 08504008-4211514 as the likely infrared counterpart with a chance coincidence probability $\approx 5 \times 10^{-6}$. We present a $1.0 - 2.5\,\mu$m spectrum of the counterpart, exhibiting features characteristic of late-type stars and an exceptionally strong He I emission line, corroborating the identification. Based on i) the strength of the $^{12}$CO(2,0) band, ii) strong CN bandheads and absent TiO bandheads at $\approx 1.1\,\mu$m and iii) equivalent width of the Mg I $1.71\,\mu$m line, we classify the counterpart to be a K3$-$K5 type RSG with an effective temperature of $3820 \pm 100$ K, located at a distance of $\approx 12$ kpc. We estimate the source X-ray luminosity to be $(4 \pm 1) \times 10^{35}$ erg s$^{-1}$, with a hard photon index ($\Gamma < 1$), arguing against a white dwarf accretor but consistent with a magnetized neutron star in the propeller phase. Our results highlight the potential of systematic NIR spectroscopy of Galactic hard X-ray sources in completing our census of the local X-ray binary population.Comment: 6 pages, 4 figures, 1 table. Submitted to MNRAS letters. Comments welcome

The Whisper and the Bang: Cosmic Fireworks in the Lives of Compact Binaries

Compact binaries, comprising of a white dwarf, neutron star or black hole in a tight orbit around another star are produced from binary evolution through a complex range of astrophysical processes -- ranging from eruptive mass loss episodes ('the whisper') to spectacular explosions ('the bang') that have shaped the universe as we see it today. In pursuit of a complete road-map of the explosive lives of high mass and low mass stars in compact binaries, I undertook two major experiments. In the first two parts of this thesis, I describe the largest volume-limited supernova classification experiment undertaken till date, using the Zwicky Transient Facility optical time domain survey. I present the identification of a new class of 'ultra-stripped' supernovae that form neutron stars in compact binary systems, that are likely direct progenitors of merging neutron stars detectable in LIGO/Virgo. Using the systematic sample of supernovae, I identify a class of helium shell explosions on low mass white dwarfs that likely represent the final fates of helium accreting white dwarfs. In the third part of this thesis, I present the development of Palomar Gattini-IR (PGIR), the first wide-field infrared survey capable of studying the dynamic infrared sky from timescales of seconds to years. I present a systematic search for nova eruptions in the Milky Way using PGIR, and show that optical surveys have systematically missed a large fraction of dust obscured novae to derive the first quantitative estimate of the Galactic nova rate. I present the first infrared constraints on the second-timescale emission from a Galactic Fast Radio Burst identified with a dust obscured Galactic magnetar. This thesis helps set the stage for a systematic exploration of the local stellar graveyard using i) the Vera Rubin Observatory to probe the lowest luminosity stellar deaths in the local universe and ii) the upcoming assemblage of infrared surveys to study variability in Galactic compact objects in conjunction with the rich landscape of X-ray and radio sky surveys. By mapping out the demographics of explosive phenomena, it helps us interpret the astrophysical populations detectable with current and future gravitational wave observatories.</p

RNO 54: A Previously Unappreciated FU Ori Star

We present evidence in support of the hypothesis that the young stellar object RNO 54 is a mature-stage FU Ori type source. The star was first cataloged as a ``red nebulous object" in the 1980s but appears to have undergone its outburst prior to the 1890s. Present-day optical and near-infrared spectra are consistent with those of other FU Ori type stars, both in the details of spectral line presence and shape, and in the overall change in spectral type from an FGK-type in the optical, to the M-type presented in the near-infrared. In addition, the spectral energy distribution of RNO 54 is well-fit by a pure-accretion disk model with parameters: $\dot{M} = 10^{-3.45\pm0.06}$ $M_\odot$ yr$^{-1}$, $M_* = 0.23\pm0.06 \ M_\odot$, and $R_\mathrm{inner} = 3.68\pm0.76 \ R_\odot$, though we believe $R_\mathrm{inner}$ is likely close to its upper range of $4.5 R_\odot$ in order to produce a $T_\mathrm{max} = 7000$ K that is consistent with the optical to near-infrared spectra. The resulting $L_\mathrm{acc}$ is $\sim 265 \ L_\odot$. To find these values, we adopted a source distance $d=1400$ pc and extinction $A_V=3.9$ mag, along with disk inclination $i=50$ deg based on consideration of confidence intervals from our initial disk model, and in agreement with observational constraints. The new appreciation of a well-known source as an FU Ori type object suggests that other such examples may be lurking in extant samples.Comment: to appear in AAS Journal

Detection of Polarized Quasi-periodic Microstructure Emission in Millisecond Pulsars

Microstructure emission, involving short timescale, often quasi-periodic, intensity fluctuations in subpulse emission, is well known in normal period pulsars. In this Letter, we present the first detections of quasi-periodic microstructure emission from millisecond pulsars (MSPs), from Giant Metrewave Radio Telescope observations of two MSPs at 325 and 610 MHz. Similar to the characteristics of microstructure observed in normal period pulsars, we find that these features are often highly polarized and exhibit quasi-periodic behavior on top of broader subpulse emission, with periods of the order of a few μs. By measuring their widths and periodicities from single pulse intensity profiles and their autocorrelation functions, we extend the microstructure timescale–rotation period relationship by more than an order of magnitude down to rotation periods ~5 ms, and find it to be consistent with the relationship derived earlier for normal pulsars. The similarity of behavior is remarkable, given the significantly different physical properties of MSPs and normal period pulsars, and rules out several previous speculations about the possible different characteristics of microstructure in MSP radio emission. We discuss the possible reasons for the non-detection of these features in previous high time resolution MSP studies along with the physical implications of our results, both in terms of a geometric beam sweeping model and temporal modulation model for micropulse production

Revealing the geometry of gravitational wave event GW 170817 with radio observations

GW 170817 marks the first gravitational wave detection of a binary neutron-star (BNS) merger by the Advanced LIGO and the Advanced VIRGO detectors. This is also the first gravitational wave event from which the electromagnetic (EM) emission was seen all the way from gamma-rays to radio bands. We detected it in radio bands 16 days after the detection [2]