GRB 160821A is the third most energetic gamma-ray burst observed by the Fermi gamma-ray space telescope. Based on the observations made by the Cadmium Zinc Telluride Imager on board AstroSat, here we ...report the most conclusive evidence to date of (i) high linear polarization ( detection), and (ii) variation of polarization angle with time, occurring twice during the rise and decay phase of the burst at 3.5 and 3.1 detections, respectively. All confidence levels are reported for two parameters of interest. These observations strongly suggest synchrotron radiation produced in magnetic field lines that are highly ordered on angular scales of 1/Γ, where Γ is the Lorentz factor of the outflow.
ABSTRACT
GRB 160325A is the only bright burst detected by AstroSat CZT Imager in its primary field of view to date. In this work, we present the spectral and polarimetric analysis of the prompt ...emission of the burst using AstroSat, Fermi, and Niel Gehrels Swift observations. The prompt emission consists of two distinct emission episodes separated by a few seconds of quiescent/ mild activity period. The first emission episode shows a thermal component as well as a low polarization fraction of $PF \lt 37\, {{\ \rm per\ cent}}$ at $1.5\, \sigma$ confidence level. On the other hand, the second emission episode shows a non-thermal spectrum and is found to be highly polarized with $PF \gt 43\, {{\ \rm per\ cent}}$ at 1.5σ confidence level. We also study the afterglow properties of the jet using Swift/XRT data. The observed jet break suggests that the jet is pointed towards the observer and has an opening angle of 1.2° for an assumed redshift, z = 2. With composite modelling of polarization, spectrum of the prompt emission, and the afterglow, we infer that the first episode of emission originates from the photosphere with localized dissipation happening below it, and the second from the optically thin region above the photosphere. The photospheric emission is generated mainly by inverse Compton scattering, whereas the emission in the optically thin region is produced by the synchrotron process. The low radiation efficiency of the burst suggests that the outflow remains baryonic dominated throughout the burst duration with only a subdominant Poynting flux component, and the kinetic energy of the jet is likely dissipated via internal shocks which evolves from an optically thick to optically thin environment within the jet.
From the first two seasons of the Palomar Transient Factory, we identify three peculiar transients (PTF 09dav, PTF 10iuv, and PTF 11bij) with five distinguishing characteristics: peak luminosity in ...the gap between novae and supernovae (M sub(R) approx = -15.5 to -16.5 mag), rapid photometric evolution (t sub(rise)approx = 12-15 days), large photospheric velocities (approx =6000-11,000 km s super(-1)), early spectroscopic evolution into nebular phase (approx =1-3 months), and peculiar nebular spectra dominated by calcium. We also culled the extensive decade-long Lick Observatory Supernova Search database and identified an additional member of this group, SN 2007ke. Our choice of photometric and spectroscopic properties was motivated by SN 2005E (Perets et al.). To our surprise, as in the case of SN 2005E, all four members of this group are also clearly offset from the bulk of their host galaxy. Given the well-sampled early- and late-time light curves, we derive ejecta masses in the range of 0.4-0.7 M sub(middot in circle). Spectroscopically, we find that there may be a diversity in the photospheric phase, but the commonality is in the unusual nebular spectra. Our extensive follow-up observations rule out standard thermonuclear and standard core-collapse explosions for this class of "calcium-rich gap" transients. If the progenitor is a white dwarf, we are likely seeing a detonation of the white dwarf core and perhaps even shock-front interaction with a previously ejected nova shell. If the progenitor is a massive star, a nonstandard channel specific to a low-metallicity environment needs to be invoked (e.g., ejecta fallback leading to black hole formation). Detection (or the lack thereof) of a faint underlying host (dwarf galaxy and cluster) will provide a crucial and decisive diagnostic to choose between these alternatives.
ABSTRACT PSR J2129−0429 is a "redback" eclipsing millisecond pulsar binary with an unusually long 15.2 hr orbit. It was discovered by the Green Bank Telescope in a targeted search of unidentified ...Fermi gamma-ray sources. The pulsar companion is optically bright (mean mR = 16.6 mag), allowing us to construct the longest baseline photometric data set available for such a system. We present 10 years of archival and new photometry of the companion from the Lincoln Near-Earth Asteroid Research Survey, the Catalina Real-time Transient Survey, the Palomar Transient Factory, the Palomar 60 inch, and the Las Cumbres Observatory Global Telescope. Radial velocity spectroscopy using the Double-Beam Spectrograph on the Palomar 200 inch indicates that the pulsar is massive: 1.74 0.18 . The G-type pulsar companion has mass 0.44 0.04 , one of the heaviest known redback companions. It is currently 95 1% Roche-lobe filling and only mildly irradiated by the pulsar. We identify a clear 13.1 mmag yr−1 secular decline in the mean magnitude of the companion as well as smaller-scale variations in the optical light curve shape. This behavior may indicate that the companion is cooling. Binary evolution calculations indicate that PSR J2129−0429 has an orbital period almost exactly at the bifurcation period between systems that converge into tighter orbits as black widows and redbacks and those that diverge into wider pulsar-white dwarf binaries. Its eventual fate may depend on whether it undergoes future episodes of mass transfer and increased irradiation.
We report positions, velocities, and metallicities of 50 ab-type RR Lyrae (RRab) stars observed in the vicinity of the Orphan stellar stream. Using about 30 RRab stars classified as being likely ...members of the Orphan stream, we study the metallicity and the spatial extent of the stream. We find that RRab stars in the Orphan stream have a wide range of metallicities, from -1.5 dex to -2.7 dex. The average metallicity of the stream is -2.1 dex, identical to the value obtained by Newberg et al. using blue horizontal branch stars. We find that the most distant parts of the stream (40-50 kpc from the Sun) are about 0.3 dex more metal-poor than the closer parts (within ~30 kpc), suggesting a possible metallicity gradient along the stream's length. We have extended the previous studies and have mapped the stream up to 55 kpc from the Sun. Even after a careful search, we did not identify any more distant RRab stars that could plausibly be members of the Orphan stream. If confirmed with other tracers, this result would indicate a detection of the end of the leading arm of the stream. We have compared the distances of Orphan stream RRab stars with the best-fit orbits obtained by Newberg et al. We find that model 6 of Newberg et al. cannot explain the distances of the most remote Orphan stream RRab stars, and conclude that the best fit to distances of Orphan stream RRab stars and to the local circular velocity is provided by potentials where the total mass of the Galaxy within 60 kpc is M sub(60) ~ 2.7 x 10 super(11) M sub(middot in circle), or about 60% of the mass found by previous studies. More extensive modeling that would consider non-spherical potentials and the possibility of misalignment between the stream and the orbit is highly encouraged.
We present spectral and timing analysis of NuSTAR observations of RX J0520.5 - 6932 in the 3-79 keV band collected during its outburst in 2014 January. The target was observed on two epochs and we ...report the detection of a cyclotron resonant scattering feature with central energies of E sub(CRSF) = 31.3 super(+0.8) sub(-0.7) keV and 31.5 super(+0.7) sub(-0.6)keV during the two observations, respectively, corresponding to a magnetic field of B approximately 2 x 10 super(12) G. The 3-79 keV luminosity of the system during the two epochs, assuming a nominal distance of 50 kpc, was 3.667 + or - 0.007 x 10 super(38) erg s super(-1) and 3.983 + or - 0.007 x 10 super(38) erg s super(-1). Both values are much higher than the critical luminosity of approximately 1.5 x 10 super(37)ergs super(-1), above which a radiation-dominated shock front may be expected. This adds a new object to the sparse set of three systems that have a cyclotron line observed at luminosities in excess of 10 super(38) erg s super(-1). A broad (sigma approximately 0.45 keV) Fe emission line is observed in the spectrum at a central energy of 6.58 super(+0.05) sub(-0.05) keV in both epochs. The pulse profile of the pulsar was observed to be highly asymmetric with a sharply rising and slowly falling profile of the primary peak. We also observed minor variations in the cyclotron line energy and width as a function of the rotation phase.
We present optical spectroscopic measurements of the eclipsing high-mass X-ray binary (HMXB) XMMU J013236.7+303228 in M 33. Based on spectra taken at multiple epochs of the 1.73 day binary orbital ...period we determine physical as well as orbital parameters for the donor star. We find the donor to be a B1.5IV subgiant with effective temperature T = 22,000-23,000 K. Using the spectroscopically derived B star mass we find the neutron star companion mass to be M sub(x) = 2.0 + or - 0.4 M sub(middot in circle), consistent with the neutron star mass in the HMXB Vela X-l, but heavier than the canonical value of 1.4M sub(middot in circle) found for many millisecond pulsars. We attempt to use as an additional constraint that the B star radius inferred from temperature, flux, and distance should equate to the Roche radius, since the system accretes by Roche lobe overflow. We conclude that precise constraints require detailed modeling of the shape of the Roche surface.
We report the optical discovery of the companion to the 2 M millisecond pulsar PSR J1614--2230. The optical colors show that the 0.5 M companion is a 2.2 Gyr old He-CO white dwarf. We infer that ...during the accretion phase is . We show that the pulsar was born with a spin close to its current value, well below the rebirth line. The spin-down parameters, the mass of the pulsar, and the age of the system challenge the simple recycling model for the formation of millisecond pulsars.
We report the discovery of 1RXS J173006.4+033813, a polar cataclysmic variable with a period of 120.21 minutes. The white dwarf primary has a magnetic field of B = 42{sup +6}{sub -5} MG and the ...secondary is an M3 dwarf. The system shows highly symmetric double-peaked photometric modulation in the active state as well as in quiescence. These arise from a combination of cyclotron beaming and ellipsoidal modulation. The projected orbital velocity of the secondary is K{sub 2} = 390 {+-} 4 km s{sup -1}. We place an upper limit of 830 {+-} 65 pc on the distance.
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