arXiv subjects

Y. Cao

Publications and source records attributed to Y. Cao.

At least 109 records · Page 6Linked to original sources

arXiv2016

PTF 13efv - An outburst 500 days prior to the SNHunt 275 explosion and its radiative efficiency

The progenitors of some supernovae (SNe) exhibit outbursts with super-Eddington luminosities prior to their final explosions. This behavior is common among Type IIn SNe, but the driving mechanisms of these precursors are not yet well understood. SNHunt 275 was announced as a possible new SN during May 2015. Here we report on pre-explosion observations of the location of this event by the Palomar Transient Factory (PTF) and report the detection of a precursor about 500 days prior to the 2015 May activity (PTF 13efv). The observed velocities in the 2015 transient and its 2013 precursor absorption spectra are low (1000-2000 km/s), so it is not clear yet if the recent activity indeed marks the final disruption of the progenitor. Regardless of the nature of this event, we use the PTF photometric and spectral observations, as well as Swift-UVOT observations, to constrain the efficiency of the radiated energy relative to the total kinetic energy of the precursor. We find that, using an order-of-magnitude estimate and under the assumption of spherical symmetry, the ratio of the radiated energy to the kinetic energy is in the range of 0.04 to 3400.

astro-ph.HE↗

arXiv2016

Time-Varying Sodium Absorption in the Type Ia Supernova 2013gh

Temporal variability of narrow absorption lines in high-resolution spectra of Type Ia supernovae (SNe Ia) is studied to search for circumstellar matter. Time series which resolve the profiles of absorption lines such as Na I D or Ca II H&K are expected to reveal variations due to photoionisation and subsequent recombination of the gases. The presence, composition, and geometry of circumstellar matter may hint at the elusive progenitor system of SNe Ia and could also affect the observed reddening law. To date, there are few known cases of time-varying Na I D absorption in SNe Ia, all of which occurred during relatively late phases of the supernova evolution. Photoionisation, however, is predicted to occur during the early phases of SNe Ia, when the supernova peaks in the ultraviolet. We therefore attempt to observe early-time absorption-line variations by obtaining high-resolution spectra of SNe before maximum light. We have obtained photometry and high-resolution spectroscopy of SNe Ia 2013gh and iPTF 13dge, to search for absorption- line variations. Furthermore, we study interstellar absorption features in relation to the observed photometric colours of the SNe. Results. Both SNe display deep Na I D and Ca II H&K absorption features. Furthermore, small but significant variations are detected in a feature of the Na I D profile of SN 2013gh. The variations are consistent with either geometric effects of rapidly moving or patchy gas clouds or photoionisation of Na I gas at R \approx 1019 cm from the explosion. Our analysis indicates that it is necessary to focus on early phases to detect photoionisation effects of gases in the circumstellar medium of SNe Ia. Different absorbers such as Na I and Ca II can be used to probe for matter at different distances from the SNe.

astro-ph.SR↗

arXiv2016

The bolometric light curves and physical parameters of stripped-envelope supernovae

The optical and optical/near-infrared pseudo-bolometric light curves of 85 stripped-envelope supernovae (SNe) are constructed using a consistent method and a standard cosmology. The light curves are analysed to derive temporal characteristics and peak luminosity $L_{\mathrm{p}}$, enabling the construction of a luminosity function. Subsequently, the mass of $^{56}$Ni synthesised in the explosion, along with the ratio of ejecta mass to ejecta kinetic energy, are found. Analysis shows that host-galaxy extinction is an important factor in accurately determining luminosity values as it is significantly greater than Galactic extinction in most cases. It is found that broad-lined SNe~Ic (SNe~Ic-BL) and gamma-ray burst SNe are the most luminous subtypes with a combined median $L_{\mathrm{p}}$, in erg s$^{-1}$, of log($L_{\mathrm{p}})=43.00$ compared to $42.51$ for SNe Ic, $42.50$ for SNe~Ib, and $42.36$ for SNe~IIb. It is also found that SNe~Ic-BL synthesise approximately twice the amount of $^{56}$Ni compared with SNe~Ic, Ib, and IIb, with median $M_{\mathrm{Ni}} = 0.34$, 0.16, 0.14, and 0.11 M$_{\odot}$, respectively. SNe~Ic-BL, and to a lesser extent SNe~Ic, typically rise from $L_{\mathrm{p}}/2$ to $L_{\mathrm{p}}$ more quickly than SNe~Ib/IIb; consequently, their light curves are not as broad.

astro-ph.HE↗

arXiv2016

Macroscopic self-reorientation of interacting two-dimensional crystals

Microelectromechanical systems, which can be moved or rotated with nanometre precision, already find applications in such fields like radio-frequency electronics, micro-attenuators, sensors and many others. Especially interesting are those which allow fine control over the motion on atomic scale due to self-alignment mechanisms and forces acting on the atomic level. Such machines can produce well-controlled movements as a reaction to small changes of the external parameters. Here we demonstrate that, for the system of graphene on hexagonal boron nitride, the interplay between the van der Waals and elastic energies results in graphene mechanically self-rotating towards the hexagonal boron nitride crystallographic directions. Such rotation is macroscopic (for graphene flakes of tens of micrometres the tangential movement can be on hundreds of nanometres) and can be used for reproducible manufacturing of aligned van der Waals heterostructures.

cond-mat.mes-hall↗

arXiv2016

iPTF Search for an Optical Counterpart to Gravitational Wave Trigger GW150914

The intermediate Palomar Transient Factory (iPTF) autonomously responded to and promptly tiled the error region of the first gravitational wave event GW150914 to search for an optical counterpart. Only a small fraction of the total localized region was immediately visible in the Northern night sky, due both to sun-angle and elevation constraints. Here, we report on the transient candidates identified and rapid follow-up undertaken to determine the nature of each candidate. Even in the small area imaged of 126 sq deg, after extensive filtering, 8 candidates were deemed worthy of additional follow-up. Within two hours, all 8 were spectroscopically classified by the Keck II telescope. Curiously, even though such events are rare, one of our candidates was a superluminous supernova. We obtained radio data with the Jansky Very Large Array and X-ray follow-up with the Swift satellite for this transient. None of our candidates appear to be associated with the gravitational wave trigger, which is unsurprising given that GW150914 came from the merger of two stellar-mass black holes. This end-to-end discovery and follow-up campaign bodes well for future searches in this post-detection era of gravitational waves.

astro-ph.IM↗

arXiv2016

Metallicity from Type II Supernovae from the (i)PTF

Type IIP supernovae (SNe IIP) have recently been proposed as metallicity ($Z$) probes. The spectral models of Dessart et al. (2014) showed that the pseudo-equivalent width of Fe II $λ$5018 (pEW$_{5018}$) during the plateau phase depends on the primordial $Z$, but there was a paucity of SNe IIP exhibiting pEW$_{5018}$ that were compatible with $Z < 0.4$ ${\rm Z}_{\odot}$. This lack might be due to some physical property of the SN II population or to the fact that those SNe have been discovered in luminous, metal-rich targeted galaxies. Here we use SN II observations from the untargeted (intermediate) Palomar Transient Factory [(i)PTF] survey, aiming to investigate the pEW$_{5018}$ distribution of this SN population and, in particular, to look for the presence of SNe II at lower $Z$. We perform pEW$_{5018}$ measurements on the spectra of a sample of 39 (i)PTF SNe II, selected to have well-constrained explosion epochs and light-curve properties. Based on the comparison with the pEW$_{5018}$ spectral models, we subgrouped our SNe into four $Z$ bins from $Z \approx 0.1$ $Z_{\odot}$ up to $Z \approx 2$ $Z_{\odot}$. We also independently investigated the $Z$ of the hosts by using their absolute magnitudes and colors and, in a few cases, using strong-line diagnostics from spectra. We searched for possible correlations between SN observables, such as their peak magnitudes and the $Z$ inferred from pEW$_{5018}$. We found 11 events with pEW$_{5018}$ that were small enough to indicate $Z \approx 0.1$ $Z_{\odot}$. The trend of pEW$_{5018}$ with $Z$ matches the $Z$ estimates obtained from the host-galaxy photometry, although the significance of the correlation is weak. We also found that SNe with brighter peak magnitudes have smaller pEW$_{5018}$ and occur at lower $Z$.

astro-ph.HE↗

arXiv2016

Mott Electrons in an Artificial Graphenelike Crystal of Rare-Earth Nickelate

Deterministic control over the periodic geometrical arrangement of the constituent atoms is the backbone of the material properties, that along with the interactions define the electronic and magnetic ground state. Following this notion, a bilayer of a prototypical rare-earth nickelate, NdNiO$_3$, combined with a dielectric spacer, LaAlO$_3$, has been layered along the pseudo cubic [111] direction. The resulting artificial graphene-like Mott crystal with magnetic 3$d$ electrons has antiferromagnetic correlations. In addition, a combination of resonant X-ray linear dichroism measurements and \textit{ab-initio} calculations reveal the presence of an ordered orbital pattern, which is unattainable in either bulk nickelates or nickelate based heterostructures grown along the [001] direction. These findings highlight another promising venue towards designing new quantum many-body states by virtue of geometrical engineering.

cond-mat.str-el↗

arXiv2016

The Caltech-NRAO Stripe 82 Survey (CNSS) Paper I: The Pilot Radio Transient Survey In 50 deg$^2$

We have commenced a multi-year program, the Caltech-NRAO Stripe 82 Survey (CNSS), to search for radio transients with the Jansky VLA in the SDSS Stripe 82 region. The CNSS will deliver five epochs over the entire $\sim$270 deg$^2$ of Stripe 82, an eventual deep combined map with a rms noise of $\sim$40 $μ$Jy and catalogs at a frequency of 3 GHz, and having a spatial resolution of 3". This first paper presents the results from an initial pilot survey of a 50 deg$^2$ region of Stripe 82, involving four epochs spanning logarithmic timescales between one week and 1.5 years, with the combined map having a median rms noise of 35 $μ$Jy. This pilot survey enabled the development of the hardware and software for rapid data processing, as well as transient detection and follow-up, necessary for the full 270 deg$^2$ survey. Classification of variable and transient sources relied heavily on the wealth of multi-wavelength data in the Stripe 82 region, supplemented by repeated mapping of the region by the Palomar Transient Factory. $3.9^{+0.5}_{-0.9}$% of the detected point sources were found to vary by greater than 30%, consistent with similar studies at 1.4 GHz and 5 GHz. Multi-wavelength photometric data and light curves suggest that the variability is mostly due to shock-induced flaring in the jets of AGN. Although this was only a pilot survey, we detected two bona fide transients, associated with an RS CVn binary and a dKe star. Comparison with existing radio survey data revealed additional highly variable and transient sources on timescales between 5-20 years, largely associated with renewed AGN activity. The rates of such AGN possibly imply episodes of enhanced accretion and jet activity occurring once every $\sim$40,000 years in these galaxies. We compile the revised radio transient rates and make recommendations for future transient surveys and joint radio-optical experiments. (Abridged)

astro-ph.HE↗

arXiv2016

Optical and near-infrared observations of SN 2013dx associated with GRB 130702A

We present optical and near-infrared light curves and optical spectra of SN 2013dx, associated with the nearby (redshift 0.145) gamma-ray burst GRB 130702A. The prompt isotropic gamma-ray energy released from GRB 130702A is measured to be $E_{γ,iso}=6.4_{-1.0}^{+1.3}\times10^{50}$erg (1keV-10MeV in the rest frame), placing it intermediate between low-luminosity GRBs like GRB 980425/SN 1998bw and the broader cosmological population. We compare the observed $g'r'i'z'$ light curves of SN 2013dx to a SN 1998bw template, finding that SN 2013dx evolves ~20% faster (steeper rise time), with a comparable peak luminosity. Spectroscopically, SN 2013dx resembles other broad-lined Type Ic supernovae, both associated with (SN 2006aj and SN 1998bw) and lacking (SN 1997ef, SN 2007I, and SN 2010ah) gamma-ray emission, with photospheric velocities around peak of ~21,000km s$^{-1}$. We construct a quasi-bolometric ($g'r'i'z'yJ$) light curve for SN 2013dx, only the fifth GRB-associated SN with extensive NIR coverage and the third with a bolometric light curve extending beyond $Δt>40$d. Together with the measured photospheric velocity, we derive basic explosion parameters using simple analytic models. We infer a $^{56}$Ni mass of $M_{\mathrm{Ni}}=0.37\pm0.01$M$_{\odot}$, an ejecta mass of $M_{\mathrm{ej}}=3.1\pm0.1$M$_{\odot}$, and a kinetic energy of $E_{\mathrm{K}}=(8.2\pm0.43)\times10^{51}$ erg (statistical uncertainties only), consistent with previous GRB-associated SNe. When considering the ensemble population of GRB-associated SNe, we find no correlation between the mass of synthesized $^{56}$Ni and high-energy properties, despite clear predictions from numerical simulations that $M_{\mathrm{Ni}}$ should correlate with the degree of asymmetry. On the other hand, $M_{\mathrm{Ni}}$ clearly correlates with the kinetic energy of the supernova ejecta across a wide range of core-collapse events.

astro-ph.HE↗

arXiv2015

Phonon-assisted resonant tunneling of electrons in graphene-boron nitride transistors

We observe a series of sharp resonant features in the differential conductance of graphene-hexagonal boron nitride-graphene tunnel transistors over a wide range of bias voltages between $\sim$10 and 200 mV. We attribute them to electron tunneling assisted by the emission of phonons of well-defined energy. The bias voltages at which they occur are insensitive to the applied gate voltage and hence independent of the carrier densities in the graphene electrodes, so plasmonic effects can be ruled out. The phonon energies corresponding to the resonances are compared with the lattice dispersion curves of graphene-boron nitride heterostructures and are close to peaks in the single phonon density of states.

cond-mat.mes-hall↗

arXiv2015

Flash Spectroscopy: Emission Lines from the Ionized Circumstellar Material around $<10$-Day-Old Type II Supernovae

Supernovae (SNe) embedded in dense circumstellar material (CSM) may show prominent emission lines in their early-time spectra ($\leq 10$ days after the explosion), owing to recombination of the CSM ionized by the shock-breakout flash. From such spectra ("flash spectroscopy"), we can measure various physical properties of the CSM, as well as the mass-loss rate of the progenitor during the year prior to its explosion. Searching through the Palomar Transient Factory (PTF and iPTF) SN spectroscopy databases from 2009 through 2014, we found 12 Type II SNe showing flash-ionized (FI) signatures in their first spectra. All are younger than 10 days. These events constitute 14\% of all 84 SNe in our sample having a spectrum within 10 days from explosion, and 18\% of SNe~II observed at ages $<5$ days, thereby setting lower limits on the fraction of FI events. We classified as "blue/featureless" (BF) those events having a first spectrum which is similar to that of a black body, without any emission or absorption signatures. It is possible that some BF events had FI signatures at an earlier phase than observed, or that they lack dense CSM around the progenitor. Within 2 days after explosion, 8 out of 11 SNe in our sample are either BF events or show FI signatures. Interestingly, we found that 19 out of 21 SNe brighter than an absolute magnitude $M_R=-18.2$ belong to the FI or BF groups, and that all FI events peaked above $M_R=-17.6$ mag, significantly brighter than average SNe~II.

astro-ph.HE↗

arXiv2015

Longitudinal Spin Seebeck Effect in Silver Strip on CoFe Film

We report the experimental observation of the spin Seebeck effect (SSE) in Ag/CoFe noble metal/magnetic metal bilayers with a longitudinal structure. Thermal voltages jointly generated by the anomalous Nernst effect (ANE) and the SSE were detected across the Ag/CoFe/Cu strip with a perpendicular thermal gradient. To effectively separate the SSE and the ANE part of the thermal voltages, we compared the experimental results between the Ag/CoFe/Cu strip and Cu/CoFe/Cu strip, where two samples processed with the heating power instead of the temperature difference through the thin CoFe film. The respective contributions of the ANE and SSE to thermal voltage were determined, and they have the ratio of 4:1. The spin current injected through CoFe/Ag interface is calculated to be 1.76 mA/W.

cond-mat.mtrl-sci↗

arXiv2015

Detection of Broad H$α$ Emission Lines in the Late-time Spectra of a Hydrogen-poor Superluminous Supernova

iPTF13ehe is a hydrogen-poor superluminous supernova (SLSN) at z=0.3434, with a slow-evolving light curve and spectral features similar to SN2007bi. It rises within (83-148)days (rest-frame) to reach a peak bolometric luminosity of 1.3x$10^{44}$erg/s, then decays very slowly at 0.015mag. per day. The measured ejecta velocity is 13000km/s. The inferred explosion characteristics, such as the ejecta mass (67-220$M_\odot$), the total radiative and kinetic energy ($10^{51}$ & 2x$10^{53}$erg respectively), is typical of a slow-evolving H-poor SLSN event. However, the late-time spectrum taken at +251days reveals a Balmer Halpha emission feature with broad and narrow components, which has never been detected before among other H-poor SLSNe. The broad component has a velocity width of ~4500km/s and has a ~300km/s blue-ward shift relative to the narrow component. We interpret this broad H$α$ emission with luminosity of $\sim$2$\times10^{41}$\,erg\,s$^{-1}$ as resulting from the interaction between the supernova ejecta and a discrete H-rich shell, located at a distance of $\sim4\times10^{16}$\,cm from the explosion site. This ejecta-CSM interaction causes the rest-frame r-band LC to brighten at late times. The fact that the late-time spectra are not completely absorbed by the shock ionized CSM shell implies that its Thomson scattering optical depth is likely <1, thus setting upper limits on the CSM mass <30$M_\odot$ and the volume number density <4x$10^8cm^{-3}$. Of the existing models, a Pulsational Pair Instability Supernova model can naturally explain the observed 30$M_\odot$ H-shell, ejected from a progenitor star with an initial mass of (95-150)$M_\odot$ about 40 years ago. We estimate that at least $\sim$15\%\ of all SLSNe-I may have late-time Balmer emission lines.

astro-ph.GA↗

arXiv2015

Resonant tunnelling between the chiral Landau states of twisted graphene lattices

A new class of multilayered functional materials has recently emerged in which the component atomic layers are held together by weak van der Waals forces that preserve the structural integrity and physical properties of each layer. An exemplar of such a structure is a transistor device in which relativistic Dirac Fermions can resonantly tunnel through a boron nitride barrier, a few atomic layers thick, sandwiched between two graphene electrodes. An applied magnetic field quantises graphene's gapless conduction and valence band states into discrete Landau levels, allowing us to resolve individual inter-Landau level transitions and thereby demonstrate that the energy, momentum and chiral properties of the electrons are conserved in the tunnelling process. We also demonstrate that the change in the semiclassical cyclotron trajectories, following a tunnelling event, is a form of Klein tunnelling for inter-layer transitions.

cond-mat.mes-hall↗

arXiv2015

Coordination of the energy and temperature scales of pairing across the doping phase diagram of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

Using a new variant of photoelectron spectroscopy, we measure the homogeneous near-nodal pairing ($Δ$) and pair-breaking self-energy ($Γ_S$) processes for a wide range of doping levels of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. For all samples we find that the pairing extends above the superconducting transition T$_c$ to a scale T$_{Pair}$ that is distinct from the antinodal pseudogap scale T$^*$ and near but slightly above T$_c$. We find that $Δ$ and T$_{Pair}$ are related with a strong coupling ratio 2$Δ$ /k$_B$T$_{Pair}\approx6$ across the entire doping phase diagram, i.e. independent of the effects of antinodal pseudogaps or charge-density waves.

cond-mat.supr-con↗

arXiv2015

Power Law Liquid - A Unified Form of Low-Energy Nodal Electronic Interactions in Hole Doped Cuprate Superconductors

The strange-metal phase of the cuprate high temperature superconductors, above where the superconductivity sets in as a function of temperature, is widely considered more exotic and mysterious than the superconductivity itself. Here, based upon detailed angle resolved photoemission spectroscopy measurements of Bi$_2$Sr$_2$CaCu$_2$O$_8$$_+$$_δ$ over a wide range of doping levels, we present a new unifying phenomenology for the non-Fermi liquid normal-state interactions (scattering rates) in the nodal direction. This new phenomenology has a continuously varying power law exponent (hence named a Power Law Liquid or PLL), which with doping varies smoothly from a quadratic Fermi Liquid to a linear Marginal Fermi Liquid and beyond. Using the extracted PLL parameters we can calculate the optics and resistivity over a wide range of doping and normal-state temperature values, with the results closely matching the experimental curves. This agreement includes the presence of the $"$pseudogap$"$ temperature scale observed in the resistivity curves including the apparent quantum critical point. This breaks the direct link to the pseudogapping of antinodal spectral weight observed at similar (but here argued to be different) temperature scales, and also gives a new direction for searches of the microscopic mechanism at the heart of these and perhaps many other non-Fermi-liquid systems.

cond-mat.str-el↗

arXiv2015

Pairing, pair-breaking, and their roles in setting the Tc of cuprate high temperature superconductors

The key ingredients in any superconductor are the Cooper pairs, in which two electrons combine to form a composite boson. In all conventional superconductors the pairing strength alone sets the majority of the physical properties including the superconducting transition temperature T$_c$. In the cuprate high temperature superconductors, no such link has yet been found between the pairing interactions and T$_c$. Using a new variant of photoelectron spectroscopy we measure both the pair-forming ($Δ$) and a self energy/pair-breaking term ($Γ_s$) as a function of sample type and sample temperature, and we make the measurements over a wide range of doping and temperatures within and outside of the pseudogap/competing order doping regimes. In all cases we find that T$_c$ is approximately set by a crossover between the pair-forming strength $Δ$ and 3 times the self-energy term $Γ_s$ - a new paradigm for superconductivity. In addition to departing from conventional superconductivity in which the pairing alone sets T$_c$, these results indicate the zero-order importance of the near-nodal self-energy effects compared to competing order/pseudogap effects.

cond-mat.supr-con↗

arXiv2015

Polarization correlated photons from a positively charged quantum dot

Polarized cross-correlation spectroscopy on a quantum dot charged with a single hole shows the sequential emission of photons with common circular polarization. This effect is visible without magnetic field, but becomes more pronounced as the field along the quantization axis is increased. We interpret the data in terms of electron dephasing in the X+ state caused by the Overhauser field of nuclei in the dot. We predict the correlation timescale can be increased by accelerating the emission rate with cavity-QED.

cond-mat.mes-hall↗