42 research outputs found
Toward an Empirical Theory of Pulsar Emission XII: Exploring the Physical Conditions in Millisecond Pulsar Emission Regions
The five-component profile of the 2.7-ms pulsar J0337+1715 appears to exhibit
the best example to date of a core/double-cone emission-beam structure in a
millisecond pulsar (MSP). Moreover, three other MSPs, the Binary Pulsar
B1913+16, B1953+29 and J1022+1001, seem to exhibit core/single-cone profiles.
These configurations are remarkable and important because it has not been clear
whether MSPs and slow pulsars exhibit similar emission-beam configurations,
given that they have considerably smaller magnetospheric sizes and magnetic
field strengths. MSPs thus provide an extreme context for studying pulsar radio
emission. Particle currents along the magnetic polar flux tube connect
processes just above the polar cap through the radio-emission region to the
light-cylinder and the external environment. In slow pulsars radio-emission
heights are typically about 500 km around where the magnetic field is nearly
dipolar, and estimates of the physical conditions there point to radiation
below the plasma frequency and emission from charged solitons by the curvature
process. We are able to estimate emission heights for the four MSPs and carry
out a similar estimation of physical conditions in their much lower emission
regions. We find strong evidence that MSPs also radiate by curvature emission
from charged solitons.Comment: 14 pages, published in Ap
Spectroscopy of the inner companion of the pulsar PSR J0337+1715
The hierarchical triple system PSR J0337+1715 offers an unprecedented
laboratory to study secular evolution of interacting systems and to explore the
complicated mass-transfer history that forms millisecond pulsars and
helium-core white dwarfs. The latter in particular, however, requires knowledge
of the properties of the individual components of the system. Here we present
precise optical spectroscopy of the inner companion in the PSR J0337+1715
system. We confirm it as a hot, low-gravity DA white dwarf with
Teff=15,800+/-100 K and log(g)=5.82+/-0.05. We also measure an inner mass ratio
of 0.1364+/-0.0015, entirely consistent with that inferred from pulsar timing,
and a systemic radial velocity of 29.7+/-0.3 km/s. Combined with the mass
(0.19751 Msun) determined from pulsar timing, our measurement of the surface
gravity implies a radius of 0.091+/-0.005 Rsun; combined further with the
effective temperature and extinction, the photometry implies a distance of
1300+/-80 pc. The high temperature of the companion is somewhat puzzling: with
current models, it likely requires a recent period of unstable hydrogen
burning, and suggests a surprisingly short lifetime for objects at this phase
in their evolution. We discuss the implications of these measurements in the
context of understanding the PSR J0337+1715 system, as well as of low-mass
white dwarfs in general.Comment: ApJ Letters, in press. 6 pages, two figures. v2 fixes typ
X-ray Variability and Evidence for Pulsations from the Unique Radio Pulsar/X-ray Binary Transition Object FIRST J102347.6+003841
We report on observations of the unusual neutron-star binary system FIRST
J102347.6+003841 carried out using the XMM-Newton satellite. This system
consists of a radio millisecond pulsar in an 0.198-day orbit with a ~0.2
solar-mass Roche-lobe-filling companion, and appears to have had an accretion
disk in 2001. We observe a hard power-law spectrum (\Gamma = 1.26(4)) with a
possible thermal component, and orbital variability in X-ray flux and possibly
hardness of the X-rays. We also detect probable pulsations at the pulsar period
(single-trial significance ~4.5 sigma from an 11(2)% modulation), which would
make this the first system in which both orbital and rotational X-ray
pulsations are detected. We interpret the emission as a combination of X-rays
from the pulsar itself and from a shock where material overflowing the
companion meets the pulsar wind. The similarity of this X-ray emission to that
seen from other millisecond pulsar binary systems, in particular 47 Tuc W (PSR
J0024-7204W) and PSR J1740-5340, suggests that they may also undergo disk
episodes similar to that seen in J1023 in 2001.Comment: 14 pages, 5 figures, 1 table; accepted to Ap
A Radio Pulsar/X-ray Binary Link
Radio pulsars with millisecond spin periods are thought to have been spun up
by transfer of matter and angular momentum from a low-mass companion star
during an X-ray-emitting phase. The spin periods of the neutron stars in
several such low-mass X-ray binary (LMXB) systems have been shown to be in the
millisecond regime, but no radio pulsations have been detected. Here we report
on detection and follow-up observations of a nearby radio millisecond pulsar
(MSP) in a circular binary orbit with an optically identified companion star.
Optical observations indicate that an accretion disk was present in this system
within the last decade. Our optical data show no evidence that one exists
today, suggesting that the radio MSP has turned on after a recent LMXB phase.Comment: published in Scienc
eZinCh-2: a versatile, genetically encoded FRET sensor for cytosolic and intraorganelle Zn2+ imaging
Zn2+ plays essential and diverse roles in numerous cellular processes. To get a better understanding of intracellular Zn2+ homeostasis and the putative signaling role of Zn2+, various fluorescent sensors have been developed that allow monitoring of Zn2+ concentrations in single living cells in real time. Thus far, two families of genetically encoded FRET-based Zn2+ sensors have been most widely applied, the eCALWY sensors developed by our group and the ZapCY sensors developed by Palmer and co-workers. Both have been successfully used to measure cytosolic free Zn2+, but distinctly different concentrations have been reported when using these sensors to measure Zn2+ concentrations in the ER and mitochondria. Here, we report the development of a versatile alternative FRET sensor containing a de novo Cys2His2 binding pocket that was created on the surface of the donor and acceptor fluorescent domains. This eZinCh-2 sensor binds Zn2+ with a high affinity that is similar to that of eCALWY-4 (Kd = 1 nM at pH 7.1), while displaying a substantially larger change in emission ratio. eZinCh-2 not only provides an attractive alternative for measuring Zn2+ in the cytosol but was also successfully used for measuring Zn2+ in the ER, mitochondria, and secretory vesicles. Moreover, organelle-targeted eZinCh-2 can also be used in combination with the previously reported redCALWY sensors to allow multicolor imaging of intracellular Zn2+ simultaneously in the cytosol and the ER or mitochondria
Quasi-simultaneous Radio/X-Ray Observations of the Candidate Transitional Millisecond Pulsar 3FGL J1544.6-1125 during its Low-luminosity Accretion-disk State
3FGL J1544.6-1125 is a candidate transitional millisecond pulsar (tMSP). Similar to the well-established tMSPs - PSR J1023+0038, IGR J18245-2452, and XSS J12270-4859 - 3FGL J1544.6-1125 shows γ-ray emission and discrete X-ray "low"and "high"modes during its low-luminosity accretion-disk state. Coordinated radio/X-ray observations of PSR J1023+0038 in its current low-luminosity accretion-disk state showed rapidly variable radio continuum emission - possibly originating from a compact, self-absorbed jet, the "propellering"of accretion material, and/or pulsar moding. 3FGL J1544.6-1125 is currently the only other (candidate) tMSP system in this state, and can be studied to see whether tMSPs are typically radio-loud compared to other neutron star binaries. In this work, we present a quasi-simultaneous Very Large Array and Swift radio/X-ray campaign on 3FGL J1544.6-1125. We detect 10 GHz radio emission varying in flux density from 47.7 ± 6.0 μJy down to ≲15 μJy (3σ upper limit) at four epochs spanning three weeks. At the brightest epoch, the radio luminosity is L 5 GHz = (2.17 ± 0.17) × 1027 erg s-1 for a quasi-simultaneous X-ray luminosity L 2-10 keV = (4.32 ± 0.23) × 1033 erg s-1 (for an assumed distance of 3.8 kpc). These luminosities are close to those of PSR J1023+0038, and the results strengthen the case that 3FGL J1544.6-1125 is a tMSP showing similar phenomenology to PSR J1023+0038.A.J. and J.W.T.H. acknowledge funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement nr. 337062 (DRAGNET). A.J. also acknowledges support from the NuSTAR mission. A.P. acknowledges support from an NWO Vidi Fellowship. J.C.A.M.-J. is the recipient of an Australian Research Council Future Fellowship (FT 140101082). S.B. was supported in part by NASA Swift Guest Investigator Cycle 12 program grant NNX16AN79G awarded through Columbia University