41 research outputs found
Radio Pulsars in Binary Systems
This thesis focuses on the study of binary radio pulsars, their evolution and
some specific use of their properties to investigate fundamental physics such
as general relativity and other gravitational theories. The work that we
present here is organized in three main parts. First, we report on the study of
PSR J1744-3922, a binary pulsar presenting a peculiar `flickering' flux
behavior as well as spin and orbital properties that do not correspond to the
expectations of standard evolution scenarios. Second, we conducted an in-depth
analysis of the eclipses in the relativistic double pulsar system PSR
J0737-3039A/B. From our modeling of the eclipses, we precisely determined the
geometry of pulsar B in space and used this information to study the temporal
behavior of the eclipses, which revealed that pulsar B precesses around the
angular momentum of the system in a way that is consistent with the prediction
of general relativity. Third, we searched for the signature of latitudinal
aberration in the pulse profile of pulsar A in the double pulsar system. The
non-detection of this effect allows us to put an upper limit on its amplitude,
which constrains the geometry of pulsar A with respect to our line of sight as
well as its emission geometry. (Abridged)Comment: Ph.D. Thesis, 236 pages, 76 figures, 7 table
The long-term evolution of the X-ray pulsar XTE J1814-338: a receding jet contribution to the quiescent optical emission?
We present a study of the quiescent optical counterpart of the Accreting
Millisecond X-ray Pulsar XTE J1814-338, carrying out multiband (BVR) orbital
phase-resolved photometry using the ESO VLT/FORS2. The optical light curves are
consistent with a sinusoidal variability modulated with the orbital period,
showing evidence for a strongly irradiated companion star, in agreement with
previous findings. The observed colours cannot be accounted for by the
companion star alone, suggesting the presence of an accretion disc during
quiescence. The system is fainter in all analysed bands compared to previous
observations. The R band light curve displays a possible phase offset with
respect to the B and V band. Through a combined fit of the multi-band light
curves we derive constraints on the companion star and disc fluxes, on the
system distance and on the companion star mass. The irradiation luminosity
required to account for the observed day-side temperature of the companion star
is consistent with the spin-down luminosity of a millisecond radio pulsar. The
flux decrease and spectral evolution of the quiescent optical emission observed
comparing our data with previous observations, collected over 5 years, cannot
be well explained with the contribution of an irradiated companion star and an
accretion disc alone. The progressive flux decrease as the system gets bluer
could be due to a continuum component evolving towards a lower, bluer spectrum.
While most of the continuum component is likely due to the disc, we do not
expect it to become bluer in quiescence. Hence we hypothesize that an
additional component, such as synchrotron emission from a jet was contributing
significantly in the earlier data obtained during quiescence and then
progressively fading or moving its break frequency toward longer wavelengths.Comment: 7 pages, 8 figures, accepted for publication in Section 7. Stellar
structure and evolution of Astronomy and Astrophysic
Observations of Doppler Boosting in Kepler Lightcurves
Among the initial results from Kepler were two striking lightcurves, for KOI
74 and KOI 81, in which the relative depths of the primary and secondary
eclipses showed that the more compact, less luminous object was hotter than its
stellar host. That result became particularly intriguing because a substellar
mass had been derived for the secondary in KOI 74, which would make the high
temperature challenging to explain; in KOI 81, the mass range for the companion
was also reported to be consistent with a substellar object. We re-analyze the
Kepler data and demonstrate that both companions are likely to be white dwarfs.
We also find that the photometric data for KOI 74 show a modulation in
brightness as the more luminous star orbits, due to Doppler boosting. The
magnitude of the effect is sufficiently large that we can use it to infer a
radial velocity amplitude accurate to 1 km/s. As far as we are aware, this is
the first time a radial-velocity curve has been measured photometrically.
Combining our velocity amplitude with the inclination and primary mass derived
from the eclipses and primary spectral type, we infer a secondary mass of
0.22+/-0.03 Msun. We use our estimates to consider the likely evolutionary
paths and mass-transfer episodes of these binary systems.Comment: 8 pages, 4 figures, ApJ 715, 51 (v4 is updated to match the published
version, including a note added in proof with measured projected rotational
velocities)
Discovery of the Optical Counterparts to Four Energetic Fermi Millisecond Pulsars
In the last few years, over 43 millisecond radio pulsars have been discovered
by targeted searches of unidentified gamma-ray sources found by the Fermi
Gamma-Ray Space Telescope. A large fraction of these millisecond pulsars are in
compact binaries with low-mass companions. These systems often show eclipses of
the pulsar signal and are commonly known as black widows and redbacks because
the pulsar is gradually destroying its companion. In this paper, we report on
the optical discovery of four strongly irradiated millisecond pulsar
companions. All four sources show modulations of their color and luminosity at
the known orbital periods from radio timing. Light curve modelling of our
exploratory data shows that the equilibrium temperature reached on the
companion's dayside with respect to their nightside is consistent with about
10-30% of the available spin-down energy from the pulsar being reprocessed to
increase the companion's dayside temperature. This value compares well with the
range observed in other irradiated pulsar binaries and offers insights about
the energetics of the pulsar wind and the production of gamma-ray emission. In
addition, this provides a simple way of estimating the brightness of irradiated
pulsar companions given the pulsar spin-down luminosity. Our analysis also
suggests that two of the four new irradiated pulsar companions are only
partially filling their Roche lobe. Some of these sources are relatively bright
and represent good targets for spectroscopic follow-up. These measurements
could enable, among other things, mass determination of the neutron stars in
these systems.Comment: 11 pages, 5 tables, 1 figure, 4 online tables. ApJ submitted and
referee
X-Ray and Optical Properties of Black Widows and Redbacks
Black widows and redbacks are binary systems consisting of a millisecond
pulsar in a close binary with a companion having matter driven off of its
surface by the pulsar wind. X-rays due to an intra-binary shock have been
observed from many of these systems, as well as orbital variations in the
optical emission from the companion due to heating and tidal distortion. We
have been systematically studying these systems in radio, optical and X-rays.
Here we will present an overview of X-ray and optical studies of these systems,
including new XMM-Newton and NuStar data obtained from several of them, along
with new optical photometry.Comment: 4 pages, 1 figure, Proceedings of IAU Symposium 337 "Pulsar
Astrophysics - The Next 50 Years" held in Jodrell Bank Observatory, UK Sept.
4-8 201
Multiband Studies of the Optical Periodic Modulation in the X-Ray Binary SAX~J1808.4-3658 during Its Quiescence and 2008 Outburst
We report on time-resolved optical imaging of the X-ray binary SAX
J1808.4-3658 during its quiescent state and 2008 outburst. The binary,
containing an accretion-powered millisecond pulsar, has a large sinusoidal-like
modulation in its quiescent optical emission. We employ a Markov chain Monte
Carlo technique to fit our multi-band light curve data in quiescence with an
irradiated star model, and derive a tight constraint of deg on
the inclination angle of the binary system. The pulsar and its companion
are constrained to have masses of 0.97^{+0.31}_{-0.22} M_{\sun} and
0.04^{+0.02}_{-0.01} M_{\sun} (both 1 ranges), respectively. The
dependence of these results on the measurements of the companion's projected
radial velocity is discussed. We also find that the accretion disk had nearly
constant optical fluxes over a 500 day period in the quiescent state our
data covered, but started brightening 1.5 months before the 2008 outburst.
Variations in modulation during the outburst were detected in our four
observations made 7-12 days after the start of the outburst, and a
sinusoidal-like modulation with 0.2 mag amplitude changed to have a smaller
amplitude of 0.1 mag. The modulation variations are discussed. We estimate the
albedo of the companion during its quiescence and the outburst, which was
approximately 0 and 0.8 (for isotropic emission), respectively. This large
difference probably provides additional evidence that the neutron star in the
binary turns on as a radio pulsar in quiescence.Comment: 13 pages, 9 figures, 1 table, accepted for publication in Ap
An irradiated brown-dwarf companion to an accreting white dwarf
Interacting compact binary systems provide a natural laboratory in which to study irradiated substellar objects. As the mass-losing secondary (donor) in these systems makes a transition from the stellar to the substellar regime, it is also irradiated by the primary (compact accretor)1, 2. The internal and external energy fluxes are both expected to be comparable in these objects, providing access to an unexplored irradiation regime. The atmospheric properties of donors are largely unknown3, but could be modified by the irradiation. To constrain models of donor atmospheres, it is necessary to obtain accurate observational estimates of their physical properties (masses, radii, temperatures and albedos). Here we report the spectroscopic detection and characterization of an irradiated substellar donor in an accreting white-dwarf binary system. Our near-infrared observations allow us to determine a model-independent mass estimate for the donor of 0.055â±â0.008 solar masses and an average spectral type of L1â±â1, supporting both theoretical predictions and model-dependent observational constraints that suggest that the donor is a brown dwarf. Our time-resolved data also allow us to estimate the average irradiation-induced temperature difference between the dayside and nightside of the substellar donor (57 kelvin) and the maximum difference between the hottest and coolest parts of its surface (200 kelvin). The observations are well described by a simple geometric reprocessing model with a bolometric (Bond) albedo of less than 0.54 at the 2Ï confidence level, consistent with high reprocessing efficiency, but poor lateral heat redistribution in the atmosphere of the brown-dwarf donor4, 5. These results add to our knowledge of binary evolution, in that the donor has survived the transition from the stellar to the substellar regime, and of substellar atmospheres, in that we have been able to test a regime in which the irradiation and the internal energy of a brown dwarf are comparable
Properties and Evolution of the Redback Millisecond Pulsar Binary PSR J2129-0429
PSR J2129â0429 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 m_R = 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 M_â. The G-type pulsar companion has mass 0.44 ± 0.04 M_â, 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^(â1) 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 J2129â0429 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 pulsarâwhite dwarf binaries. Its eventual fate may depend on whether it undergoes future episodes of mass transfer and increased irradiation
The Gravitational-wave Optical Transient Observer (GOTO)
The Gravitational-wave Optical Transient Observer (GOTO) is a wide-field telescope project focused on detecting optical counterparts to gravitational-wave sources. Each GOTO robotic mount holds eight 40 cm telescopes, giving an overall field of view of 40 square degrees. As of 2022 the first two GOTO mounts have been commissioned at the Roque de los Muchachos Observatory on La Palma, Canary Islands, and construction of the second node with two additional 8-telescope mounts has begin at Siding Spring Observatory in New South Wales, Australia. Once fully operational each GOTO mount will be networked to form a robotic, multi-site observatory, which will survey the entire visible sky every two nights and enable rapid follow-up detections of transient sources
LOFAR discovery of a quiet emission mode in PSR B0823+26
15 pages, 8 figures, 2 tables, accepted for publication in MNRASInternational audiencePSR B0823+26, a 0.53-s radio pulsar, displays a host of emission phenomena over timescales of seconds to (at least) hours, including nulling, subpulse drifting, and mode-changing. Studying pulsars like PSR B0823+26 provides further insight into the relationship between these various emission phenomena and what they might teach us about pulsar magnetospheres. Here we report on the LOFAR discovery that PSR B0823+26 has a weak and sporadically emitting 'quiet' (Q) emission mode that is over 100 times weaker (on average) and has a nulling fraction forty-times greater than that of the more regularly-emitting 'bright' (B) mode. Previously, the pulsar has been undetected in the Q-mode, and was assumed to be nulling continuously. PSR B0823+26 shows a further decrease in average flux just before the transition into the B-mode, and perhaps truly turns off completely at these times. Furthermore, simultaneous observations taken with the LOFAR, Westerbork, Lovell, and Effelsberg telescopes between 110 MHz and 2.7 GHz demonstrate that the transition between the Q-mode and B-mode occurs within one single rotation of the neutron star, and that it is concurrent across the range of frequencies observed