40 research outputs found
Evidence for hot clumpy accretion flow in the transitional millisecond pulsar PSR J1023+0038
We present simultaneous optical and near-infrared (IR) photometry of the millisecond pulsar PSR J1023+0038 during its low-mass X-ray binary phase. The r'- and K-s-band light curves show rectangular, flat-bottomed dips, similar to the X-ray mode-switching (active passive state transitions) behaviour observed previously. The cross-correlation function (CCF) of the optical and near-IR data reveals a strong, broad negative anticorrelation at negative lags, a broad positive correlation at positive lags, with a strong, positive narrow correlation superimposed. The shape of the CCF resembles the CCF of black hole X-ray binaries but the time-scales are different. The features can be explained by reprocessing and a hot accretion flow close to the neutron star's magnetospheric radius. The optical emission is dominated by the reprocessed component, whereas the near-IR emission contains the emission from plasmoids in the hot accretion flow and a reprocessed component. The rapid active passive state transition occurs when the hot accretion flow material is channelled on to the neutron star and is expelled from its magnetosphere. During the transition the optical reprocessing component decreases resulting in the removal of a blue spectral component. The accretion of clumpy material through the magnetic barrier of the neutron star produces the observed near-IR/optical CCF and variability. The dip at negative lags corresponds to the suppression of the near-IR synchrotron component in the hot flow, whereas the broad positive correlation at positive lags is driven by the increased synchrotron emission of the outflowing plasmoids. The narrow peak in the CCF is due to the delayed reprocessed component, enhanced by the increased X-ray emission
Pulsating in unison at optical and X-ray energies: simultaneous high-time resolution observations of the transitional millisecond pulsar PSR J1023+0038
PSR J1023+0038 is the first millisecond pulsar discovered to pulsate in the
visible band; such a detection took place when the pulsar was surrounded by an
accretion disk and also showed X-ray pulsations. We report on the first high
time resolution observational campaign of this transitional pulsar in the disk
state, using simultaneous observations in the optical (TNG, NOT, TJO), X-ray
(XMM-Newton, NuSTAR, NICER), infrared (GTC) and UV (Swift) bands. Optical and
X-ray pulsations were detected simultaneously in the X-ray high intensity mode
in which the source spends 70% of the time, and both disappeared in the
low mode, indicating a common underlying physical mechanism. In addition,
optical and X-ray pulses were emitted within a few km, had similar pulse shape
and distribution of the pulsed flux density compatible with a power-law
relation connecting the optical and the 0.3-45 keV
X-ray band. Optical pulses were detected also during flares with a pulsed flux
reduced by one third with respect to the high mode; the lack of a simultaneous
detection of X-ray pulses is compatible with the lower photon statistics. We
show that magnetically channeled accretion of plasma onto the surface of the
neutron star cannot account for the optical pulsed luminosity (
erg/s). On the other hand, magnetospheric rotation-powered pulsar emission
would require an extremely efficient conversion of spin-down power into pulsed
optical and X-ray emission. We then propose that optical and X-ray pulses are
instead produced by synchrotron emission from the intrabinary shock that forms
where a striped pulsar wind meets the accretion disk, within a few light
cylinder radii away, 100 km, from the pulsar.Comment: 26 pages, 14 figures, first submitted to ApJ on 2019, January 1
Sgr A* near-infrared flares from reconnection events in a magnetically arrested disc
Large-amplitude Sgr A* near-infrared flares result from energy injection into
electrons near the black hole event horizon. Astrometry data show continuous
rotation of the emission region during bright flares, and corresponding
rotation of the linear polarization angle. One broad class of physical flare
models invokes magnetic reconnection. Here we show that such a scenario can
arise in a general relativistic magnetohydrodynamic simulation of a
magnetically arrested disc. Saturation of magnetic flux triggers eruption
events, where magnetically dominated plasma is expelled from near the horizon
and forms a rotating, spiral structure. Dissipation occurs via reconnection at
the interface of the magnetically dominated plasma and surrounding fluid. This
dissipation is associated with large increases in near-infrared emission in
models of Sgr A*, with durations and amplitudes consistent with the observed
flares. Such events occur at roughly the timescale to re-accumulate the
magnetic flux from the inner accretion disc, 10h for Sgr A*. We study
near-infrared observables from one sample event to show that the emission
morphology tracks the boundary of the magnetically dominated region. As the
region rotates around the black hole, the near-infrared centroid and linear
polarization angle both undergo continuous rotation, similar to the behavior
seen in Sgr A* flares.Comment: revised version, MNRAS, in pres
GMP-selected dual and lensed AGNs: selection function and classification based on near-IR colors and resolved spectra from VLT/ERIS, KECK/OSIRIS, and LBT/LUCI
The Gaia-Multi-Peak (GMP) technique can be used to identify large numbers of
dual or lensed AGN candidates at sub-arcsec separation, allowing us to study
both multiple SMBHs in the same galaxy and rare, compact lensed systems. The
observed samples can be used to test the predictions of the models of SMBH
merging once 1) the selection function of the GMP technique is known, and 2)
each system has been classified as dual AGN, lensed AGN, or AGN/star alignment.
Here we show that the GMP selection is very efficient for separations above
0.15'' when the secondary (fainter) object has magnitude G<20.5. We present the
spectroscopic classification of five GMP candidates using VLT/ERIS and
Keck/OSIRIS, and compare them with the classifications obtained from: a) the
near-IR colors of 7 systems obtained with LBT/LUCI, and b) the analysis of the
total, spatially-unresolved spectra. We conclude that colors and integrated
spectra can already provide reliable classifications of many systems. Finally,
we summarize the confirmed dual AGNs at z>0.5 selected by the GMP technique,
and compare this sample with other such systems from the literature, concluding
that GMP can provide a large number of confirmed dual AGNs at separations below
7 kpc.Comment: 14 pages,A&A, in pres
GMP-selected dual and lensed AGNs: Selection function and classification based on near-IR colors and resolved spectra from VLT/ERIS, Keck/OSIRIS, and LBT/LUCI
The Gaia Multipeak (GMP) technique can be used to identify large numbers of dual or lensed active galactic nucleus (AGN) candidates at subarcsec separation, allowing us to study both multiple supermassive black holes (SMBHs) in the same galaxy and rare, compact lensed systems. The observed samples can be used to test the predictions of the models of SMBH merging when (1) the selection function of the GMP technique is known, and (2) each system has been classified as a dual AGN, a lensed AGN, or an AGN/star alignment. Here we show that the GMP selection is very efficient for separations above 0:15′′ when the secondary (fainter) object has a magnitude G ≤ 20:5. We present the spectroscopic classification of five GMP candidates using VLT/ERIS and Keck/OSIRIS and compare them with the classifications obtained from (a) the near-IR colors of seven systems obtained with LBT/LUCI, and (b) the analysis of the total spatially unresolved spectra. We conclude that colors and integrated spectra can already provide reliable classifications of many systems. Finally, we summarize the confirmed dual AGNs at z > 0:5 selected by the GMP technique, and compare this sample with other such systems from the literature, concluding that GMP can provide a large number of confirmed dual AGNs at separations below 7 kpc
The Enhanced Resolution Imager and Spectrograph for the VLT
ERIS, the Enhanced Resolution Imager and Spectrograph, is an instrument that
both extends and enhances the fundamental diffraction limited imaging and
spectroscopy capability for the VLT. It replaces two instruments that were
being maintained beyond their operational lifetimes, combines their
functionality on a single focus, provides a new wavefront sensing module for
natural and laser guide stars that makes use of the Adaptive Optics Facility,
and considerably improves on their performance. The observational modes ERIS
provides are integral field spectroscopy at 1-2.5 {\mu}m, imaging at 1-5 {\mu}m
with several options for high contrast imaging, and longslit spectroscopy at
3-4 {\mu}m, The instrument is installed at the Cassegrain focus of UT4 at the
VLT and, following its commissioning during 2022, has been made available to
the community.Comment: 19 pages with 29 figures; submitted to A&
Furiously fast and red: sub-second optical flaring in V404 Cyg during the 2015 outburst peak
We present observations of rapid (sub-second) optical flux variability in V404 Cyg during its 2015 June outburst. Simultaneous three-band observations with the ULTRACAM fast imager on four nights show steep power spectra dominated by slow variations on ∼100–1000 s time-scales. Near the peak of the outburst on June 26, a dramatic change occurs and additional, persistent sub-second optical flaring appears close in time to giant radio and X-ray flaring. The flares reach peak optical luminosities of ∼ few × 1036 erg s−1. Some are unresolved down to a time resolution of 24 ms. Whereas the fast flares are stronger in the red, the slow variations are bluer when brighter. The redder slopes, emitted power and characteristic time-scales of the fast flares can be explained as optically thin synchrotron emission from a compact jet arising on size scales ∼140–500 Gravitational radii (with a possible additional contribution by a thermal particle distribution). The origin of the slower variations is unclear. The optical continuum spectral slopes are strongly affected by dereddening uncertainties and contamination by strong Hα emission, but the variations of these slopes follow relatively stable loci as a function of flux. Cross-correlating the slow variations between the different bands shows asymmetries on all nights consistent with a small red skew (i.e. red lag). X-ray reprocessing and non-thermal emission could both contribute to these. These data reveal a complex mix of components over five decades in time-scale during the outburst