10,411 research outputs found
Searching for the Donor Stars of ULX Pulsars
We report on our search for the optical counterparts of two ultraluminous
X-ray pulsars with known orbital periods, M82 X-2 and NGC 5907 X-1, in new and
archival HST observations, in an effort to characterize the donor stars in
these systems. We detect five near-infrared sources consistent with the
position of M82 X-2 that are too bright to be single stars. We also detect
seven sources in the WFC3/UVIS F336W image whose photometry matches that of
10-15 M stars turning off the main sequence. Such stars have densities
consistent with the properties of the donor star of M82 X-2 as inferred from
X-ray timing analysis, although it is also possible that the donor is a lower
mass star below our detection limit or that there is a significant contribution
from the accretion disc to the optical emission. We detect three candidate
counterparts to NGC 5907 X-1 in the near-infrared. All of these are too bright
to be the donor star of the ULX, which based on its orbital period is a red
giant. The high background at the location of NGC 5907 X-1 precludes us from
detecting this expected donor star. The recently discovered NGC 5907 ULX-2 also
falls within the field of view of the near-infrared imaging; we detect four
sources in the error circle, with photometry that matches AGB stars. The star
suggested to be the counterpart of NGC 5907 ULX-2 by Pintore et al. (2018)
falls outside our 2- error circle.Comment: 10 pages, 3 figures, accepted for publication in Ap
PCA of PCA: Principal Component Analysis of Partial Covering Absorption in NGC 1365
We analyse 400 ks of XMM-Newton data on the active galactic nucleus NGC 1365
using principal component analysis (PCA) to identify model independent spectral
components. We find two significant components and demonstrate that they are
qualitatively different from those found in MCG?6-30-15 using the same method.
As the variability in NGC 1365 is known to be due to changes in the parameters
of a partial covering neutral absorber, this shows that the same mechanism
cannot be the driver of variability in MCG-6-30-15. By examining intervals
where the spectrum shows relatively low absorption we separate the effects of
intrinsic source variability, including signatures of relativistic reflection,
from variations in the intervening absorption. We simulate the principal
components produced by different physical variations, and show that PCA
provides a clear distinction between absorption and reflection as the drivers
of variability in AGN spectra. The simulations are shown to reproduce the PCA
spectra of both NGC 1365 and MCG-6-30-15, and further demonstrate that the
dominant cause of spectral variability in these two sources requires a
qualitatively different mechanism.Comment: 8 pages, 10 figures. Accepted for publication in MNRA
Broad Iron Emission from Gravitationally Lensed Quasars Observed by Chandra
Recent work has demonstrated the potential of gravitationally lensed quasars
to extend measurements of black hole spin out to high-redshift with the current
generation of X-ray observatories. Here we present an analysis of a large
sample of 27 lensed quasars in the redshift range 1.0<z<4.5 observed with
Chandra, utilizing over 1.6 Ms of total observing time, focusing on the
rest-frame iron K emission from these sources. Although the X-ray
signal-to-noise (S/N) currently available does not permit the detection of iron
emission from the inner accretion disk in individual cases in our sample, we
find significant structure in the stacked residuals. In addition to the narrow
core, seen almost ubiquitously in local AGN, we find evidence for an additional
underlying broad component from the inner accretion disk, with a clear red wing
to the emission profile. Based on simulations, we find the detection of this
broader component to be significant at greater than the 3-sigma level. This
implies that iron emission from the inner disk is relatively common in the
population of lensed quasars, and in turn further demonstrates that, with
additional observations, this population represents an opportunity to
significantly extend the sample of AGN spin measurements out to high-redshift.Comment: 5 pages, 2 figures, accepted for publication in Ap
Spectral Changes in the Hyperluminous Pulsar in NGC 5907 as a Function of Super-Orbital Phase
We present broad-band, multi-epoch X-ray spectroscopy of the pulsating
ultra-luminous X-ray source (ULX) in NGC 5907. Simultaneous XMM-Newton and
NuSTAR data from 2014 are best described by a multi-color black-body model with
a temperature gradient as a function of accretion disk radius significantly
flatter than expected for a standard thin accretion disk (T(r) ~ r^{-p}, with
p=0.608^{+0.014}_{-0.012}). Additionally, we detect a hard power-law tail at
energies above 10 keV, which we interpret as being due to Comptonization. We
compare this observation to archival XMM-Newton, Chandra, and NuSTAR data from
2003, 2012, and 2013, and investigate possible spectral changes as a function
of phase over the 78d super-orbital period of this source. We find that
observations taken around phases 0.3-0.4 show very similar temperature
profiles, even though the observed flux varies significantly, while one
observation taken around phase 0 has a significantly steeper profile. We
discuss these findings in light of the recent discovery that the compact object
is a neutron star and show that precession of the accretion disk or the neutron
star can self-consistently explain most observed phenomena.Comment: 7 pages, 5 figures, submitted to ApJ; comments welcom
Baseline design of the filters for the LAD detector on board LOFT
The Large Observatory for X-ray Timing (LOFT) was one of the M3 missions
selected for the phase A study in the ESA's Cosmic Vision program. LOFT is
designed to perform high-time-resolution X-ray observations of black holes and
neutron stars. The main instrument on the LOFT payload is the Large Area
Detector (LAD), a collimated experiment with a nominal effective area of ~10 m
2 @ 8 keV, and a spectral resolution of ~240 eV in the energy band 2-30 keV.
These performances are achieved covering a large collecting area with more than
2000 large-area Silicon Drift Detectors (SDDs) each one coupled to a collimator
based on lead-glass micro-channel plates. In order to reduce the thermal load
onto the detectors, which are open to Sky, and to protect them from out of band
radiation, optical-thermal filter will be mounted in front of the SDDs.
Different options have been considered for the LAD filters for best compromise
between high quantum efficiency and high mechanical robustness. We present the
baseline design of the optical-thermal filters, show the nominal performances,
and present preliminary test results performed during the phase A study.Comment: Proc. SPIE 9144, Space Telescopes and Instrumentation 2014:
Ultraviolet to Gamma Ray, 91446
EMCCDs for space applications
This paper describes a qualification programme for Electron-Multiplication Charge Coupled Devices (EMCCDs) for use in space applications. While the presented results are generally applicable, the programme was carried out in the context of CCD development for the Radial Velocity Spectrometer (RVS) instrument on the European Space Agency's cornerstone Gaia mission. We discuss the issues of device radiation tolerance, charge transfer efficiency at low signal levels and life time effects on the electron-multiplication gain. The development of EMCCD technology to allow operation at longer wavelengths using high resistivity silicon, and the cryogenic characterisation of EMCCDs are also described
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