8 research outputs found
Discovery of hard phase lags in the pulsed emission of GRO J1744-28
We report on the discovery and energy dependence of hard phase lags in the
2.14 Hz pulsed profiles of GRO J1744-28. We used data from XMM-Newton and
NuSTAR. We were able to well constrain the lag spectrum with respect to the
softest (0.3--2.3 keV) band: the delay shows increasing lag values reaching a
maximum delay of 12 ms, between 6 and 6.4 keV. After this maximum, the
value of the hard lag drops to 7 ms, followed by a recovery to a plateau at 9
ms for energies above 8 keV. NuSTAR data confirm this trend up to 30 keV, but
the measurements are statistically poorer, and therefore, less constraining.
The lag-energy pattern up to the discontinuity is well described by a
logarithmic function. Assuming this is due to a Compton reverberation
mechanism, we derive a size for the Compton cloud 120
, consistent with previous estimates on the magnetospheric radius.
In this scenario, the sharp discontinuity at 6.5 keV appears difficult
to interpret and suggests the possible influence of the reflected component in
this energy range. We therefore propose the possible coexistence of both
Compton and disk reverberation to explain the scale of the lags and its energy
dependence.Comment: Accepted for publication in MNRAS Letters on 2016 June 0
Study of the reflection spectrum of the accreting neutron star GX 3+1 using XMM-Newton and INTEGRAL
Broad emission features of abundant chemical elements, such as Iron, are
commonly seen in the X-ray spectra of accreting compact objects and their
studies can provide useful information about the geometry of the accretion
processes. In this work, we focus our attention on GX 3+1, a bright, persistent
accreting low mass X-ray binary, classified as an atoll source. Its spectrum is
well described by an accretion disc plus a stable comptonizing, optically thick
corona which dominates the X-ray emission in the 0.3-20 keV energy band. In
addition, four broad emission lines are found and we associate them with
reflection of hard photons from the inner regions of the accretion disc where
doppler and relativistic effects are important. We used self-consistent
reflection models to fit the spectra of the 2010 XMM-Newton observation and the
stacking of the whole datasets of 2010 INTEGRAL observations. We conclude that
the spectra are consistent with reflection produced at ~10 gravitational radii
by an accretion disc with an ionization parameter of xi~600 erg cm/s and viewed
under an inclination angle of the system of ~35{\deg}. Furthermore, we detected
for the first time for GX 3+1, the presence of a powerlaw component dominant at
energies higher than 20 keV, possibly associated with an optically thin
component of non-thermal electrons.Comment: Accepted to appear on MNRAS, 9 pages, 5 figur
Broad-band spectral analysis of the accreting millisecond X-ray pulsar SAX J1748.9-2021
We analyzed a 115 ks XMM-Newton observation and the stacking of 8 days of
INTEGRAL observations, taken during the raise of the 2015 outburst of the
accreting millisecond X-ray pulsar SAX J1748.9-2021. The source showed numerous
type-I burst episodes during the XMM-Newton observation, and for this reason we
studied separately the persistent and burst epochs. We described the persistent
emission with a combination of two soft thermal components, a cold thermal
Comptonization component (~2 keV) and an additional hard X-ray emission
described by a power-law (photon index ~2.3). The continuum components can be
associated with an accretion disc, the neutron star (NS) surface and a thermal
Comptonization emission coming out of an optically thick plasma region, while
the origin of the high energy tail is still under debate. In addition, a number
of broad (~0.1-0.4 keV) emission features likely associated to reflection
processes have been observed in the XMM-Newton data. The estimated 1.0-50 keV
unabsorbed luminosity of the source is ~5x10^37 erg/s, about 25% of the
Eddington limit assuming a 1.4 solar mass NS. We suggest that the spectral
properties of SAX J1748.9-2021 are consistent with a soft state, differently
from many other accreting X-ray millisecond pulsars which are usually found in
the hard state. Moreover, none of the observed type-I burst reached the
Eddington luminosity. Assuming that the burst ignition and emission are
produced above the whole NS surface, we estimate a neutron star radius of ~7-8
km, consistent with previous results.Comment: Accepted for publication in MNRAS; 12 pages, 9 figures, 2 table
Disk precession to explain the super-orbital modulation of LMC X-4: results from the Swift monitoring campaign
We studied the spectral changes of the high-mass X-ray binary system LMC X-4
to understand the origin and mechanisms beyond its super-orbital modulation
(30.4 days). To this aim, we obtained a monitoring campaign with Swift/XRT
(0.3-10 keV) and complemented these data with the years-long Swift/BAT survey
data (15-60 keV). We found a self-consistent, physically motivated, description
of the broadband X-ray spectrum using a Swift/XRT and a NuSTAR observation at
the epoch of maximum flux. We decomposed the spectrum into the sum of a
bulk+thermal Comptonization, a disk-reflection component and a soft
contribution from a standard Shakura-Sunyaev accretion disk. We applied this
model to 20 phase-selected Swift spectra along the super-orbital period. We
found a phase-dependent flux ratio of the different components, whereas the
absorption column does not significantly vary. The disk emission is decoupled
with respect to the hard flux. We interpret this as a geometrical effect in
which the inner parts of the disk are tilted with respect to the obscuring
outer regions.Comment: 14 pages, 15 figures, Accepted for publication in MNRA
A search for optical and near-infrared counterparts of the compact binary merger GW190814
We report on our observing campaign of the compact binary merger GW190814,
detected by the Advanced LIGO and Advanced Virgo detectors on August 14th,
2019. This signal has the best localisation of any observed gravitational wave
(GW) source, with a 90% probability area of 18.5 deg, and an estimated
distance of ~ 240 Mpc. We obtained wide-field observations with the Deca-Degree
Optical Transient Imager (DDOTI) covering 88% of the probability area down to a
limiting magnitude of = 19.9 AB. Nearby galaxies within the high
probability region were targeted with the Lowell Discovery Telescope (LDT),
whereas promising candidate counterparts were characterized through
multi-colour photometry with the Reionization and Transients InfraRed (RATIR)
and spectroscopy with the Gran Telescopio de Canarias (GTC). We use our optical
and near-infrared limits in conjunction with the upper limits obtained by the
community to constrain the possible electromagnetic counterparts associated
with the merger. A gamma-ray burst seen along its jet's axis is disfavoured by
the multi-wavelength dataset, whereas the presence of a burst seen at larger
viewing angles is not well constrained. Although our observations are not
sensitive to a kilonova similar to AT2017gfo, we can rule out high-mass (> 0.1
M) fast-moving (mean velocity >= 0.3c) wind ejecta for a possible
kilonova associated with this merger.Comment: 17 pages, 11 figures, 5 tables; updated acknowledgement section.
Accepted for publication in MNRAS (10 September 2020
GROWTH on GW190425: Searching thousands of square degrees to identify an optical or infrared counterpart to a binary neutron star merger with the Zwicky Transient Facility and Palomar Gattini IR
The beginning of the third observing run by the network of gravitational-wave
detectors has brought the discovery of many compact binary coalescences.
Prompted by the detection of the first binary neutron star merger in this run
(GW190425 / LIGO/Virgo S190425z), we performed a dedicated follow-up campaign
with the Zwicky Transient Facility (ZTF) and Palomar Gattini-IR telescopes. As
it was a single gravitational-wave detector discovery, the initial skymap
spanned most of the sky observable from Palomar Observatory, the site of both
instruments. Covering 8000 deg of the inner 99\% of the initial skymap over
the next two nights, corresponding to an integrated probability of 46\%, the
ZTF system achieved a depth of \,21 in - and
-bands. Palomar Gattini-IR covered a total of 2200 square degrees in
-band to a depth of 15.5\,mag, including 32\% of the integrated probability
based on the initial sky map. However, the revised skymap issued the following
day reduced these numbers to 21\% for the Zwicky Transient Facility and 19\%
for Palomar Gattini-IR. Out of the 338,646 ZTF transient "alerts" over the
first two nights of observations, we narrowed this list to 15 candidate
counterparts. Two candidates, ZTF19aarykkb and ZTF19aarzaod were particularly
compelling given that their location, distance, and age were consistent with
the gravitational-wave event, and their early optical lightcurves were
photometrically consistent with that of kilonovae. These two candidates were
spectroscopically classified as young core-collapse supernovae. The remaining
candidates were photometrically or spectroscopically ruled-out as supernovae.
Palomar Gattini-IR identified one fast evolving infrared transient after the
merger, PGIR19bn, which was later spectroscopically classified as an M-dwarf
flare. [abridged
GROWTH on S190425z: Searching Thousands of Square Degrees to Identify an Optical or Infrared Counterpart to a Binary Neutron Star Merger with the Zwicky Transient Facility and Palomar Gattini-IR
The third observing run by LVC has brought the discovery of many compact binary coalescences. Following the detection of the first binary neutron star merger in this run (LIGO/Virgo S190425z), we performed a dedicated follow-up campaign with the Zwicky Transient Facility (ZTF) and Palomar Gattini-IR telescopes. The initial skymap of this single-detector gravitational wave (GW) trigger spanned most of the sky observable from Palomar Observatory. Covering 8000 deg2 of the initial skymap over the next two nights, corresponding to 46% integrated probability, ZTF system achieved a depth of ≈21 m AB in g- and r-bands. Palomar Gattini-IR covered 2200 square degrees in J-band to a depth of 15.5 mag, including 32% integrated probability based on the initial skymap. The revised skymap issued the following day reduced these numbers to 21% for the ZTF and 19% for Palomar Gattini-IR. We narrowed 338,646 ZTF transient "alerts" over the first two nights of observations to 15 candidate counterparts. Two candidates, ZTF19aarykkb and ZTF19aarzaod, were particularly compelling given that their location, distance, and age were consistent with the GW event, and their early optical light curves were photometrically consistent with that of kilonovae. These two candidates were spectroscopically classified as young core-collapse supernovae. The remaining candidates were ruled out as supernovae. Palomar Gattini-IR did not identify any viable candidates with multiple detections only after merger time. We demonstrate that even with single-detector GW events localized to thousands of square degrees, systematic kilonova discovery is feasible
Observatory science with eXTP
International audienceIn this White Paper we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to Observatory Science targets. These include flaring stars, supernova remnants, accreting white dwarfs, low and high mass X-ray binaries, radio quiet and radio loud active galactic nuclei, tidal disruption events, and gamma-ray bursts. eXTP will be excellently suited to study one common aspect of these objects: their often transient nature. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s