52 research outputs found
Quantum clock: A critical discussion on spacetime
We critically discuss the measure of very short time intervals. By means of a
Gedankenexperiment, we describe an ideal clock based on the occurrence of
completely random events. Many previous thought experiments have suggested
fundamental Planck-scale limits on measurements of distance and time. Here we
present a new type of thought experiment, based on a different type of clock,
that provide further support for the existence of such limits. We show that the
minimum time interval that this clock can measure scales as the
inverse of its size . This implies an uncertainty relation between
space and time: ; where G, and c
are the gravitational constant, the reduced Planck constant, and the speed of
light, respectively. We outline and briefly discuss the implications of this
uncertainty conjecture.Comment: 10 pages, published in Physical Review
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
A method to constrain the neutron star magnetic field in Low Mass X-ray Binaries
We describe here a method to put an upper limit to the strength of the magnetic field of neutron stars in low mass X‐ray binaries for which the spin period and the X‐ray luminosity during X‐ray quiescent periods are known. This is obtained using simple considerations about the position of the magnetospheric radius during quiescent periods. We applied this method to the accreting millisecond pulsar SAX J1808.4‐3658, which shows coherent X‐ray pulsations at a frequency of ∼ 400 Hz and a quiescent X‐ray luminosity of ∼ 5 × 1031 ergs/s, and found that B ⩽ 5 × 108 Gauss in this source. Combined with the lower limit inferred from the presence of X‐ray pulsations, this constrains the SAX J1808.4‐3658 neutron star magnetic field in the quite narrow range (1 – 5) × 108 Gauss. Similar considerations applied to the case of Aql X‐1 and KS 1731‐260 give neutron star magnetic fields lower than ∼ 109 Gauss
A Complex Environment around Circinus X-1
We present the results of an archival 54 ks long Chandra observation of the peculiar source Cir X-1 during the phase passage 0.223-0.261. We focus on the study of detected emission and absorption features using the HETGS. A comparative analysis of X-ray spectra, selected at different flux levels of the source, allows us to distinguish between a very hard state, at a low count rate, and a brighter, softer, highly absorbed spectrum during episodes of flaring activity. The spectrum of the hard state clearly shows emission lines of highly ionized elements, while, during the flaring state, the spectrum also shows strong resonant absorption lines. The most intense and interesting feature in this latter state is present in the Fe K alpha region: a very broadened absorption line at energies similar to 6.5 keV that could result from a smeared blending of resonant absorption lines of moderately ionized iron ions (Fe XX-Fe XXIV). We also observe strong resonant absorption lines of Fe XXV and Fe XXVI, together with a smeared absorption edge above 7 keV. We argue that the emitting region during the quiescent/hard state is constituted of a purely photoionized medium, possibly present above an accretion disk, or of a photoionized plasma present in a beamed outflow. During the flaring states the source undergoes enhanced turbulent accretion that modifies both the accretion geometry and the optical depth of the gas surrounding the primary X- ray source
The Zoo of emission lines in the spectrum of Cir X-1 observed by XMM-Newton
We present the preliminary analysis of a 10 ks XMM-Newton EPIC/pn observation of Cir X-1 immediately after the zero phase. The continuum emission is modeled using a blackbody component partially absorbed by neutral matter probably located around the binary system. We detect a forest of emission lines associated to highly ionized ions
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
General relativistic effects on the evolution of binary systems.
When a radio pulsar brakes down due to magnetodipole emission,its gravitational mass decreases accordingly. If the pulsar is hosted in a binary system, this mass loss will Increase the orbital period of the system. We show that this relativistic effect can be indeed observable if the neutron star is fast and magnetized enough and that, if observed, it will help to put tight constraints to the equation of state of ultradense matter. Moreover, in Low Mass X-ray Binaries that evolve towards short periods, the neutron star lights up as a radio pulsar during the "period gap". As the effect we consider contrasts the orbital period decay, the system spends a longer time in this phase. As a consequence, the neutron star can survive this phase only if it is non-supramassive, Since in such bianries similar to 0.84M(circle dot) can be accreted onto the neutron star, short period (P <= 2 h) millisecond Xray pulsars like SAX J1808.4-3658 can be formed only if either a large part of the accreting matter has been ejected from the system, or the equation of state of ultradense matter is very stiff
Relativistically Smeared Iron Lines in the Spectra of Bright NS LMXB
We present preliminary results of a study on three bright accreting low-mass X-ray binaries hosting a neutron star, based on XMM-Newton observations. These sources (GX 340+0, GX 349+2 and SAX J1808.4-3658) show a broad Fe K alpha iron line in their spectra. This feature can be well described by relativistic line profile in each case; the good spectral resolution of the EPIC/PN and the high statistics spectra allow to put very good constraints on the disk geometry and ionization stage of the reflecting matter
Resolving the Fe XXV triplet with Chandra in Centaurus X-3
We present the results of a 45 ks Chandra observation of the high-mass X-ray binary Cen X-3 at orbital phases between 0.13 and 0.40 (in the eclipse post-egress phases). Here we concentrate on the study of discrete features in the energy spectrum at energies between 6 and 7 keV, that is, on the iron K alpha line region, using the High Energy Transmission Grating Spectrometer (HETGS) on board the Chandra satellite. We clearly see a K alpha neutral iron line at similar to 6.40 keV and were able to distinguish the three lines of the Fe xxv triplet at 6.61, 6.67, and 6.72 keV, with equivalent widths of 6, 9, and 5 eV, respectively. The equivalent width of the Ka neutral iron line is 13 eV, an order of magnitude lower than previous measures. We discuss the possibility that the small equivalent width is due to a decrease of the solid angle subtended by the reflector
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