622 research outputs found
Spectral and Rotational Changes in the Isolated Neutron Star RX J0720.4-3125
RX J0720.4-3125 is an isolated neutron star that, uniquely in its class, has
shown changes in its thermal X-ray spectrum. We use new spectra taken with
Chandra's Low Energy Transmission Grating Spectrometer, as well as archival
observations, to try to understand the timescale and nature of these changes.
We construct lightcurves, which show both small, slow variations on a timescale
of years, and a larger event that occurred more quickly, within half a year.
From timing, we find evidence for a `glitch' coincident with this larger
event, with a fractional increase in spin frequency of 5x10^{-8}. We compare
the `before' and `after' spectra with those from RX J1308.6+2127, an isolated
neutron star with similar temperature and magnetic field strength, but with a
much stronger absorption feature in its spectrum. We find that the `after'
spectrum can be represented remarkably well by the superposition of the
`before' spectrum, scaled by two thirds, and the spectrum of RX J1308.6+2127,
thus suggesting that the event affected approximately one third of the surface.
We speculate the event reflects a change in surface composition caused by,
e.g., an accretion episode.Comment: 4 pages, 2 figures, 2 tables, emulateapj format. ApJL, accepte
Modeling Phase-resolved Observations of the Surfaces of Magnetic Neutron Stars
Recent observations by XMM-Newton detected rotational pulsations in the total
brightness and spectrum of several neutron stars. To properly interpret the
data, accurate modeling of neutron star emission is necessary. Detailed
analysis of the shape and strength of the rotational variations allows a
measurement of the surface composition and magnetic field, as well as
constrains the nuclear equation of state. We discuss our models of the spectra
and light curves of two of the most observed neutron stars, RX J1856.5-3754 and
1E 1207.4-5209, and discuss some implications of our results and the direction
of future work.Comment: 5 pages, 6 figures; Proceedings of "40 Years of Pulsars", eds. C.
Bassa, Z. Wang, A. Cumming, V. Kaspi, AIP, submitte
Evidence for a Mid-Atomic-Number Atmosphere in the Neutron Star 1E1207.4-5209
Recently Sanwal et al. (2002) reported the first clear detection of
absorption features in an isolated neutron star, 1E1207.4-5209. Remarkably
their spectral modeling demonstrates that the atmosphere cannot be Hydrogen.
They speculated that the neutron star atmosphere is indicative of ionized
Helium in an ultra-strong (~1.5x10^{14} G) magnetic field. We have applied our
recently developed atomic model (Mori & Hailey 2002) for strongly-magnetized
neutron star atmospheres to this problem. We find that this model, along with
some simp le atomic physics arguments, severely constrains the possible
composition of the atmosphere. In particular we find that the absorption
features are naturally associated with He-like Oxygen or Neon in a magnetic
field of ~10^{12} G, comparable to the magnetic field derived from the spin
parameters of the neutron star. This interpretation is consistent with the
relative line strengths and widths and is robust. Our model predicts possible
substructure in the spectral features, which has now been reported by
XMM-Newton (Mereghetti et al. 2002). However we show the Mereghetti et al.
claim that the atmosphere is Iron or some comparable high-Z element at ~
10^{12} G is easily ruled out by the Chandra and XMM-Newton data.Comment: 5 pages, AASTeX, Revised version. Accepted for publication in ApJ
Letter
NuSTAR and XMM-Newton observations of the Arches cluster in 2015: fading hard X-ray emission from the molecular cloud
We present results of long NuSTAR (200 ks) and XMM-Newton (100 ks)
observations of the Arches stellar cluster, a source of bright thermal (kT~2
keV) X-rays with prominent Fe XXV K_alpha 6.7 keV line emission and a nearby
molecular cloud, characterized by an extended non-thermal hard X-ray continuum
and fluorescent Fe K_alpha 6.4 keV line of a neutral or low ionization state
material around the cluster. Our analysis demonstrates that the non-thermal
emission of the Arches cloud underwent a dramatic change, with its homogeneous
morphology, traced by fluorescent Fe K_alpha line emission, vanishing after
2012, revealing three bright clumps. The declining trend of the cloud emission,
if linearly fitted, is consistent with half-life decay time of ~8 years. Such
strong variations have been observed in several other molecular clouds in the
Galactic Centre, including the giant molecular cloud Sgr B2, and point toward a
similar propagation of illuminating fronts, presumably induced by the past
flaring activity of Sgr A*.Comment: 15 pages, 14 figures, 7 tables, submitted to MNRAS; comments welcom
Preparation of Rat Gingival Mitochondria with an Improved Isolation Method
In order to establish a method of obtaining rat gingival mitochondria (Mt), Mt fractions were prepared in various combinations of homogenizing time with collagenase concentration. Rat gingival tissues were excised, minced, treated with collagenase, homogenized, and subjected to differential centrifugation rates. Both the respiratory control ratio (RCR) and adenosine diphosphate/oxygen (ADP/O) ratio of the Mt fraction prepared in a combination of 40, 50, or 60 sec homogenization with collagenase in a concentration range of 0.115%–0.130% (w/v) were measured. The values for the RCR and ADP/O ratio of the Mt fraction obtained in an optimal condition was 1.80 ± 0.05 and 1.65 ± 0.03, respectively. These results suggest that Mt of fairly high quality can be obtained through this refined combination of the homogenizing time and collagenase concentration
Isolated magnetar spin-down, soft X-ray emission and RXJ1856.5-3754
When an isolated magnetar with magnetic dipole field B ~ 10^15 G moves at
high velocity (v > 10^7 cm/s) through the ISM, its transition into propeller
effect driven spin-down may occur in less than 10^6 years. We propose that the
nearby neutron star RXJ1856.5-3754 is such a magnetar, and has spun down by the
propeller effect to a period greater than 10^4 sec within ~5*10^5 years. This
magnetar scenario is consistent with observed thermal X-ray emission properties
and the absence of detectable spin-modulations of them. Detection of other
rapidly moving long period (> 100 sec) magnetars with known ages would strongly
constrain the very great variety of predicted propeller-effect torque
magnitudes.Comment: 5 pages, 1 figure, accepted for publication in ApJ
Multi-wavelength observations of 2HWC J1928+177: dark accelerator or new TeV gamma-ray binary?
2HWC J1928+177 is a Galactic TeV gamma-ray source detected by the High
Altitude Water Cherenkov (HAWC) Observatory up to ~ 56 TeV. The HAWC source,
later confirmed by H.E.S.S., still remains unidentified as a dark accelerator
since there is no apparent supernova remnant or pulsar wind nebula detected in
the lower energy bands. The radio pulsar PSR J1928+1746, coinciding with the
HAWC source position, has no X-ray counterpart. Our SED modeling shows that
inverse Compton scattering in the putative pulsar wind nebula can account for
the TeV emission only if the unseen nebula is extended beyond r ~ 4 [arcmin].
Alternatively, TeV gamma rays may be produced by hadronic interactions between
relativistic protons from an undetected supernova remnant associated with the
radio pulsar and a nearby molecular cloud G52.9+0.1. NuSTAR and Chandra
observations detected a variable X-ray point source within the HAWC error
circle, potentially associated with a bright IR source. The X-ray spectra can
be fitted with an absorbed power-law model with cm and and exhibit
long-term X-ray flux variability over the last decade. If the X-ray source,
possibly associated with the IR source (likely an O star), is the counterpart
of the HAWC source, it may be a new TeV gamma-ray binary powered by collisions
between the pulsar wind and stellar wind. Follow-up X-ray observations are
warranted to search for diffuse X-ray emission and determine the nature of the
HAWC source.Comment: accepted to ApJ, 8 pages, 7 figure
A broadband X-ray study of the Rabbit pulsar wind nebula powered by PSR J1418-6058
We report on broadband X-ray properties of the Rabbit pulsar wind nebula
(PWN) associated with the pulsar PSR J1418-6058 using archival Chandra and
XMM-Newton data, and a new NuSTAR observation. NuSTAR data above 10 keV allowed
us to detect the 110-ms spin period of the pulsar, characterize its hard X-ray
pulse profile, and resolve hard X-ray emission from the PWN after removing
contamination from the pulsar and other overlapping point sources. The extended
PWN was detected up to 20 keV and is well described by a power-law model
with a photon index 2. The PWN shape does not vary significantly
with energy, and its X-ray spectrum shows no clear evidence of softening away
from the pulsar. We modeled the spatial profile of X-ray spectra and broadband
spectral energy distribution in the radio to TeV band to infer the physical
properties of the PWN. We found that a model with low magnetic field strength
( G) and efficient diffusion ( cm s)
fits the PWN data well. The extended hard X-ray and TeV emission, associated
respectively with synchrotron radiation and inverse Compton scattering by
relativistic electrons, suggests that particles are accelerated to very high
energies ( TeV), indicating that the Rabbit PWN is a Galactic
PeVatron candidate.Comment: 21 pages, 10 figures. ApJ accepte
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