415 research outputs found
Continued Neutron Star Crust Cooling of the 11 Hz X-Ray Pulsar in Terzan 5: A Challenge to Heating and Cooling Models?
The transient neutron star low-mass X-ray binary and 11 Hz X-ray pulsar IGR
J17480-2446 in the globular cluster Terzan 5 exhibited an 11-week accretion
outburst in 2010. Chandra observations performed within five months after the
end of the outburst revealed evidence that the crust of the neutron star became
substantially heated during the accretion episode and was subsequently cooling
in quiescence. This provides the rare opportunity to probe the structure and
composition of the crust. Here, we report on new Chandra observations of Terzan
5 that extend the monitoring to ~2.2 yr into quiescence. We find that the
thermal flux and neutron star temperature have continued to decrease, but
remain significantly above the values that were measured before the 2010
accretion phase. This suggests that the crust has not thermally relaxed yet,
and may continue to cool. Such behavior is difficult to explain within our
current understanding of heating and cooling of transiently accreting neutron
stars. Alternatively, the quiescent emission may have settled at a higher
observed equilibrium level (for the same interior temperature), in which case
the neutron star crust may have fully cooled.Comment: Accepted to ApJ without revision. Updated references and fixed few
typos to match published version. 7 pages, 3 figures, 3 table
Faint X-ray Sources in the Globular Cluster Terzan 5
We report our analysis of a Chandra X-ray observation of the rich globular
cluster Terzan 5, in which we detect 50 sources to a limiting 1.0-6 keV X-ray
luminosity of 3*10^{31} ergs/s within the half-mass radius of the cluster.
Thirty-three of these have L_X>10^{32} ergs/s, the largest number yet seen in
any globular cluster. In addition to the quiescent low-mass X-ray binary (LMXB,
identified by Wijnands et al.), another 12 relatively soft sources may be
quiescent LMXBs. We compare the X-ray colors of the harder sources in Terzan 5
to the Galactic Center sources studied by Muno and collaborators, and find the
Galactic Center sources to have harder X-ray colors, indicating a possible
difference in the populations. We cannot clearly identify a metallicity
dependence in the production of low-luminosity X-ray binaries in Galactic
globular clusters, but a metallicity dependence of the form suggested by Jordan
et al. for extragalactic LMXBs is consistent with our data.Comment: 15 pages, 10 figures (3 color). Resubmitted to ApJ after
incorporating referee comments. v2: Added references to introductio
Two years of INTEGRAL monitoring of GRS 1915+105 Part 1: multiwavelength coverage with INTEGRAL, RXTE, and the Ryle radio Telescope
(Abridged) We report the results of monitoring observations of the Galactic
microquasar GRS 1915+105 performed simultaneously with INTEGRAL and RXTE Ryle .
We present the results of the whole \integral campaign, report the sources that
are detected and their fluxes and identify the classes of variability in which
GRS 1915+105 is found. The accretion ejection connections are studied in a
model independent manner through the source light curves, hardness ratio, and
color color diagrams. During a period of steady ``hard'' X-ray state (the
so-called class chi) we observe a steady radio flux. We then turn to 3
particular observations during which we observe several types of soft X-ray
dips and spikes cycles, followed by radio flares. During these observations GRS
1915+105 is in the so-called nu, lambda, and beta classes of variability. The
observation of ejections during class lambda are the first ever reported. We
generalize the fact that a (non-major) discrete ejection always occurs, in GRS
1915+105, as a response to an X-ray sequence composed of a spectrally hard
X-ray dip terminated by an X-ray spike marking the disappearance of the hard
X-ray emission above 18 keV. We also identify the trigger of the ejection as
this X-ray spike. A possible correlation between the amplitude of the radio
flare and the duration of the X-ray dip is found in our data. In this case the
X-ray dips prior to ejections could be seen as the time during which the source
accumulates energy and material that is ejected later.Comment: 17 pages, 14 figures. Accepted for publication in ApJ, scheduled for
the March 20, 2008, vol676 issue. Table 3 has been degrade
SN1993J VLBI (I): The Center of the Explosion and a Limit on Anisotropic Expansion
Phase-referenced VLBI observations of supernova 1993J at 24 epochs, from 50
days after shock breakout to the present, allowed us to determine the
coordinates of the explosion center relative to the quasi-stationary core of
the host galaxy M81 with an accuracy of 45 micro-arcsec, and to determine the
nominal proper motion of the geometric center of the radio shell with an
accuracy of 9micro-arcsec/yr. The uncertainties correspond to 160 AU for the
position and 160 km/s for the proper motion at the distance of the source of
3.63 Mpc. After correcting for the expected galactic proper motion of the
supernova around the core of M81 using HI rotation curves, we obtain a peculiar
proper motion of the radio shell center of only 320 +/- 160 km/s to the south,
which limits any possible one-sided expansion of the shell. We also find that
the shell is highly circular, the outer contours in fact being circular to
within 3%. Combining our proper motion values with the degree of circular
symmetry, we find that the expansion of the shockfront from the explosion
center is isotropic to within 5.5% in the plane of the sky. This is a more
fundamental result on isotropic expansion than can be derived from the
circularity of the images alone. The brightness of the radio shell, however,
varies along the ridge and systematically changes with time. The degree of
isotropy in the expansion of the shockfront contrasts with the asymmetries and
polarization found in optical spectral lines. Asymmetric density distributions
in the ejecta or more likely in the circumstellar medium, are favored to
reconcile the radio and optical results. We see no sign of any disk-like
density distribution of the circumstellar material, with the average axis ratio
of the radio shell of SN1993J being less than 1.04.Comment: 21 pages, LaTex + 5 Figures (encapulsated PostScript), Accepted for
Publication in the Astrophysical Journa
Discovery of Radio Outbursts in the Active Nucleus of M81
The low-luminosity active galactic nucleus of M81 has been monitored at
centimeter wavelengths since early 1993 as a by-product of radio programs to
study the radio emission from Supernova 1993J. The extensive data sets reveal
that the nucleus experienced several radio outbursts during the monitoring
period. At 2 and 3.6 cm, the main outburst occurred roughly in the beginning of
1993 September and lasted for approximately three months; at longer
wavelengths, the maximum flux density decreases, and the onset of the burst is
delayed. These characteristics qualitatively resemble the standard model for
adiabatically expanding radio sources, although certain discrepancies between
the observations and the theoretical predictions suggest that the model is too
simplistic. In addition to the large-amplitude, prolonged variations, we also
detected milder changes in the flux density at 3.6 cm and possibly at 6 cm on
short (less than 1 day) timescales. We discuss a possible association between
the radio activity and an optical flare observed during the period that the
nucleus was monitored at radio wavelengths.Comment: To appear in The Astronomical Journal. Latex, 18 pages including
embedded figures and table
X-ray variability during the quiescent state of the neutron-star X-ray transient in the globular cluster NGC 6440
The globular cluster NGC 6440 is known to harbor a bright neutron-star X-ray
transient. We observed the globular cluster with Chandra on two occasions when
the bright transient was in its quiescent state in July 2000 and June 2003
(both observations were made nearly 2 years after the end of their preceding
outbursts). The quiescent spectrum during the first observation is well
represented by a two component model (a neutron-star atmosphere model plus a
power-law component which dominates at energies above 2 keV). During the second
observation (which was roughly of equal duration to the first observation) we
found that the power-law component could no longer be detected. Our spectral
fits indicate that the effective temperature of the neutron-star surface was
consistent between the two observations. We conclude that the effect of the
change in power-law component caused the 0.5-10 keV flux to be a factor of ~2
lower during the second observation compared to the first observation. We
discuss plausible explanations for the variations, including variable residual
accretion onto the neutron star magnetosphere or some variation in the
interaction of the pulsar wind with the matter still outflowing from the
companion star.Comment: 18 pages, 3 color figs, 1 b&w figures, 3 tables; discussion expanded;
accepted for publication in Ap
A One-sided, Highly Relativistic Jet from Cygnus X-3
Very Long Baseline Array images of the X-ray binary, Cygnus X-3, were
obtained 2, 4 and 7 days after the peak of a 10 Jy flare on 4 February 1997.
The first two images show a curved one-sided jet, the third a scatter-broadened
disc, presumably at the position of the core. The jet curvature changes from
the first to the second epoch, which strongly suggests a precessing jet. The
ratio of the flux density in the approaching to that in the (undetected)
receding jet is > 330; if this asymmetry is due to Doppler boosting, the
implied jet speed is > 0.81c. Precessing jet model fits, together with the
assumptions that the jet is intrinsically symmetric and was ejected during or
after the major flare, yield the following constraints: the jet inclination to
the line of sight must be < 14 degrees; the cone opening angle must be < 12
degrees; and the precession period must be > 60 days.Comment: 12 pages 7 figures, accepted by Ap
The Unusual X-ray Binaries of the Globular Cluster NGC 6652
Our 5 ks Chandra ACIS-S observation of the globular cluster NGC 6652 detected
7 X-ray sources, 3 of which are previously unidentified. This cluster hosts a
well-known bright low-mass X-ray binary, source A (or XB 1832-330). Source B
shows unusual rapid flaring variability, with an average L_X(0.5-10 keV)
~2*10^{34} ergs/s, but with minutes-long flares up to L_X=9*10^{34} ergs/s. Its
spectrum can be fit by an absorbed power-law of photon index Gamma~1.24, and
hardens as the countrate decreases. This suggests that part or all of the
variation might be due to obscuration by the rim of a highly inclined accretion
disk. Sources C and D, with L_X ~10^{33} ergs/s, have soft and unusual spectra.
Source C requires a very soft component, with a spectrum peaking at 0.5 keV,
which might be the hot polar cap of a magnetically accreting polar cataclysmic
variable.
Source D shows a soft spectrum (fit by a power-law of photon index ~2.3) with
marginal evidence for an emission line around 1 keV; its nature is unclear. The
faint new sources E, F, and G have luminosities of 1-2*10^{32} ergs/s, if
associated with the cluster (which is likely). E and F have relatively hard
spectra (consistent with power-laws with photon index ~1.5). G lacks soft
photons, suggesting absorption with N_H>10^{22} cm^{-2}.Comment: ApJ, in press. 9 pages, 9 figures (3 in color
Water vapour at high redshift: Arecibo monitoring of the megamaser in MG J0414+0534
The study of water masers at cosmological distances would allow us to
investigate the parsec-scale environment around powerful radio sources, to
probe the physical conditions of the molecular gas in the inner parsecs of
quasars, and to estimate their nuclear engine masses in the early universe. To
derive this information, the nature of the maser source, jet or disk-maser,
needs to be assessed through a detailed investigation of the observational
characteristics of the line emission. We monitored the maser line in the lensed
quasar MGJ0414+0534 at z = 2.64 with the 300-m Arecibo telescope for ~15 months
to detect possible additional maser components and to measure a potential
velocity drift of the lines. In addition, we follow the maser and continuum
emissions to reveal significant variations in their flux density and to
determine correlation or time-lag, if any, between them. The main maser line
profile is complex and can be resolved into a number of broad features with
line widths of 30-160 km/s. A new maser component was tentatively detected in
October 2008 that is redshifted by 470 km/s w.r.t the systemic velocity of the
quasar. The line width of the main maser feature increased by a factor of two
between the Effelsberg and EVLA observations reported by Impellizzeri et al.
(2008) and the first epoch of the Arecibo monitoring campaign. After correcting
for the lens magnification, we find that the total H2O isotropic luminosity of
the maser in MGJ0414+0534 is now ~30,000 Lsun, making this source the most
luminous ever discovered.[Abridged]Comment: 8 pages, 6 figures, accepted for publication in A&
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