157 research outputs found
Chandra localization of XTE J1906+090 and discovery of its optical and infrared counterparts
We present the Chandra identification and localization of the transient X-ray source XTE J1906+090 and the discovery of its optical and infrared counterparts. Our analysis of archival Chandra ACIS-I observations of the field found the source approximately 8 away from the position determined earlier with the RXTE PCA. We have confirmed the source identification with timing analysis of the X-ray data, which detected the source spin period of 89.6 s. The best Chandra position for the source is R.A. = 19h04m47491, decl. = +09024140. Subsequently, we performed optical observations of the field around the new location and discovered a coincident optical source with R-band magnitude of 18.7. A search in the Two Micron All Sky Survey catalog revealed an infrared point source with J = 15.2, H = 14.2, and K = 13.5, whose location is also coincident with our Chandra and optical positions. Our results add fresh evidence for a Be/X-ray transient nature for XTE J1906+090
RXTE Observations of Soft Gamma Repeater Bursts
The spectra of short soft gamma repeater (SGR) bursts at photon energies above 15 keV are often well described by an optically thin thermal bremsstrahlung model (i.e., F(E) ~ E^−1 exp(−E/kT) ) with kT=20−40 keV. However, the spectral shape burst continuum at lower photon energies (down to 2 keV) is not well established. It is important to better understand the SGR burst spectral properties at lower energies since inadequate description of the burst spectral continuum could lead to incorrect conclusions, such as existence of spectral lines. Here, we present detailed spectral investigations (in 2-200 keV) of 163 bursts from SGR 1806-20, all detected with Rossi X-ray Timing Explorer during the 2004 active episode that included the giant flare on 27 December 2004. We find that the great majority of burst spectra are well represented by the combination of a blackbody plus a OTTB models
A possible magnetar nature for IGR J16358-4726
We present detailed spectral and timing analysis of the hard X-ray transient IGR J16358-4726 using multisatellite archival observations. A study of the source flux time history over 6 yr suggests that lower luminosity transient outbursts can be occurring in intervals of at most 1 yr. Joint spectral fits of the higher luminosity outburst using simultaneous Chandra ACIS and INTEGRAL ISGRI data reveal a spectrum well described by an absorbed power-law model with a high-energy cutoff plus an Fe line. We detected the 1.6 hr pulsations initially reported using Chandra ACIS also in the INTEGRAL ISGRI light curve and in subsequent XMM-Newton observations. Using the INTEGRAL data, we identified a spin-up of 94 s ( = 1.6 × 10-4), which strongly points to a neutron star nature for IGR J16358-4726. Assuming that the spin-up is due to disk accretion, we estimate that the source magnetic field ranges between 1013 and 1015 G, depending on its distance, possibly supporting a magnetar nature for IGR J16358-4726
Chandra Observations of the Anomalous X-ray Pulsar 1E 2259+58.6
We present X-ray imaging, timing, and phase resolved spectroscopy of the anomalous X-ray pulsar 1E 2259+58.6 using the Chandra X-ray Observatory. The spectrum is well described by a power law plus blackbody model with power law index = 3.6(1), kT_BB = 0.412(6) keV, and N_H=0.93(3) x 10^{22} cm^{-2}; we find no evidence for spectral features (0.5-7.0 keV). We derive a new, precise X-ray position for the source and determine its spin period, P=6.978977(24) s. Time resolved X-ray spectra show no significant variation as a function of pulse phase. We have detected excess emission beyond 4 arcsec from the central source extending to beyond 100 arcsec, due to the supernova remnant and possibly dust scattering from the interstellar medium
John Schuster, Descartes-agonistes: Physico-mathematics, method and corpuscular-mechanism, 1618–1633
We report on a 10 ks simultaneous Chandra/HETG-NuSTAR observation of the
Bursting Pulsar, GRO J1744-28, during its third detected outburst since
discovery and after nearly 18 years of quiescence. The source is detected up to
60 keV with an Eddington persistent flux level. Seven bursts, followed by dips,
are seen with Chandra, three of which are also detected with NuSTAR. Timing
analysis reveals a slight increase in the persistent emission pulsed fraction
with energy (from 10% to 15%) up to 10 keV, above which it remains constant.
The 0.5-70 keV spectra of the persistent and dip emission are the same within
errors, and well described by a blackbody (BB), a power-law with an exponential
rolloff, a 10 keV feature, and a 6.7 keV emission feature, all modified by
neutral absorption. Assuming that the BB emission originates in an accretion
disc, we estimate its inner (magnetospheric) radius to be about 4x10^7 cm,
which translates to a surface dipole field B~9x10^10 G. The Chandra/HETG
spectrum resolves the 6.7 keV feature into (quasi-)neutral and highly ionized
Fe XXV and Fe XXVI emission lines. XSTAR modeling shows these lines to also
emanate from a truncated accretion disk. The burst spectra, with a peak flux
more than an order of magnitude higher than Eddington, are well fit with a
power-law with an exponential rolloff and a 10~keV feature, with similar fit
values compared to the persistent and dip spectra. The burst spectra lack a
thermal component and any Fe features. Anisotropic (beamed) burst emission
would explain both the lack of the BB and any Fe components.Comment: 15 pages, 11 figures, Accepted in Ap
Prompt Optical Detection of GRB 050401 with ROTSE-IIIa
The ROTSE-IIIa telescope at Siding Spring Observatory, Australia, detected
prompt optical emission from Swift GRB 050401. In this letter, we present
observations of the early optical afterglow, first detected by the ROTSE-IIIa
telescope 33 s after the start of gamma-ray emission, contemporaneous with the
brightest peak of this emission. This GRB was neither exceptionally long nor
bright. This is the first prompt optical detection of a GRB of typical duration
and luminosity. We find that the early afterglow decay does not deviate
significantly from the power-law decay observable at later times, and is
uncorrelated with the prompt gamma-ray emission. We compare this detection with
the other two GRBs with prompt observations, GRB 990123 and GRB 041219a. All
three bursts exhibit quite different behavior at early times.Comment: 4 pages, 3 figures. Accepted for publication in ApJ Letter
Thermonuclear X-ray Bursts with late secondary peaks observed from 4U 1608-52
We report the temporal and spectral analysis of three thermonuclear X-ray
bursts from 4U 1608-52, observed by the Neutron Star Interior Composition
Explorer (NICER) during and just after the outburst observed from the source in
2020. In two of the X-ray bursts, we detect secondary peaks, 30 and 18 seconds
after the initial peaks. The secondary peaks show a fast rise exponential
decay-like shape resembling a thermonuclear X-ray burst. Time-resolved X-ray
spectral analysis reveals that the peak flux, blackbody temperature, and
apparent emitting radius values of the initial peaks are in agreement with
X-ray bursts previously observed from 4U 1608-52, while the same values for the
secondary peaks tend toward the lower end of the distribution of bursts
observed from this source. The third X-ray burst, which happened during much
lower accretion rates did not show any evidence for a deviation from an
exponential decay and was significantly brighter than the previous bursts. We
present the properties of the secondary peaks and discuss the events within the
framework of short recurrence time bursts or bursts with secondary peaks. We
find that the current observations do not fit in standard scenarios and
challenge our understanding of flame spreading.Comment: Accepted for publication in the Astrophysical Journa
Optical Lightcurve & Cooling Break of GRB 050502A
We present lightcurves of the afterglow of GRB050502A, including very early
data at t-t_{GRB} < 60s. The lightcurve is composed of unfiltered ROTSE-IIIb
optical observations from 44s to 6h post-burst, R-band MDM observations from
1.6 to 8.4h post-burst, and PAIRITEL J H K_s observations from 0.6 to 2.6h
post-burst. The optical lightcurve is fit by a broken power law, where
t^{alpha} steepens from alpha = -1.13 +- 0.02 to alpha = -1.44 +- 0.02 at
\~5700s. This steepening is consistent with the evolution expected for the
passage of the cooling frequency nu_c through the optical band. Even in our
earliest observation at 44s post-burst, there is no evidence that the optical
flux is brighter than a backward extrapolation of the later power law would
suggest. The observed decay indices and spectral index are consistent with
either an ISM or a Wind fireball model, but slightly favor the ISM
interpretation. The expected spectral index in the ISM interpretation is
consistent within 1 sigma with the observed spectral index beta = -0.8 +- 0.1;
the Wind interpretation would imply a slightly (~2 sigma) shallower spectral
index than observed. A small amount of dust extinction at the source redshift
could steepen an intrinsic spectrum sufficiently to account for the observed
value of beta. In this picture, the early optical decay, with the peak at or
below 4.7e14 Hz at 44s, requires very small electron and magnetic energy
partitions from the fireball.Comment: 22 pages, including 3 tables and 1 figure, Accepted by Ap
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