209 research outputs found
Spectral Comparison of Weak Short Bursts to the Persistent X-rays from the Magnetar 1E 1547.0-5408 in its 2009 Outburst
In January 2009, the 2.1-sec anomalous X-ray pulsar 1E 1547.0-5408 evoked
intense burst activity. A follow-up Suzaku observation on January 28 recorded
enhanced persistent emission both in soft and hard X-rays (Enoto et al. 2010b).
Through re-analysis of the same Suzaku data, 18 short bursts were identified in
the X-ray events recorded by the Hard X-ray Detector (HXD) and the X-ray
Imaging Spectrometer (XIS). Their spectral peaks appear in the HXD-PIN band,
and their 10-70 keV X-ray fluences range from ~2e-9 erg cm-2 to 1e-7 erg cm-2.
Thus, the 18 events define a significantly weaker burst sample than was ever
obtained, ~1e-8-1e-4 erg cm-2. In the ~0.8 to ~300 keV band, the spectra of the
three brightest bursts can be represented successfully by a two-blackbody
model, or a few alternative ones. A spectrum constructed by stacking 13 weaker
short bursts with fluences in the range (0.2-2)e-8 erg s-1 is less curved, and
its ratio to the persistent emission spectrum becomes constant at ~170 above ~8
keV. As a result, the two-blackbody model was able to reproduce the stacked
weaker-burst spectrum only after adding a power-law model, of which the photon
index is fixed at 1.54 as measured is the persistent spectrum. These results
imply a possibility that the spectrum composition employing an optically-thick
component and a hard power-law component can describe wide-band spectra of both
the persistent and weak-burst emissions, despite a difference of their fluxes
by two orders of magnitude. Based on the spectral similarity, a possible
connection between the unresolved short bursts and the persistent emission is
discussed.Comment: 21 pages, 18 figures and 3 tables. Accepted for publication in
Monthly Notices of the Royal Astronomical Society Main Journa
The Suzaku Discovery of A Hard Power-Law Component in the Spectra of Short Bursts from SGR 0501+4516
Using data with the Suzaku XIS and HXD, spectral studies of short bursts from
the soft gamma repeater SGR 0501+4516 were performed. In total, 32 bursts were
detected during the ~60 ks of observation conducted in the 2008 August
activity. Excluding the strongest one, the remaining 31 bursts showed an
average 2--40 keV fluence of 1.0(-0.5,+0.3)*10^-9 erg cm^-2. A 1--40 keV
spectrum summed over them leaves significant positive residuals in the HXD-PIN
band with chi^2/d.o.f. = 74/50, when fitted with a two-blackbody function. By
adding a power law model, the fit became acceptable with chi^2/d.o.f. = 56/48,
yielding a photon index of Gamma=1.0(-0.3,+0.4). This photon index is
comparable to Gamma=1.33(-0.16,+0.23) (Enoto et al. 2010a) for the persistent
emission of the same object obtained with Suzaku. The two-blackbody components
showed very similar ratios, both in the temperature and the emission radii, to
those comprising the persistent emission. However, the power-law to
two-blackbody flux ratio was possibly higher than that of the persistent
emission at 2.6 sigma level. Based on these measurements, average wide-band
properties of these relatively weak bursts are compared with those of the
persistent emission.Comment: 11 pages, 8 figures, 2 tables, Accepted for publication in PASJ
(Suzaku & MAXI special issue
Soft and Hard X-Ray Emissions from the Anomalous X-ray Pulsar 4U 0142+61 Observed with Suzaku
The anomalous X-ray pulsar 4U 0142+61 was observed with Suzaku on 2007 August
15 for a net exposure of -100 ks, and was detected in a 0.4 to ~70 keV energy
band. The intrinsic pulse period was determined as 8.68878 \pm 0.00005 s, in
agreement with an extrapolation from previous measurements. The broadband
Suzaku spectra enabled a first simultaneous and accurate measurement of the
soft and hard components of this object by a single satellite. The former can
be reproduced by two blackbodies, or slightly better by a resonant cyclotron
scattering model. The hard component can be approximated by a power-law of
photon index \Gamma h ~0.9 when the soft component is represented by the
resonant cyclotron scattering model, and its high-energy cutoff is constrained
as >180 keV. Assuming an isotropic emission at a distance of 3.6 kpc, the
unabsorbed 1-10 keV and 10-70 keV luminosities of the soft and hard components
are calculated as 2.8e+35 erg s^{-1} and 6.8e+34 erg s^{-1}, respectively.
Their sum becomes ~10^3 times as large as the estimated spin-down luminosity.
On a time scale of 30 ks, the hard component exhibited evidence of variations
either in its normalization or pulse shape.Comment: 24 pages, 8 figures, accepted for publication in Publications of the
Astronomical Society of Japa
Further Evidence for the ~ 9 s Pulsation in LS 5039 from NuSTAR and ASCA
The present study aims to reinforce the evidence for the ~9 s pulsation in
the gamma-ray binary LS 5039, derived with a Suzaku observation in 2007 and
that with NuSTAR in 2016 (Yoneda et al 2000). Through a reanalysis of the
NuSTAR data incorporating the orbital Doppler correction, the 9.0538 s
pulsation was confirmed successfully even in the 3--10 keV range, where it was
undetectable previously. This was attained by perceiving an energy-dependent
drift in the pulse phase below 10 keV, and correcting the pulse timing of
individual photons for that effect. Similarly, an archival 0.7--12 keV data set
of LS 5039, taken with the ASCA GIS in 1999 October, was analyzed. The data
showed possible periodicity at about 8.882 s, but again the energy-dependent
phase drift was noticed below 10 keV. By correcting for this effect, and for
the orbital Doppler delays in the LS 5039 system, the 2.8--12 keV periodicity
became statistically significant at 8.891+- 0.001 s. The periods measured with
ASCA, Suzaku, and NuSTAR approximately follow an average period derivative of
dP/dt = 3.0 e-10 s/s. These results provide further evidence for the pulsation
in this object, and strengthen the scenario by (Yoneda et al 2000), that the
compact object in LS 5039 is a strongly magnetized neutron star.Comment: 20 pages, 16 figures, accepted for publication in The Astrophysical
Journa
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