348 research outputs found
GBM Observations of V404 Cyg During its 2015 Outburst
V404 Cygni was discovered in 1989 by the X-ray satellite during its
only previously observed X-ray outburst and soon after confirmed as a black
hole binary. On June 15, 2015, the Gamma Ray Burst Monitor (GBM) triggered on a
new outburst of V404 Cygni. We present 13 days of GBM observations of this
outburst including Earth occultation flux measurements, spectral and temporal
analysis. The Earth occultation fluxes reached 30 Crab with detected emission
to 100 keV and determined, via hardness ratios, that the source was in a hard
state. At high luminosity, spectral analysis between 8 and 300 keV showed that
the electron temperature decreased with increasing luminosity. This is expected
if the protons and electrons are in thermal equilibrium during an outburst with
the electrons cooled by the Compton scattering of softer seed photons from the
disk. However, the implied seed photon temperatures are unusually high,
suggesting a contribution from another source, such as the jet. No evidence of
state transitions is seen during this time period. The temporal analysis
reveals power spectra that can be modeled with two or three strong, broad
Lorentzians, similar to the power spectra of black hole binaries in their hard
state
Earth Occultation Imaging of the Low Energy Gamma-Ray Sky with GBM
The Earth Occultation Technique (EOT) has been applied to Fermi's Gamma-ray
Burst Monitor (GBM) to perform all-sky monitoring for a predetermined catalog
of hard X-ray/soft gamma-ray sources. In order to search for sources not in the
catalog, thus completing the catalog and reducing a source of systematic error
in EOT, an imaging method has been developed -- Imaging with a Differential
filter using the Earth Occultation Method (IDEOM). IDEOM is a tomographic
imaging method that takes advantage of the orbital precession of the Fermi
satellite. Using IDEOM, all-sky reconstructions have been generated for ~sim 4
years of GBM data in the 12-50 keV, 50-100 keV and 100-300 keV energy bands in
search of sources otherwise unmodeled by the GBM occultation analysis. IDEOM
analysis resulted in the detection of 57 sources in the 12-50 keV energy band,
23 sources in the 50-100 keV energy band, and 7 sources in the 100-300 keV
energy band. Seventeen sources were not present in the original GBM-EOT catalog
and have now been added. We also present the first joined averaged spectra for
four persistent sources detected by GBM using EOT and by the Large Area
Telescope (LAT) on Fermi: NGC 1275, 3C 273, Cen A, and the Crab
A 0535+26: an X-ray/Optical Tour
We compiled X-ray and Optical observations of the accreting X-ray binary
system A 0535+262 since its discovery in 1975, that will allow us to shed light
on the unpredictable behavior of this binary system. We present the data in
terms of the Be-disk interaction with the neutron star companion. In addition,
we show recent results from the continuous monitoring of this source by the
Gamma-ray Burst Monitor(GBM), on board the Fermi observatory, since its launch
in 2008 June 11.Comment: 4 pages, 5 figures, Conference proceeding of "Astrophysics of Neutron
Stars 2010 - a conference in honor of M. Ali Alpar", 2-6 August 2010, Cesme,
Izmir, Turke
Studying the accretion geometry of EXO 2030+375 at luminosities close to the propeller regime
The Be X-ray binary EXO 2030+375 was in an extended low luminosity state
during most of 2016. We observed this state with NuSTAR and Swift, supported by
INTEGRAL observations as well as optical spectroscopy with the NOT. We present
a comprehensive spectral and timing analysis of these data here to study the
accretion geometry and investigate a possible onset of the propeller effect.
The H-alpha data show that the circumstellar disk of the Be-star is still
present. We measure equivalent widths similar to values found during more
active phases in the past, indicating that the low-luminosity state is not
simply triggered by a smaller Be disk. The NuSTAR data, taken at a 3-78 keV
luminosity of ~6.8e35 erg/s (for a distance of 7.1 kpc), are well described by
standard accreting pulsar models, such as an absorbed power-law with a
high-energy cutoff. We find that pulsations are still clearly visible at these
luminosities, indicating that accretion is continuing despite the very low mass
transfer rate. In phase-resolved spectroscopy we find a peculiar variation of
the photon index from ~1.5 to ~2.5 over only about 3% of the rotational period.
This variation is similar to that observed with XMM-Newton at much higher
luminosities. It may be connected to the accretion column passing through our
line of sight. With Swift/XRT we observe luminosities as low as 1e34 erg/s
during which the data quality did not allow us to search for pulsations, but
the spectrum is much softer and well described by either a blackbody or soft
power-law continuum. This softer spectrum might be due to the fact that
accretion has been stopped by the propeller effect and we only observe the
neutron star surface cooling.Comment: 11 pages, 6 figures, accepted for publication in A&A (v2 including
language edits
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