56 research outputs found
EXO 2030+375 Restarts in Reverse
The Be X-ray binary pulsar EXO 2030+375, first detected in 1985, has shown a
significant detected X-ray outburst at nearly every periastron passage of its
46-day orbit for the past ~25 years, with one low state accompanied by a torque
reversal in the 1990s. In early 2015 the outbursts progressively became fainter
and less regular while the monotonic spin-up flattened. At the same time a
decrease in the H line equivalent width was reported, indicating a
change in the disk surrounding the mass donor.
In order to explore the source behaviour in the poorly explored low-flux
state with a possible transition to a state of centrifugal inhibition of
accretion we have undertaken an observing campaign with Swift/XRT, NuSTAR and
the Nordic Optical Telescope (NOT). This conference contribution reports the
preliminary results obtained from our campaign.Comment: 11th INTEGRAL Conference Gamma-Ray Astrophysics in Multi-Wavelength
Perspective, 10-14 October 2016, Amsterdam, The Netherlands. 7 page
Discovery of spin-phase dependent QPOs in the super-critical accretion regime from the X-ray pulsar RX J0440.9+4431
RX J0440.9+4431 is an accreting X-ray pulsar (XRP) that remained relatively
unexplored until recently, when major X-ray outburst activity enabled more
in-depth studies. Here, we report on the discovery of Hz
quasi-periodic oscillations (QPOs) from this source observed with -GBM.
The appearance of QPOs in RX J0440.9+4431 is thricely transient, that is, QPOs
appear only above a certain luminosity, only at certain pulse phases (namely
corresponding to the peak of its sine-like pulse profile), and only for a few
oscillations at time. We argue that this newly discovered phenomenon
(appearance of thricely transient QPOs -- or ATTO) occurs if QPOs are fed
through an accretion disk whose inner region viscosity is unstable to mass
accretion rate and temperature variations. Such variations are triggered when
the source switches to the super-critical accretion regime and the emission
pattern changes. We also argue that the emission region configuration is likely
responsible for the observed QPOs spin-phase dependence.Comment: 5 + 2 appendix pages. Accepted on A&A. Proofs versio
MoonBEAM: A Beyond Earth-Orbit Gamma-Ray Burst Detector for Multi-Messenger Astronomy
Moon Burst Energetics All-sky Monitor (MoonBEAM) is a CubeSat concept of deploying gamma-ray detectors in cislunar space to increase gamma-ray burst detections and improve localization precision with the timing triangulation technique. A gamma-ray instrument in cislunar orbit will have greatly reduced sky blockage compared to instruments in low Earth orbit. Working in conjunction with another instrument in low Earth orbit, MoonBEAM can also help constrain the arrival direction of the wavefront to an annulus on the sky by utilizing the light arrival times between the different orbits. This method has been demonstrated by the Interplanetary Gamma- Ray Burst Timing Network. However, delays in data downlink for instruments outside the Tracking and Data Relay Satellite network prevent rapid follow-up observations. We present here a gamma-ray CubeSat concept in Earth-Moon L3 halo orbit that is capable of faster response and provide a timing baseline for localization improvement. Such an instrument would aid in the gravitational wave follow-up observations in other wavelengths to identify the gamma-ray burst afterglow and kilonova emission. Reducing the region of interest makes identifying afterglows much faster, allowing for rapid on-source observations and monitoring of the rise and decay times. It will also prevent source confusion between two transients and enable robust association. A gamma-ray detection could also increase the confidence of a simultaneous but marginal gravitational wave signal, extending the detection horizon. MoonBEAM is a 12U CubeSat concept of deploying gamma-ray detectors in cislunar space to increase gamma-ray burst detections and improve localization precision with the timing triangulation technique. Such an instrument would probe the extreme processes in cosmic collision of compact objects and facilitate multi-messenger time-domain astronomy to explore the end of stellar life cycles and black hole formations
Synchrotron Cooling in Energetic Gamma-Ray Bursts Observed by the Fermi Gamma-Ray Burst Monitor
We study the time-resolved spectra of eight GRBs observed by Fermi GBM in its
first five years of mission, with 1 keV - 1 MeV fluence
erg cm and signal-to-noise level above 900 keV. We
aim to constrain in detail the spectral properties of GRB prompt emission on a
time-resolved basis and to discuss the theoretical implications of the fitting
results in the context of various prompt emission models. We perform
time-resolved spectral analysis using a variable temporal binning technique
according to optimal S/N criteria, resulting in a total of 299 time-resolved
spectra. We fit the Band function to all spectra and obtain the distributions
for the low-energy power-law index , the high-energy power-law index
, the peak energy in the observed spectrum , and
the difference between the low- and high-energy power-law indices . Using the distributions of and , the
electron population index is found to be consistent with the "moderately
fast" scenario which fast- and slow-cooling scenarios cannot be distinguished.
We also apply a physically motivated synchrotron model, which is a triple
power-law with constrained power-law indices and a blackbody component, to test
for consistency with a synchrotron origin for the prompt emission and obtain
the distributions for the two break energies and ,
the middle segment power-law index , and the Planck function temperature
. A synchrotron model is found consistent with the majority of
time-resolved spectra for these eight energetic Fermi GBM bursts with good
high-energy photon statistics, as long as both the cooling and injection break
are included and the leftmost spectral slope is lifted either by inclusion of a
thermal component or when an evolving magnetic field is accounted for.Comment: 20 pages, 7 figures, 8 tables, accepted for publication in A&
Three years of Fermi GBM Earth Occultation Monitoring: Observations of Hard X-ray/Soft Gamma-Ray Sources
The Gamma ray Burst Monitor (GBM) on board Fermi has been providing
continuous data to the astronomical community since 2008 August 12. In this
paper we present the results of the analysis of the first three years of these
continuous data using the Earth occultation technique to monitor a catalog of
209 sources. From this catalog, we detect 99 sources, including 40 low-mass
X-ray binary/neutron star systems, 31 high-mass X-ray binary neutron star
systems, 12 black hole binaries, 12 active galaxies, 2 other sources, plus the
Crab Nebula, and the Sun. Nine of these sources are detected in the 100-300 keV
band, including seven black-hole binaries, the active galaxy Cen A, and the
Crab. The Crab and Cyg X-1 are also detected in the 300-500 keV band. GBM
provides complementary data to other sky-monitors below 100 keV and is the only
all-sky monitor above 100 keV. Up-to-date light curves for all of the catalog
sources can be found at http://heastro.phys.lsu.edu/gbm/.Comment: 24 pages, 12 figures, accepted for publication in ApJ
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