56 research outputs found
Recent activity of the Be/X-ray binary system SAX J2103.5+4545
Aims. We present a multiwavelength study of the Be/X-ray binary system SAX
J2103.5+4545 with the goal of better characterizing the transient behaviour of
this source.
Methods. SAX J2103.5+4545 was observed by Swift-XRT four times in 2007 from
April 25 to May 5, and during quiescence in 2012 August 31. In addition, this
source has been monitored from the ground-based astronomical observatories of
El Teide (Tenerife, Spain), Roque de los Muchachos (La Palma, Spain) and Sierra
Nevada (Granada, Spain) since 2011 August, and from the TUBITAK National
Observatory (Antalya, Turkey) since 2009 June. We have performed spectral and
photometric temporal analyses in order to investigate the different states
exhibited by SAX J2103.5+4545.
Results. In X-rays, an absorbed power law model provided the best fit for all
the XRT spectra. An iron-line feature at ~6.42 keV was present in all the
observations except for that taken during quiescence in 2012. The photon
indexes are consistent with previous studies of SAX J2103.5+4545 in high/low
luminosity states. Pulsations were found in all the XRT data from 2007
(2.839(2) mHz; MJD 54222.02), but not during quiescence. Both optical outbursts
in 2010 and 2012 lasted for about 8/9 months (as the one in 2007 probably did
and the current one in 2014 might do) and were most probably caused by mass
ejection events from the Be star that eventually fed the circumstellar disc.
All of these outbursts started about 3 months before the triggering of the
X-ray activity, and about the same period before the maximum of the H_alpha
line equivalent width (in emission) was reached at only ~ -5 \AA. In this work
we found that the global correlation between the BV variability and the X-ray
intensity was also observed at longer wavelengths in the IR domain.Comment: 11 pages, 7 figures, and online material (2 tables). Submitted to A&A
in 2014 Januar
The Fermi –Gbm Three-Year X-Ray Burst Catalog
The Fermi Gamma Ray Burst Monitor (GBM) is an all sky gamma-ray monitor well known in the gamma-ray burst community. Although GBM excels in detecting the hard, bright extragalactic GRBs, its sensitivity above 8 keV and all-sky view make it an excellent instrument for the detection of rare, short-lived Galactic transients. In March 2010, we initiated a systematic search for transients using GBM data. We conclude this phase of the search by presenting a 3 year catalog of 1084 X-ray bursts. Using spectral analysis, location and spatial distributions we classified the 1084 events into 752 thermonuclear X-ray bursts, 267 transient events from accretion flares and X-ray pulses, and 65 untriggered gamma-ray bursts. All thermonuclear bursts have peak blackbody temperatures broadly consistent with photospheric radius expanison (PRE) bursts. We find an average rate of 1.4 PRE bursts per day, integrated over all Galactic bursters within about 10 kpc. These include 33 and 10 bursts from the ultra-compact X-ray binaries 4U~0614+09 and 2S~0918-549, respectively. We discuss these recurrence times and estimate the total mass ejected by PRE bursts in our Galaxy
Evidence of a Change in the Long Term Spin-down Rate of the X-ray Pulsar 4U 1907+09
We analyzed RXTE archival observations of 4U 1907+09 between 17 February 1996
and 6 March 2002. The pulse timing analysis showed that the source stayed at
almost {\bf{constant}} period around August 1998 and then started to spin-down
at a rate of Hz s which is 0.60
times lower than the long term ( years) spin-down rate (Baykal et al.
2001). Our pulse frequency measurements for the first time resolved significant
spin-down rate variations since the discovery of the source. We also presented
orbital phase resolved X-ray spectra during two stable spin down episodes
during November 1996 - December 1997 and March 2001 - March 2002. The source
has been known to have two orbitally locked flares. We found that X-ray flux
and spectral parameters except Hydrogen column density agreed with each other
during the flares.We interpreted the similar values of X-ray fluxes as an
indication of the fact that the source accretes not only via transient
retrograde accretion disc (in't Zand et al. 1998) but also via the stellar wind
of the companion (Roberts et al. 2001), so that the variation of the accretion
rate from the disc does not cause significant variation in the observed X-ray
flux. Lack of significant change in spectral parameters except Hydrogen column
density was interpreted as a sign of the fact that the change in the spin-down
rate of the source was not accompanied by a significant variation in the
accretion geometry.Comment: Revised version. Accepted for publication in MNRA
Recent RXTE/ASM and ROTSEIIId Observations of EXO 2030+375
Using the archival RXTE/ASM and SWIFT/BAT observations, the new orbital
phases of Type I outbursts of EXO 2030+375 are estimated. A possible
correlation between the Type II outburst and optical brightness variations is
investigated. In order to estimate the phases of Type I outbursts, we fitted
Gaussian profiles to the RXTE/ASM and SWIFT/BAT light curves. The time
corresponding to the maximum value of the profiles is treated as the arrival
time of Type I outburst. We used differential magnitudes in the time-series
analysis of the optical light curve. MIDAS and its suitable packages were used
to reduce and analyze the spectra. Prior to the Type II outburst, orbital
phases of Type I outbursts were delayed for 6 days after the periastron
passage, which is consistent with findings of Wilson et al., (2002, 2005).
After the giant Type II outburst, the phase of Type I outbursts underwent a
sudden shift of 13 days after the periastron passage. The amplitudes of Type I
outbursts were increased between MJD 52500 and 53500. These amplitudes then
decreased for 10 orbital cycles until the Type II outburst was triggered. If
the change of outburst amplitudes correlated with the mass accretion, then
during the decrease of these amplitudes mass should be deposited in a disk
around neutron star temporarily. The release of this stored mass may ignite the
Type II outburst. We report that the optical light curve became fainter by 0.4
mag during the decrease of amplitude of the Type I outbursts. The observed
H profiles and their equivalent widths during the decay and after the
giant outburst are consistent with previous observations of the system.Comment: to appear at Astronomy and Astrophysic
New Torque Reversal and Spin-Up of 4u 1626- 67 Observed by Fermi/GBM and Swift/BAT
After about 18 years of steadily spinning down, the accretion-powered pulsar
4U 1626-67, experienced a torque reversal at the beginning of 2008. For the
present study we have used all available Fermi/GBM data since its launch in
2008 June 11 and over 5 yr of hard X-ray Swift/BAT observations (starting from
2004 October up to the present time). This second detected torque reversal is
centered near MJD 54500 (2008 Feb 4) and it lasts approximately 150 days. From
2004 up to the end of 2007 4U 1626-67 the spin-down rate decreased at a mean
rate of ~ -5.5E-13 Hz s-1 until the source reversed torque again. Since then it
has been following a steady spin-up at a mean rate of ~ 5E-13 Hz s-1. In
addition, 4U 1626-67 increased its flux simultaneously (a ~2.5 factor). We
present detailed long-term timing analysis of this source and a long term
spectral hardness ratio study in order to see whether there are spectral
changes around this new observed torque reversal.Comment: 12 pages, 9 figure
First Results from Fermi GBM Earth Occultation Monitoring: Observations of Soft Gamma-Ray Sources Above 100 keV
The NaI and BGO detectors on the Gamma-ray Burst Monitor (GBM) on Fermi are
now being used for long-term monitoring of the hard X-ray/low energy gamma-ray
sky. Using the Earth occultation technique as demonstrated previously by the
BATSE instrument on the Compton Gamma-Ray Observatory, GBM can be used to
produce multiband light curves and spectra for known sources and transient
outbursts in the 8 keV to 1 MeV energy range with its NaI detectors and up to
40 MeV with its BGO detectors. Over 85% of the sky is viewed every orbit, and
the precession of the Fermi orbit allows the entire sky to be viewed every ~26
days with sensitivity exceeding that of BATSE at energies below ~25 keV and
above ~1.5 MeV. We briefly describe the technique and present preliminary
results using the NaI detectors after the first two years of observations at
energies above 100 keV. Eight sources are detected with a significance greater
than 7 sigma: the Crab, Cyg X-1, SWIFT J1753.5-0127, 1E 1740-29, Cen A, GRS
1915+105, and the transient sources XTE J1752-223 and GX 339-4. Two of the
sources, the Crab and Cyg X-1, have also been detected above 300 keV.Comment: 13 pages, 9 figures, submitted to Ap
THE FERMI-GBM X-RAY BURST MONITOR: THERMONUCLEAR BURSTS FROM 4U 0614+09
Thermonuclear bursts from slowly accreting neutron stars (NSs) have proven difficult to detect, yet they are potential probes of the thermal properties of the NS interior. During the first year of a systematic all-sky search for X-ray bursts using the Gamma-ray Burst Monitor aboard the Fermi Gamma-ray Space Telescope we have detected 15 thermonuclear bursts from the NS low-mass X-ray binary 4U 0614+09 when it was accreting at nearly 1% of the Eddington limit. We measured an average burst recurrence time of 12 ± 3 days (68% confidence interval) between 2010 March and 2011 March, classified all bursts as normal duration bursts and placed a lower limit on the recurrence time of long/intermediate bursts of 62 days (95% confidence level). We discuss how observations of thermonuclear bursts in the hard X-ray band compare to pointed soft X-ray observations and quantify such bandpass effects on measurements of burst radiated energy and duration. We put our results for 4U 0614+09 in the context of other bursters and briefly discuss the constraints on ignition models. Interestingly, we find that the burst energies in 4U 0614+09 are on average between those of normal duration bursts and those measured in long/intermediate bursts. Such a continuous distribution in burst energy provides a new observational link between normal and long/intermediate bursts. We suggest that the apparent bimodal distribution that defined normal and long/intermediate duration bursts during the last decade could be due to an observational bias toward detecting only the longest and most energetic bursts from slowly accreting NSs.United States. National Aeronautics and Space Administration (Fermi Guest Investigation Program Grant NNX11AO19G
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