Yupeng Chen

LONG-TERM MONITORING OF THE HIGH-ENERGY γ -RAY EMISSION FROM LS I +61 ◦ 303 AND LS 5039

The Fermi Large Area Telescope (LAT) reported the first definitive GeV detections of the binaries LS I +61°303 and LS 5039 in the first year after its launch in 2008 June. These detections were unambiguous as a consequence of the reduced positional uncertainty and the detection of modulated γ-ray emission on the corresponding orbital periods. An analysis of new data from the LAT, comprising 30 months of observations, identifies a change in the γ-ray behavior of LS I +61°303. An increase in flux is detected in 2009 March and a steady decline in the orbital flux modulation is observed. Significant emission up to 30 GeV is detected by the LAT; prior data sets led to upper limits only. Contemporaneous TeV observations no longer detected the source, or found it—in one orbit—close to periastron, far from the phases at which the source previously appeared at TeV energies. The detailed numerical simulations and models that exist within the literature do not predict or explain many of these features now observed at GeV and TeV energies. New ideas and models are needed to fully explain and understand this behavior. A detailed phase-resolved analysis of the spectral characterization of LS I +61°303 in the GeV regime ascribes a power law with an exponential cutoff spectrum along each analyzed portion of the systems orbit. The on-source exposure of LS 5039 is also substantially increased with respect to our prior publication. In this case, whereas the general γ-ray properties remain consistent, the increased statistics of the current data set allows for a deeper investigation of its orbital and spectral evolution.

A MAGNETAR-LIKE EVENT FROM LS I +61°303 AND ITS NATURE AS A GAMMA-RAY BINARY

We report on the Swift Burst Alert Telescope detection of a short burst from the direction of the TeV binary LS I + 61 degrees 303, resembling those generally labeled as magnetar-like. We show that it is likely that the short burst was indeed originating from LS I + 61 degrees 303 (although we cannot totally exclude the improbable presence of a far-away, line-of-sight magnetar) and that it is a different phenomenon with respect to the previously observed ks-long flares from this system. Accepting the hypothesis that LS I + 61 degrees 303 is the first magnetar detected in a binary system, we study those implications. We find that a magnetar-composed LS I + 61 degrees 303 system would most likely be (i.e., for the usual magnetar parameters and mass-loss rate) subject to a flip-flop behavior, from a rotationally powered regime (in the apastron) to a propeller regime (in the periastron) along each of the LS I + 61 degrees 303 eccentric orbital motion. We prove that, unlike near an apastron, where an interwind shock can lead to the normally observed LS I + 61 degrees 303 behavior, during TeV emission the periastron propeller is expected to efficiently accelerate particles only to sub-TeV energies. This flip-flop scenario would explain the systems behavior when a recurrent TeV emission only appears near the apastron, the anti-correlation of the GeV and TeV emission, and the long-term TeV variability (which seems correlated to LS I + 61 degrees 303s super-orbital period), including the appearance of a low TeV state. Finally, we qualitatively put the multi-wavelength phenomenology into the context of our proposed model and make some predictions for further testing.

Long-term X-Ray Monitoring of LS I +61°303: Analysis of Spectral Variability and Flares

We report on the full analysis of a Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA) monitoring of the {\gamma}-ray binary system LS I +61 303. The data set covers 42 contiguous cycles of the system orbital motion. Analyzing this X-ray monitoring dataset, the largest to date for this source, we report on the variability of the orbital profile and the spectral distribution, and provide strong evidence for an anti-correlation between flux and spectral index (the higher the flux, the harder the spectral index). Furthermore, we present the analysis of two newly discovered ks-timescale flares, which present significant variability also on shorter timescales, and tend to occur at orbital phases between 0.6-0.9. However, a detailed timing analysis of the flares does not show any coherent or quasi-coherent (QPO) structure in their power spectra. We also investigated the possible appearance of the radio super-orbital modulation at X-rays energies, but we could not unambiguously detect such modulation in the system flux history, nor in the evolution of its orbital modulation fraction.

Unveiling the Super-orbital Modulation of LS I +61°303 in X-Rays

We found evidence for the super-orbital modulation in the X-ray emission of LS I +61 303 from the longest monitoring date by the RXTE. The time evolution of the modulated fraction in the orbital light curves can be well fitted with a sinusoidal function having a super-orbital period of 1667 days. However, we have found a 281.8+/-44.6 day shift between the super-orbital variability found at radio frequencies and our X-ray data. We also find a super-orbital modulation in the maximum count rate of the orbital light curves, compatible with the former results, including the shift.

The hard X-ray shortages prompted by the clock bursts in GS 1826-238

We report on a study of GS 1826-238 using all available Rossi X-Ray Timing Explorer observations, concentrating on the behavior of the hard X-rays during type-I bursts. We find a hard X-ray shortage at 30-50 keV prompted by the shower of soft X-rays coming from type-I bursts. This shortage happens with a time delay after the peak of the soft flux of 3.6 ± 1.2 s. The behavior of hard X-rays during bursts indicates cooling and reheating of the corona, during which a large amount of energy is required. We speculate that this energy originates from the feedback of the type-I bursts to the accretion process, resulting in a rapid temporary increase of the accretion rate.

TYPE-I X-RAY BURSTS REVEAL A FAST CO-EVOLVING BEHAVIOR OF THE CORONA IN AN X-RAY BINARY

The coronae in X-ray binaries (XRBs) still remain poorly understood, although they have been believed for a long time to play a key role in modeling the characteristic outbursts of XRBs. Type-I X-ray bursts, the thermonuclear flashes happening on the surface of a neutron star (NS), can be used as a probe to the innermost region of an NS XRB, where the corona is believed to be located very close to the NS. We report the discovery of a tiny life cycle of the corona that is promptly co-evolved with the type-I bursts superimposed on the outburst of the NS XRB IGR J17473-2721. This finding may serve as the first evidence of directly seeing the rapid disappearance and formation of a corona in an XRB with a cooling/heating timescale of less than a second, which can strongly constrain the accretion models in XRBs at work.

Long-term monitoring of LS I +61°303 with INTEGRAL

LS I +61 � 303 is one of the few high-mass X-ray binaries that have been recently observed at TeV and GeV energies. Here we investigate the hard X-ray spectral and timing properties of this source using the IBIS/ISGRI instrument on-board the INTEGRAL satellite. We carry out a systematic analysis based on all available INTEGRAL data since December 28, 2002 up to April 30, 2008. The total exposure time analyzed amounts to 2.1 Ms, hence more than doubling the previous reported sample. The source is best detected in the 18-60 keV band, with a significance level of 12.0�. The hard X-ray data are best fit with a simple power law with a photon index of � 1.7� 0.2. We detect a periodical signal at 27 � 4 days, matching the orbital period of 26.496 days previously reported at other wavelengths. The hard X-rays orbital lightcurve is obtained and compared with those derived at other frequencies.

The 2009 outburst of H 1743-322 as observed by RXTE

Context. Six outbursts have been observed by RXTE from the source since 2003. Following “a failed outburst” with a missing high/soft state in 2008, the most recent one occurred in 2009, exhibiting all spectral transitions. Aims. An outburst was observed by RXTE from the black hole candidate H 1743-322 in 2009. We investigate this particular outburst in the context of the source outburst history. Methods. We analyze the RXTE observations of the 2009 outburst of H 1743-322, as well as the observations of the previous five outbursts for comparison. All available RXTE observations taken since 2003 sum up to a total exposure of ∼1200 ks. Results. The hardness-intensity diagram (HID) shows a complete counter-clockwise q-track for the 2009 outburst. It leaves the low/hard state but does not reach the leftmost edge of the overall HID. Near maximum, the X-ray spectra show a power law with photon index <2.4; the peak luminosity during the whole outburst reached only 20% of the Eddington luminosity. The highest disk fractional luminosity is about 0.75 for the 2009 outburst, which is close to the lower limit observed from the black hole XRB sample in their state transitions from the intermediate to the high/soft state. While the lowest hardness (6–19 keV/3–6 keV) values in the HID is about 0.3–0.4 for the 2009 outburst, the timing analysis shows that a transition to the high soft state occurred. During the low/hard state of the 2009 outburst, the inner radius of the accretion disk is found to be closer to the central black hole and anti-correlated with the disk temperature implying reprocessing of the hot corona on the disk’s soft X-rays. In the luminosity diagram of the corona versus the disk, the tracks of the outbursts in 2003 and 2009 cross the line which represents a roughly equal contribution to the emission from the thermal and the non-thermal components; the track of the 2008 outburst has its turn-over falling on this line. This may indicate an emission balance between the corona and the disk that prevents the state transition from going further than the low/hard state.

The last three outbursts of H1743−322 observed by RXTE in its latest service phase

After 2010, three outbursts of H1743-322 were detected by RXTE. We have carried out timing and spectral analysis of the data, emphasizing on the two with relatively complete evolution history presented in the RXTE/Proportional Counter Array observations. We then constitute an enlarged outburst sample for H1743-322 which allows us to investigate the spectral transitions in more details. We find that the spectral transitions to high-soft state constrain a region for four outbursts in hardness-intensity diagram. An extension of the region locates in the vicinity of the failed outburst in 2008, and excludes it from a successful group. We therefore suggest that the failed outburst in 2008 may present the first almost successful outburst sample, which is important for modelling the outburst, especially upon the conditions required for transition to high-soft state.

THE HARD X-RAY BEHAVIOR OF AQL X-1 DURING TYPE-I BURSTS

We report the discovery of an anti-correlation between the soft and hard X-ray light curves of the X-ray binary Aql X-1 when bursting. This behavior may indicate that the corona is cooled by the soft X-ray shower fed by the type-I X-ray bursts, and that this process happens within a few seconds. Stacking the Aql X-1 light curves of type-I bursts, we find a shortage in the 40-50 keV band, delayed by 4.5 ± 1.4 s with respect to the soft X-rays. The photospheric radius expansion bursts are different in that neither a shortage nor an excess shows up in the hard X-ray light curve.