arXiv subjects

Long Ji

Publications and source records attributed to Long Ji.

98 records · Page 6Linked to original sources

arXiv2018

Spectral and timing analysis of the bursting pulsar GRO J1744-28 with RXTE observations

We analyzed RXTE/PCA observations of the bursting pulsar GRO J1744-28 during its two outbursts in 1995-1997. We have found a significant transition, i.e., a sharp change, of both spectral and timing properties around a flux of $\rm 10^{-8}\ ergs\ cm^{-2}\ s^{-1}$ (3-30 keV), which corresponds to a luminosity of 2--8$\rm \times 10^{37}\ ergs\ s^{-1}$ for a distance of 4--8 kpc. We define the faint (bright) state when the flux is smaller (larger) than this threshold. In the faint state, the spectral hardness increases significantly with the increasing flux, while remaining almost constant in the bright state. In addition, we find that the pulsed fraction is positively related to both the flux and the energy (<30keV) in both states. Thanks to the very stable pulse profile shape in all energy bands, a hard X-ray lag could be measured. This lag is only significant in the faint state (reaching $\sim$ 20\,ms between 3-4 keV and 16-20 keV), and much smaller in the bright state ($\lesssim$2ms). We speculate that the discovered transition might be caused by changes of the accretion structure on the surface of the neutron star, and the hard X-ray lags are due to a geometry effect.

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arXiv2016

Search for transitions between states in redbacks and black widows using seven years of Fermi-LAT observations

Considering about seven years of Fermi-Large Area Telescope (LAT) data, we present a systematic search for variability possibly related to transitions between states in redbacks and black widow systems. Transitions are characterized by sudden and significant changes in the gamma-ray flux that persist on a timescale much larger than the orbital period. This phenomenology was already detected in the case of two redback systems, PSR J1023+0038 and PSR J1227-4853, for which we present here a dedicated study. We show the existence of only one transition for each of these systems over the past seven years. We determine their spectra, establishing high-energy cutoffs at a few GeV for the high gamma-ray state of PSR J1023+0038 and for both states of PSR J1227-4853. The surveying capability of the Fermi-LAT allows studying whether similar phenomenology has occurred in other sources. Although we have not found any hint for a state transition for most of the studied pulsars, we note two black-widow systems, PSR J2234+0944 and PSR J1446-4701, whose apparent variability is reminiscent of the transitions in PSR J1023+0038 and PSR J1227-4853. For the other systems we set limits on potential transitions in their measured gamma-ray light curves.

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arXiv2015

Diagnosing the burst influence upon accretion in the clocked burster GS 1826--238

Type-I X-ray bursts on the surface of a neutron star are a unique probe to the accretion in X-ray binary systems. However, we know little about the feedback of the burst emission upon accretion. Hard X-ray shortages and enhancements of the persistent emission at soft X-rays have been observed. To put these findings in context with the aim of understanding the possible mechanism underneath, we investigated 68 bursts seen by RXTE from the clocked burster GS 1826--238. We diagnosed jointly the burst influence at both soft and hard X-rays, and found that the observations can be described as the CompTT model with variable normalization, electron temperature and optical depth. Putting these results in a scenario of coronal Compton cooling via the burst emission would lead to a shortage of the cooling power, which may suggest that additional consideration like the influence of the burst on the corona formation should be accounted for as well.

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arXiv2015

The short bursts in SGR 1806-20, 1E 1048-5937 and SGR 0501+4516

We analyzed temporal and spectral properties, focusing on the short bursts, for three anomalous X-ray pulsars (AXPs) and soft gamma repeaters (SGRs), including SGR 1806-20, 1E 1048-5937 and SGR 0501+4516. Using the data from XMM-Newton, we located the short bursts by Bayesian blocks algorithm. The short bursts' duration distributions for three sources were fitted by two lognormal functions. The spectra of shorter bursts ($< 0.2~\rm s$) and longer bursts ($\geq 0.2~\rm s$) can be well fitted in two blackbody components model or optically thin thermal bremsstrahlung model for SGR 0501+4516. We also found that there is a positive correlation between the burst luminosity and the persistent luminosity with a power law index $γ= 1.23 \pm 0.18 $. The energy ratio of this persistent emission to the time averaged short bursts is in the range of $10 - 10^3$, being comparable to the case in Type I X-ray burst.

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arXiv2014

A state-dependent influence of the type-I bursts on the accretion in 4U 1608--52?

We investigated the possible feedback of type-I burst to the accretion process during the spectral evolution of the atoll source 4U 1608--52. By fitting the burst spectrum with a blackbody and an adjustable, persistent spectral component, we found that the latter is significant state-dependent. In the banana state the persistent flux increases along the burst evolution, while in the island state this trend holds only when the bursts are less luminous and starts to reverse at higher burst luminosities. We speculate that, by taking into account both the Poynting-Robertson drag and radiation pressure, these phenomena may arise from the interactions between the radiation field of the type-I burst and the inner region of the accretion disc.

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arXiv2014

Possible hard X-ray shortages in bursts from KS 1731-260 and 4U 1705-44

Aims: A hard X-ray shortage, implying the cooling of the corona, was observed during bursts of IGR J17473-272, 4U 1636-536, Aql X-1, and GS 1826-238. Apart from these four sources, we investigate here an atoll sample, in which the number of bursts for each source is larger than 5, to explore the possible additional hard X-ray shortage during {\it Rossi X-ray timing explorer (RXTE)} era. Methods: According to the source catalog that shows type-I bursts, we analyzed all the available pointing observations of these sources carried out by the {\it RXTE} proportional counter array (PCA). We grouped and combined the bursts according to their outburst states and searched for the possible hard X-ray shortage while bursting. Results: We found that the island states of KS 1731-260 and 4U 1705-44 show a hard X-ray shortage at significant levels of 4.5 and 4.7 $σ$ and a systematic time lag of $0.9 \pm 2.1$ s and $2.5 \pm 2.0$ s with respect to the soft X-rays, respectively. While in their banana branches and other sources, we did not find any consistent shortage.

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arXiv2013

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 the hard X-ray lightcurves 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 lightcurves of type-I bursts, we find a shortage in the 40--50 keV band, delayed by 4.5$\pm$1.4 s with respect to the soft X-rays. The photospheric radius expansion (PRE) bursts are different in that neither a shortage nor an excess shows up in the hard X-ray lightcurve.

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arXiv2013

X-ray bursts as a probe of the corona: the case of XRB 4U 1636-536

To investigate the possible cooling of the corona by soft X-rays bursts, we have studied 114 bursts embedded in the known X-ray evolution of 4U 1636-536. We have grouped these bursts according to the ratio of the flux in the 1.5--12 keV band with respect to that in the 15--50 keV band, as monitored by RXTE/ASM and Swift/BAT, respectively. We have detected a shortage at hard X-rays while bursting. This provides hints for a corona cooling process driven by soft X-rays fed by the bursts that occurred on the surface of neutron star. The flux shortage at 30--50 keV has a time lag of 2.4$\pm$1.5 seconds with respect to that at 2--10 keV, which is comparable to that of 0.7$\pm$0.5 seconds reported in bursts of IGR 17473-2721. We comment on the possible origin of these phenomena and on the implications for the models on the location of the corona.

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