We carry out a detailed study of the spectral and the timing properties of the stellar-mass black hole candidate XTE J1752-223 during its 2009-10 outburst using RXTE PCA data in the $2.5-25$ keV energy range. Low frequency quasi-periodic oscillations (LFQPOs) are seen in the power density spectrum (PDS). The spectral analysis is done using two types of models: one is the combined disk black body plus power-law model and the other is Transonic flow solution based Two Component Advective Flow (TCAF) model. RXTE PCA was non-operational during 2009 Nov. 16 to 2010 Jan. 18 and thus we study light curve profiles and evolution of hardness ratios using MAXI GSC and Swift BAT data. Based on the evolution of the temporal and the spectral properties of the source during its 2009-10 outburst, we find that the object evolved through the following spectral states: hard, hard-intermediate and soft-intermediate/soft. From the TCAF model fitted spectral analysis, we also estimate the probable mass of the black hole to be in the range of $8.1-11.9$ $M_\odot$, and more precisely, the mass appears to be is $10\pm{1.9}~M_\odot$.
We study the X-ray emission of the microquasar XTE J1550-564 and analyze the properties of its light curves using the recurrence analysis method. The indicators for non-linear dynamics of the accretion flow are found in the very high state and soft state of this source. The significance of deterministic variability depends on the energy band. We discuss the non-linear dynamics of the accretion flow in the context of the disc-corona geometry and propagating oscillations in the accretion flow.
We study the timing properties of XTE J1858+034 using the Nuclear Spectroscopic Telescope Array (NuSTAR) and Burst Alert Telescope onboard Swift during the outburst in October--November 2019. We have investigated for Quasi-Periodic Oscillation (QPO) during the outburst and detected a low-frequency QPO at $\sim$196 mHz with $\sim$6% RMS variability from the NuSTAR observation. The QPO is fitted and explained with the model - power law and a Lorentzian component. We have also studied the variation of QPO frequency with energy. The beat frequency model and Keplerian frequency model both are suita…
We report on new X-ray outbursts observed with Swift from three Supergiant Fast X-ray Transients (SFXTs): XTEJ1739-302, IGRJ17544-2619 and IGRJ08408-4503. The former two outbursts were caught during the monitoring campaign we have been performing with the Swift satellite since October 2007: XTEJ1739-302 underwent a new outburst on 2008, August 13, IGRJ17544-2619 on 2008, September 4, while IGRJ08408-4503 on 2008, September 21. While XTEJ1739-302 and IGRJ08408-4503 bright emission triggered the Swift/Burst Alert Telescope, IGRJ17544-2619 did not, thus we could perform a spectral investigation o…
We study spectral and temporal properties of Galactic short orbital period transient black hole XTE J1118+480 during its 2005 outburst using archival data of RXTE PCA and HEXTE instruments in the combined energy range of $3-100$ keV. Spectral analysis with the physical two-component advective flow (TCAF) model allows us to understand the accretion flow properties of the source. We found that this outburst of XTE J1118+480 is an unconventional outburst as the source was only in the hard state (HS). Our spectral analysis suggests that during the entire outburst, the source was highly dominated b…
We present the timing and spectral studies of the Be/X-ray binary XTE J1946+274 during its 2018 and 2021 giant outbursts using observations with the SXT and the LAXPC instruments on the AstroSat satellite. Unlike the 1998 and 2010 outbursts, where a giant outburst was followed by several low intensity periodic outbursts, the 2018 and 2021 outbursts were single outbursts. The X-ray pulsations are detected over a broad energy band covering 0.5-80 keV from the compact object. We construct the spin evolution history of the pulsar over two decades and find that the pulsar spins-up during the outbur…
XTE J1810-189 underwent an outburst in 2008, and was observed over $\sim 100$ d by RXTE. Performing a time-resolved spectral analysis on the photospheric radius expansion burst detected on 2008 May 4, we obtain the source distance in the range of 3.5--8.7 kpc for the first time. During its outburst, XTE J1810-189 did not enter into the high/soft state, and both the soft and hard colours decreased with decreasing flux. The fractional rms remained at high values ($\sim 30$ per cent). The RXTE/PCA spectra for 3-25 keV can be described by an absorbed power-law component with an additional Gaussian…
After 15 years, in late 2018, the magnetar XTE J1810-197 underwent a second recorded X-ray outburst event and reactivated as a radio pulsar. We initiated an X-ray monitoring campaign to follow the timing and spectral evolution of the magnetar as its flux decays using Swift, XMM-Newton, NuSTAR, and NICER observations. During the year-long campaign, the magnetar reproduced similar behaviour to that found for the first outburst, with a factor of two change in its spin-down rate from $\sim7.2\times10^{-12}$ s s$^{-1}$ to $\sim1.5\times10^{-11}$ s s$^{-1}$ after two months. Unique to this outburst,…
We revisit the discovery outburst of the X-ray transient XTE J1550-564 during which relativistic jets were observed in 1998 September, and review the radio images obtained with the Australian Long Baseline Array, and lightcurves obtained with the Molonglo Observatory Synthesis Telescope and the Australia Telescope Compact Array. Based on HI spectra, we constrain the source distance to between 3.3 and 4.9 kpc. The radio images, taken some two days apart, show the evolution of an ejection event. The apparent separation velocity of the two outermost ejecta is at least 1.3c and may be as large as …
The RXTE and Swift observations on the 2009 outburst from a newly discovered transient and black-hole candidate XTE J1652$-$453 are analyzed. The source was observed by RXTE to behave a sequence of spectral states that are typical to the black hole XRBs. During the first 7 observations, the source is diagnosed to stay in a high/soft state, from the spectrum dominated by soft thermal component which contributes an average of $\sim$ 85% to the X-ray flux at 2-20keV, and from the hardness $\sim$ 0.1 showing up in the hardness-intensity diagram (HID). For the last 20 observations the spectral stat…
X-ray time lags are complicated in nature. The exact reasons for complex lag spectra are yet to be known. However, the hard lags, in general, are believed to be originated due to the inverse Comptonization process. But, the origin of soft lags remained mischievous. Recent studies on "Disk-Jet Connections" revealed that the jets are also contributing to the X-ray spectral and timing properties in a magnitude which was more than what was predicted earlier. In this article, we first show an exact anti-correlation between X-ray time lag and radio flux for XTE J1550-546 during its 1998 outburst. We…
We present the earliest available soft X-ray observations of XTE J1810-187, the prototypical transient magnetar, obtained 75--84 days after its 2018 outburst with the Neutron Star Interior Composition Explorer (NICER). Using a series of observations covering eight days we find that its decreasing X-ray flux is well-described by either a blackbody plus power-law or a two-blackbody spectral model. The 2-10 keV flux of the source varied from (1.206+/-0.007)x10^{-10} to (1.125+/-0.004)x10^{-10} erg s^{-1} cm^{-2}, a decrease of about 7% within our observations and 44% from that measured 7-14 days …
Evolution of variability properties of Galactic transient sources is a diagnostic tool to understand various regimes of the accretion flow and its dynamics close to the central black hole. In this paper, we concentrate on the variability properties of the X-ray transient XTE J1650-500 and investigate the evolution of viscous delay, time lag, QPO frequency, and their energy dependence throughout the rising phase as observed by RXTE during its outburst in 2001. Our analysis reveals (1) a delay of 12 \pm 1 days between one day averaged hard (5-12 keV) and soft (1.5-3 keV) photon light-curves as o…
We present Swift broadband observations of the recently discovered black hole candidate, X-ray transient, XTE J1752-223, obtained over the period of outburst from October 2009 to June 2010. From Swift-UVOT data we confirm the presence of an optical counterpart which displays variability correlated, in the soft state, to the X-ray emission observed by Swift-XRT. The optical counterpart also displays hysteretical behaviour between the states not normally observed in the optical bands, suggesting a possible contribution from a synchrotron emitting jet to the optical emission in the rising hard st…
XTE J1908+094 is an X-ray transient black hole candidate in the Galactic plane that was observed in outburst in 2002 and 2013. Here we present multi-frequency radio and X-ray data, including radio polarimetry, spanning the entire period of the 2013 outburst. We find that the X-ray behaviour of XTE J1908+094 traces the standard black hole hardness-intensity path, evolving from a hard state, through a soft state, before returning to a hard state and quiescence. Its radio behaviour is typical of a compact jet that becomes quenched before discrete ejecta are launched during the late stages of X-ra…
While the sources of X-ray and radio emission in the different states of low-mass X-ray binaries are relatively well understood, the origin of the near-infrared (NIR) and optical emission is more often debated. It is likely that the NIR/optical flux originates from an amalgam of different emission regions, because it occurs at the intersecting wavelengths of multiple processes. We aim to identify the NIR/optical emission region(s) of one such low-mass X-ray binary and black hole candidate, XTE J1650-500, via photometric, timing, and spectral analyses. We present unique NIR/optical images and s…
Galactic black hole candidate (BHC) XTE~J1118+480 during its 2000 outburst has been studied in a broad energy range using the archival data of PCA and HEXTE payloads of {\it Rossi X-ray Timing Explorer}. Detailed spectral and temporal properties of the source are studied. Low and very low frequency quasi-periodic oscillations (QPOs), with a general trend of increasing frequency are observed during the outburst. Spectral analysis is done using the combined data of PCA and HEXTE instruments with two types of models: the well known phenomenological power-law model and the current version of the {…
We report on a series of outbursts of the high mass X-ray binary XTE J1946+274 in 2010/2011 as observed with INTEGRAL, RXTE, and Swift. We discuss possible mechanisms resulting in the extraordinary outburst behavior of this source. The X-ray spectra can be described by standard phenomenological models, enhanced by an absorption feature of unknown origin at about 10 keV and a narrow iron K alpha fluorescence line at 6.4 keV, which are variable in flux and pulse phase. We find possible evidence for the presence of a cyclotron resonance scattering feature at about 25 keV at the 93% level. The pre…
Here we summarise the Swift broadband observations of the recently discovered X-ray transient and black hole candidate, XTE J1752-223,obtained over the period of outburst from October 2009 to June 2010. We offer a phenomenological treatment of the spectra as an indication of the canonical spectral state of the source during different periods of the outburst. We find that the high energy hardness-intensity diagrams over two separate bands follows the canonical behavior, confirming the spectral states. From Swift-UVOT data we confirm the presence of an optical counterpart which displays variabil…
A number of low mass X-ray binaries (LMXBs) undergo "failed outbursts" in which, instead of evolving through the canonical states, they remain in a hard state throughout the outburst. While the sources of X-ray and radio emission in the hard state are relatively well understood, the origin of the near infrared (NIR) and optical emission is more complex though it likely stems from an amalgam of different emission processes, occurring as it does, at the intersecting wavelengths of those processes. We aim to identify the NIR/optical emission region(s) during a number of failed outbursts of one su…
XTE J1810-197 is the first transient Anomalous X-ray Pulsar ever discovered. Its highly variable X-ray flux allowed us to study the timing and spectral emission properties of a magnetar candidate over a flux range of about two orders of magnitude. We analyzed nine XMM-Newton observations of XTE J1810-197 collected over a four years baseline (September 2003 - September 2007). EPIC PN and MOS data were reduced and used for detailed timing and spectral analysis. Pulse phase spectroscopic studies were also carried out for observations with sufficiently high signal to noise. We find that: (i) a thr…
X-ray irradiation in X-ray binaries is thought to control the behavior at the outer disc, which is observable mainly at optical wavelengths. It is generally parameterized phenomenologically, but it can also be predicted from theoretical models of irradiated discs and their coronae/winds. We test these models using five multi-wavelength Hubble Space Telescope (HST) and quasi simultaneous Rossi X-Ray Timing Explorer (RXTE) datasets from the black-hole binary XTE J1859$+$226. These show how the reprocessed optical emission changes during outburst as the source fades from the very high/intermediat…
We present radio coverage of the 2019 outburst of the accreting millisecond X-ray pulsar SAX J1808.4-3658, obtained with MeerKAT. We compare these data to contemporaneous X-ray and optical measurements in order to investigate the coupling between accretion and jet formation in this system, while the optical lightcurve provides greater detail of the outburst. The reflaring activity following the main outburst peak was associated with a radio re-brightening, indicating a strengthening of the jet in this phase of the outburst. We place quasi-simultaneous radio and X-ray measurements on the global…
We examine the dynamical behavior of accretion flow around XTE J1859+226 during the 1999 outburst by analyzing the entire outburst data ($\sim$ 166 days) from RXTE Satellite. Towards this, we study the hysteresis behavior in the hardness intensity diagram (HID) based on the broadband ($3 - 150$ keV) spectral modeling, spectral signature of jet ejection and the evolution of Quasi-periodic Oscillation (QPO) frequencies using the two-component advective flow model around a black hole. We compute the flow parameters, namely Keplerian accretion rate (${\dot m}_d$), sub-Keplerian accretion rate (${\…
We report results of an extensive analysis of the X-ray nova XTE J1859+226 observed with BeppoSAX and RXTE during its 1999 outburst. We modelled the source spectrum with a multicolour blackbody-like feature plus the generic Comptonization model BMC which has the advantage of providing spectral description of the emitted-radiation properties without assumptions on the underlying physical process. The multicolour component is attributed to the geometrically thin accretion disk, while the Comptonization spectrum is claimed to originate in the innermost sub-Keplerian region of the system (transiti…
The black hole candidate XTE J1908+094 went into outburst for the first time since 2003 in October 2013. We report on an observation with the Nuclear Spectroscopic Telescope Array (NuSTAR) and monitoring observations with Swift during the outburst. NuSTAR caught the source in the soft state: the spectra show a broad relativistic iron line, and the light curves reveal a ~40 ks flare with the count rate peaking about 40% above the non-flare level and with significant spectral variation. A model combining a multi-temperature thermal component, a power-law, and a reflection component with an iron …
Galactic short orbital period black hole candidate (BHC) XTE~J1752-223 was discovered on 2009 Oct 21 by the Rossi X-ray Timing Explorer (RXTE). We study the spectral properties of this outburst using transonic flow solution based two component advective flow (TCAF) model. TCAF model fitted spectrum gives an estimation of the physical flow parameters, such as the Keplerian disk rate, sub-Keplerian halo rate, properties of the so-called {\it{Compton cloud}}, other than the mass of the source and normalization ($N$). $N$ is a standardized ratio of emitted to observed photon flux in TCAF which doe…
We re-investigated the `spectro-temporal' behaviour of the source XTE J1859+226 in X-rays during its outburst phase in 1999, by analysing the RXTE PCA/HEXTE data in 2 - 150 keV spectral band. Detailed analysis shows that although the evolution pattern of the outburst followed the typical q-shaped profile, an absence of `canonical' soft state and a weak presence of `secondary' emission are observed. The broad-band spectra, modeled with high energy cutoff, shows that the fold-energy increases monotonically in the hard and hard-intermediate states followed by a random variation in the soft-interm…
Galactic black hole transients show many interesting phenomena during outburst decays. We present simultaneous X-ray (RXTE, Swift, and INTEGRAL), and optical/near-infrared (O/NIR) observations (SMARTS) of the X-ray transient XTE J1752-223 during its outburst decay in 2010. The multiwavelength observations over 150 days in 2010 cover the transition from soft to hard spectral state. We discuss the evolution of radio emission is with respect to the O/NIR light curve which shows several flares. One of those flares is bright and long, starting about 60 days after the transition in X-ray timing prop…
We present the earliest X-ray observations of the 2018 outburst of XTE J1810-197, the first outburst since its 2003 discovery as the prototypical transient and radio-emitting anomalous X-ray pulsar (AXP). The Monitor of All-sky X-ray Image (MAXI) detected XTE J1810-197 immediately after a November 20-26 visibility gap, contemporaneous with its reactivation as a radio pulsar, first observed on December 8. On December 13 the Nuclear Spectroscopic Telescope Array (NUSTAR) detected X-ray emission up to at least 30 keV, with a spectrum well-characterized by a blackbody plus power-law model with tem…
It is now established that thermal disc emission and non-thermal jet emission can both play a role at optical/infrared (OIR) wavelengths in X-ray transients. The spectra of the jet and disc components differ, as do their dependence on mass accretion properties. Here we demonstrate that the OIR colour-magnitude diagrams (CMDs) of the evolution of the X-ray transient XTE J1550-564 in outburst can be used to separate the disc from the jet. Monitoring in two wavebands is all that is required. This outburst in 2000 was well studied, and both disc and jet were known to contribute. During the outburs…
We present optical, X-ray and radio observations of the black hole transient (BHT) XTE J1752-223 towards and in quiescence. Optical photometry shows that the quiescent magnitude of XTE J1752-223 is fainter than 24.4 magnitudes in the i'-band. A comparison with measurements of the source during its 2009-2010 outburst shows that the outburst amplitude is more than 8 magnitudes in the i'-band. Known X-ray properties of the source combined with the faintness of the quiescence optical counterpart and the large outburst optical amplitude point towards a short orbital period system (Porb<~6.8 h) with…
Context. Of the 13 known accreting millisecond pulsars, only a few showed more than one outburst during the RXTE era. XTE J1751-305 showed, after the main outburst in 2002, other three dim outbursts. We report on the timing analysis of the latest one, occurred on October 8, 2009 and serendipitously observed from its very beginning by RXTE. Aims. The detection of the pulsation during more than one outburst permits to obtain a better constraint of the orbital parameters and their evolution as well as to track the secular spin frequency evolution of the source. Methods. Using the RXTE data of the…
Timing analysis of ~12.4 Ms of INTEGRAL/IBIS data has revealed a period of 51.47 +/- 0.02 days in the supergiant fast X-ray transient source XTE J1739-302/IGR J17391-3021 that can be interpreted as an orbital period. An outburst history showing 35 epochs of activity has been produced, showing X-ray outbursts throughout the orbit of XTE J1739-302. Possible indications of an enhanced equatorial density region within the supergiant stellar wind are present in the phase-folded lightcurve. It is found that many orbital configurations are possible within this system with eccentricities of up to e ~0…
The black hole X-ray transient, XTE J1118+480, has now twice been observed in outburst - 2000 and 2005 - and on both occasions remained in the low/hard X-ray spectral state. Here we present radio, infrared, optical, soft X-ray and hard X-ray observations of the more recent outburst. We find that the lightcurves have very different morphologies compared with the 2000 event and the optical decay is delayed relative to the X-ray/radio. We attribute this lesser degree of correlation to contributions of emission from multiple components, in particular the jet and accretion disc. Whereas the jet see…
XTE J1810-197 (PSR J1809-1943) was the first ever magnetar which was found to emit transient radio emission. It has recently undergone another radio and high-energy outburst. This is only the second radio outburst that has been observed from this source. We observed J1810-197 soon after its recent radio outburst at low radio frequencies using the Giant Metrewave Radio Telescope. We present the 650 MHz flux density evolution of the source in the early phases of the outburst, and its radio spectrum down to frequencies as low as 300 MHz. The magnetar also exhibits radio emission in the form of st…
The black hole candidate, XTE J1752-223, was discovered in 2009 October when it entered an outburst. We obtained radio data from the Australia Telescope Compact Array for the duration of the ~9 month event. The lightcurves show that the radio emission from the compact jet persisted for the duration of an extended hard state and through the transition to the intermediate state. The flux then rose rapidly by a factor of 10 and the radio source entered a series of at least 7 maxima, the first of which was likely to be emission associated with the compact jet. The subsequent 6 flares were accompan…
XTE J1818-245 is an X-ray nova that experienced an outburst in 2005, first seen by the RXTE satellite. The source was observed simultaneously at various wavelengths up to soft gamma-rays with the INTEGRAL satellite, from 2005 February to September. X-ray novae are extreme systems that often harbor a black hole, and are known to emit throughout the electromagnetic spectrum when in outburst. We analyzed radio, (N)IR, optical, X-ray and soft gamma-ray observations and constructed simultaneous broad-band X-ray spectra. Analyzing both the light curves in various energy ranges and the hardness-inten…
Transient LMXBs that host neutron stars (NSs) provide excellent laboratories for probing the dense matter physics present in NS crusts. During accretion outbursts in LMXBs, exothermic reactions may heat the NS crust, disrupting the crust-core equilibrium. When the outburst ceases, the crust cools to restore thermal equilibrium with the core. Monitoring this evolution allows us to probe the dense matter physics in the crust. Properties of the deeper crustal layers can be probed at later times after the end of the outburst. We report on the unexpected late-time temperature evolution (>2000 days …
We study the properties of the faint X-ray activity of Galactic transient black hole candidate XTE~J1908+094 during its 2019 outburst. Here, we report the results of detailed spectral and temporal analysis during this outburst using observations from {\it Nuclear Spectroscopic Telescope Array (NuSTAR)}. We have not observed any quasi-periodic-oscillations (QPOs) in the power density spectrum (PDS). The spectral study suggests that the source remained in the softer (more precisely in soft-intermediate) spectral state during this short period of the X-ray activity. We notice a faint but broad Fe…
We present the results of two years of radio and X-ray monitoring of the magnetar XTE J1810$-$197 since the radio re-activation in late 2018. Single pulse analysis of radio observations from the Lovell and MkII telescopes at 1564 MHz and the Effelsberg telescope at 6 GHz has resulted in the detection of a total of 91 giant pulses (GPs) between MJDs 58858 and 59117. These GPs appear to be confined to two specific phase ranges (0.473 <= ϕ<= 0.502$ and 0.541 <= ϕ<= 0.567). We also observe that the first detection of GP emission corresponds to a minimum in the spin-down rate. Simultaneous radio an…
We present observations of the 9.8 s X-ray pulsar XTE J1859+083 made with the ASM and PCA on board RXTE, and the WFC on board BeppoSAX. The ASM data cover a 12 year time interval and show that an extended outburst occurred between approximately MJD 50,250 and 50,460 (1996 June 16 to 1997 January 12). The ASM data excluding this outburst interval suggest a possible modulation with a period of 60.65 +/- 0.08 days. Eighteen sets of PCA observations were obtained over an approximately one month interval in 1999. The flux variability measured with the PCA appears consistent with the possible period…
Low and intermediate frequency quasi-periodic oscillations (QPOs) are thought to be due to oscillations of Comptonizing regions or hot regions embedded in Keplerian discs. Observational evidence of evolutions of QPOs would therefore be very important as they throw lights on the dynamics of the hotter region. Our aim is to find systems in which there is a well-defined correlation among the frequencies of the QPOs over a range of time so as to understand the physical picture. In this paper, we concentrate on the archival data of XTE J1550-564 obtained during 1998 outburst, and study the systemat…
This is an overview of pulsar accretion modeling. The physics of pulsar accretion, i.e., the process of plasma flow onto the neutron star surface, can be constrained from the spectral properties of the X-ray source. We discuss a new implementation of the physical continuum model developed by Becker and Wolff (2007, ApJ 654, 435). The model incorporates Comptonized blackbody, bremsstrahlung, and cyclotron emission. We discuss preliminary results of applying the new tool to the test cases of Suzaku data of Cen X-3 and XTE J1946+274. Cen X-3 is a persistent accreting pulsar with an O-star compani…
Outbursts of the black hole (BH) X-ray binaries are dramatic events occurring in our Galaxy approximately once a year. They are detected by the X-ray telescopes and often monitored at longer wavelengths. We analyse the X-ray and optical/infrared (OIR) light curves of the BH binary XTE J1550-564 during the 2000 outburst. By using the observed extreme colours as well as the characteristic decay time-scales of the OIR and X-ray light curves, we put strong constraints on the extinction towards the source. We accurately separate the contributions to the OIR flux of the irradiated accretion disc and…
Using the data from the Rossi X-Ray Timing Explorer satellite, we investigate the spectral evolution along a "Z" track and a "v" track on the hardness-intensity diagrams of the first transient Z source XTE J1701-462. The spectral analyses suggest that the inner disk radius depends on the mass accretion rate, in agreement with the model prediction, R_in \propto ((dM/dt)_disk)^{2/7}, for a radiation pressure dominated accretion disk interacting with the magnetosphere of a neutron star (NS). The changes in the disk mass accretion rate (dM/dt)_disk are responsible for the evolution of the "Z" or "…
We present a coherent timing analysis of the 2003 outburst of the accreting millisecond pulsar XTE J1807-294. We find an upper limit for the spin frequency derivative of 5E-14 Hz/s. The sinusoidal fractional amplitudes of the pulsations are the highest observed among the accreting millisecond pulsars and can reach values of up to 27% (2.5-30 keV). The pulse arrival time residuals of the fundamental follow a linear anti-correlation with the fractional amplitudes that suggests hot spot motion over the surface of the neutron star both in longitude and latitude. An anti-correlation between residua…
We study the temporal and energy spectral properties of the unique neutron star low-mass X-ray binary XTE J1701-462. After assuming the HB/NB vertex as a reference position of accretion rate, the horizontal branch oscillation (HBO) of the HB/NB vertex is roughly 50 Hz. It indicates that the HBO is independent with the accretion rate or the source intensity. The spectral analysis shows $R_{\rm{in}}\propto\dot{M}_{\rm{Disk}}^{2.9\pm0.09}$ in the HB/NB vertex and $R_{\rm{in}}\propto\dot{M}_{\rm{Disk}}^{1.7\pm0.06}$ in the NB/FB vertex, which implies that different accretion rate may be produced i…
We have analyzed XMM-Newton and Chandra observations of the transient magnetar XTE J1810-197, spanning more than 11 years, from the initial phases of the 2003 outburst to the current quiescent level. We investigated the evolution of the pulsar spin period and we found evidence for two distinct regimes: during the outburst decay, the spin derivative (nu_dot) was highly variable in the range -(2-4.5)x10^-13 Hz/s, while during quiescence the spin-down rate was more stable at an average value of -1x10^{-13} Hz/s. Only during ~3000 days (from MJD 54165 to MJD 56908) in the quiescent stage, it was p…
The X-ray transient XTE J1719-291 was discovered with RXTE/PCA during its outburst in 2008 March, which lasted at least 46 days. Its 2-10 keV peak luminosity is 7E35 erg/s assuming a distance of 8 kpc, which classifies the system as a very faint X-ray transient. The outburst was monitored with Swift, RXTE, Chandra and XMM-Newton. We analysed the X-ray spectral evolution during the outburst. We fitted the overall data with a simple power-law model corrected for absorption and found that the spectrum softened with decreasing luminosity. However, the XMM-Newton spectrum can not be fitted with a s…
We present near-infrared and optical imaging of fifteen candidate black hole X-ray binaries. In addition to quiescent observations for all sources, we also observed two of these sources (IGR J17451-3022 and XTE J1818-245) in outburst. We detect the quiescent counterpart for twelve out of fifteen sources, and for the remaining three we report limiting magnitudes. The magnitudes of the detected counterparts range between $K_s$ = 17.59 and $K_s$ = 22.29 mag. We provide (limits on) the absolute magnitudes and finding charts of all sources. Of these twelve detections in quiescence, seven represent …
With Rossi X-ray Timing Explorer (RXTE) data, we systematically study the cross-correlation and time lag in all spectral states of black hole X-ray binary (BHXB) XTE J1859+226 in detail during its entire 1999-2000 outburst that lasted for 166 days. Anti-correlations and positive correlations and their respective soft and hard X-ray lags are only detected in the first 100 days of the outburst when the luminosity is high. This suggests that the cross-correlations may be related to high luminosity. Positive correlations are detected in every state of XTE J1859+226, viz., hard state, hard-intermed…
Accreting X-ray pulsars (XRPs) undergo luminous X-ray outbursts during which the spectral and timing behavior of the neutron star can be studied in detail. We analyze a $NuSTAR$ observation of the XRP XTE J1858+034 during its outburst in 2019. The spectrum is fit with a phenomenological, a semi-empirical and a physical spectral model. A candidate cyclotron line is found at $48\,$keV, implying a magnetic field of $5.4\times10^{\rm 12}\,$G at the site of emission. This is also supported by the physical best-fit model. We propose an orbital period of about $81$ days based on the visual inspection…
We observed the new X-ray transient and black-hole candidate XTE J1652-453 simultaneously with XMM-Newton and the Rossi X-ray Timing Explorer (RXTE). The observation was done during the decay of the 2009 outburst, when XTE J1652-453 was in the hard-intermediate state. The spectrum shows a strong and broad iron emission line with an equivalent width of ~ 450 eV. The profile is consistent with that of a line being produced by reflection off the accretion disk, broadened by relativistic effects close to the black hole. The best-fitting inner radius of the accretion disk is ~ 4 gravitational radii…
