Swift J1644 Research Articles

Understanding the spectral and timing behaviour of a newly discovered transient X-ray pulsar Swift J0243.6+6124

We present the results obtained from timing and spectral studies of the newly discovered accreting X-ray binary pulsar Swift J0243.6+6124 using a NuSTAR observation in 2017 October at a flux level of ~280 mCrab. Pulsations at 9.85423(5) s were detected in the X-ray light curves of the pulsar. Pulse profiles of the pulsar were found to be strongly energy dependent. A broad profile at lower energies was found to evolve into a double peaked profile in $\ge$30keV. The 3-79 keV continuum spectrum of the pulsar was well described with a negative and positive exponential cutoff or high energy cutoff power law models modified with a hot blackbody at $\sim$3 keV. An iron emission line was also detected at 6.4 keV in the source spectrum. We did not find any signature of cyclotron absorption line in our study. Results obtained from phase-resolved and time-resolved spectroscopy are discussed in the paper.

Properties of X-Ray Flux of Jets during the 2005 Outburst of Swift J1753.5−0127 Using the TCAF Solution

Abstract Galactic black hole candidate Swift J1753.5−0127 was discovered on 2005 June 30 by the Swift Burst Alert Telescope. We study the accretion flow properties during its very first outburst through careful analysis of the evolution of the spectral and the temporal properties using the two-component advective flow (TCAF) paradigm. RXTE proportional counter array spectra in are fitted with the current version of the TCAF model fits file to estimate physical flow parameters, such as two-component (Keplerian disk and sub-Keplerian halo) accretion rates, properties of the Compton cloud, probable mass of the source, etc. The source is found to be in harder (hard and hard-intermediate) spectral states during the entire phase of the outburst with very significant jet activity. Since in the TCAF solution the model normalization is constant for any particular source, any requirement of significantly different normalization to have a better fit on certain days would point to an X-ray contribution from components not taken into account in the current TCAF model fits file. By subtracting the contribution using actual normalization, we derive the contribution of X-rays from the jets and outflows. We study its properties, such as its magnitude and spectra. We find that on some days, up to about 32% of X-ray flux is emitted from the base of the jet itself.

Swift J164449.3+573451 and Swift J2058.4+0516: Black hole mass estimates for tidal disruption event sources

A tidal disruption event (TDE) is an astronomical phenomenon in which a previously dormant black hole (BH) destroys a star passing too close to its central part. We analyzed the flaring episode detected from the TDE sources, Swift~J1644+57 and Swift J2058+05 using RXTE, Swift and Suzaku data. The spectra are well fitted by the so called Bulk Motion Comptonization model for which the best-fit photon index Gamma varies from 1.1 to 1.8. We have firmly established the saturation of Gamma versus mass accretion rate at Gamma_{sat} about 1.7 -- 1.8. The saturation of Gamma is usually identified as a signature of a BH now established in Swift~J1644+57 and Swift J2058+05. In Swift~J1644+57 we found the relatively low Gamma_{sat} values which indicate a high electron (plasma) temperature, kT_e ~ 30 -- 40 keV. This is also consistent with high cutoff energies, E_{cut} ~ 60 -- 80 keV found using best fits of the RXTE spectra. Swift~J2058+05 shows a lower electron temperature, kT_e ~ 4-10 keV than that for Swift~J1644+57. For the BH mass estimate we used the scaling technique taking the Galactic BHs, GRO J1655--40, GX~339--4, Cyg~X--1 and 4U~1543--47 as reference sources and found that the BH mass in Swift~J1644+57 is M_{BH}> 7x10^6 solar masses assuming the distance to this of 1.5 Gpc. For Swift J2058+05 we obtain M_{BH}> 2x 10^7 solar masses assuming the distance to this source of 3.7 Gpc.

High-energy cosmic ray nuclei from tidal disruption events: Origin, survival, and implications

Tidal disruption events (TDEs) by supermassive or intermediate mass black holes have been suggested as candidate sources of ultrahigh-energy cosmic rays (UHECRs) and high-energy neutrinos. Motivated by the recent measurements from the Pierre Auger Observatory, which indicates a metal-rich cosmic-ray composition at ultrahigh energies, we investigate the fate of UHECR nuclei loaded in TDE jets. First, we consider the production and survival of UHECR nuclei at internal shocks, external forward and reverse shocks, and nonrelativistic winds. Based on the observations of Swift J1644+57, we show that the UHECRs can survive for external reverse and forward shocks, and disk winds. On the other hand, UHECR nuclei are significantly disintegrated in internal shocks, although they could survive for low-luminosity TDE jets. Assuming that UHECR nuclei can survive, we consider implications of different composition models of TDEs. We find that the tidal disruption of main sequence stars or carbon-oxygen white dwarfs does not successfully reproduce UHECR observations, namely the observed composition or spectrum. The observed mean depth of the shower maximum and its deviation could be explained by oxygen-neon-magnesium white dwarfs, but they may be too rare to be the sources of UHECRs.

Radiative interaction between the relativistic jet and optically thick envelope in tidal disruption events

Reverberation observations have uncovered an Fe K{\alpha} fluorescence line in the tidal disruption event (TDE) Swift J1644+57 (Kara et al. 2016). The discovery paper used the lag spectrum to argue that the X-ray continuum source was located very close to the blackhole (~30 gravitational radii) and moved sub-relativistically. We reanalyze the lag spectrum, pointing out that dilution effects cause it to indicate a geometric scale an order of magnitude larger than previously inferred. If the X-ray continuum is produced by a relativistic jet, as suggested by rapid variability, high luminosity and hard spectrum, this larger scale predicts an Fe ionization state consistent with efficient K{\alpha} production. Moreover, the momentum of the jet radiation impinging on the surrounding accretion flow on this larger scale accelerates a layer of gas to speeds ~0.1-0.2c, consistent with the blueshifted line profile. Implications of our results on the global picture of jetted TDEs are discussed. A power-law {\gamma}/X-ray spectrum may be produced by external UV-optical photons being repetitively inverse-Compton scattered by cold electrons in the jet, although our model for the K{\alpha} reverberation does not depend on the jet radiation mechanism (magnetic reconnection in a Poynting jet is still a viable mechanism). The non-relativistic wind driven by jet radiation may explain the late-time radio rebrightening in Swift J1644+57. This energy injection may also cause the thermal UV-optical emission from jetted TDEs to be systematically brighter than in non-jetted ones.

Can we observe neutrino flares in coincidence with explosive transients?

The new generation of powerful instruments is reaching sensitivities and temporal resolutions that will allow multi-messenger astronomy of explosive transient phenomena, with high-energy neutrinos as a central figure. We derive general criteria for the detectability of neutrinos from powerful transient sources for given instrument sensitivities. In practice, we provide the minimum photon flux necessary for neutrino detection based on two main observables: the bolometric luminosity and the time variability of the emission. This limit can be compared to the observations in specified wavelengths in order to target the most promising sources for follow-ups. Our criteria can also help distinguishing false associations of neutrino events with a flaring source. We find that relativistic transient sources such as high- and low-luminosity gamma-ray bursts (GRBs), blazar flares, tidal disruption events, and magnetar flares could be observed with IceCube, as they have a good chance to occur within a detectable distance. Of the nonrelativistic transient sources, only luminous supernovae appear as promising candidates. We caution that our criterion should not be directly applied to low-luminosity GRBs and type Ibc supernovae, as these objects could have hosted a choked GRB, leading to neutrino emission without a relevant counterpart radiation. We treat a set of concrete examples and show that several transients, some of which are being monitored by IceCube, are far from meeting the criterion for detectability (e.g., Crab flares or Swift J1644+57).

Magnetic field evolution in tidal disruption events

When a star gets tidally disrupted by a supermassive black hole, its magnetic field is expected to pervade its debris. In this paper, we study this process via smoothed particle magnetohydrodynamical simulations of the disruption and early debris evolution including the stellar magnetic field. As the gas stretches into a stream, we show that the magnetic field evolution is strongly dependent on its orientation with respect to the stretching direction. In particular, an alignment of the field lines with the direction of stretching induces an increase of the magnetic energy. For disruptions happening well within the tidal radius, the star compression causes the magnetic field strength to sharply increase by an order of magnitude at the time of pericentre passage. If the disruption is partial, we find evidence for a dynamo process occurring inside the surviving core due to the formation of vortices. This causes an amplification of the magnetic field strength by a factor of $\sim 10$. However, this value represents a lower limit since it increases with numerical resolution. For an initial field strength of 1 G, the magnetic field never becomes dynamically important. Instead, the disruption of a star with a strong 1 MG magnetic field produces a debris stream within which magnetic pressure becomes similar to gas pressure a few tens of hours after disruption. If the remnant of one or multiple partial disruptions is eventually fully disrupted, its magnetic field could be large enough to magnetically power the relativistic jet detected from Swift J1644+57. Magnetized streams could also be significantly thickened by magnetic pressure when it overcomes the confining effect of self-gravity.

Expanding hot flow in the black hole binary SWIFT J1753.5−0127: evidence from optical timing

We describe the evolution of optical and X-ray temporal characteristics during the outburst decline of the black hole X-ray binary SWIFT J1753.5-0127. The optical/X-ray cross-correlation function demonstrates a single positive correlation at the outburst peak, then it has multiple dips and peaks during the decline stage, which are then replaced by the precognition dip plus peak structure in the outburst tail. Power spectral densities and phase lags show a complex evolution, revealing the presence of intrinsically connected optical and X-ray quasi-periodic oscillations. For the first time, we quantitatively explain the evolution of these timing properties during the entire outburst within one model, the essence of which is the expansion of the hot accretion flow towards the tail of the outburst. The pivoting of the spectrum produced by synchrotron Comptonization in the hot flow is responsible for the appearance of the anti-correlation with the X-rays and for the optical quasi-periodic oscillations. Our model reproduces well the cross-correlation and phase lag spectrum during the decline stage, which could not be understood with any model proposed before.

Radio Observations of the Tidal Disruption Event XMMSL1 J0740–85

Abstract We present radio observations of the tidal disruption event candidate (TDE) XMMSL1 J0740−85 spanning 592 to 875 days post X-ray discovery. We detect radio emission that fades from an initial peak flux density at 1.6 GHz of 1.19 ± 0.06 mJy to 0.65 ± 0.06 mJy, suggesting an association with the TDE. This makes XMMSL1 J0740−85 at d = 75 Mpc the nearest TDE with detected radio emission to date and only the fifth TDE with radio emission overall. The observed radio luminosity rules out a powerful relativistic jet like that seen in the relativistic TDE Swift J1644+57. Instead, we infer from an equipartition analysis that the radio emission most likely arises from a non-relativistic outflow similar to that seen in the nearby TDE ASASSN-14li, with a velocity of about 104 km s−1 and a kinetic energy of about 1048 erg, expanding into a medium with a density of about 102 cm−3. Alternatively, the radio emission could arise from a weak initially relativistic but decelerated jet with an energy of erg, or (for an extreme disruption geometry) from the unbound debris. The radio data for XMMSL1 J0740−85 continues to support the previous suggestion of a bimodal distribution of common non-relativistic isotropic outflows and rare relativistic jets in TDEs (in analogy with the relation between Type Ib/c supernovae and long-duration gamma-ray bursts). The radio data also provide a new measurement of the circumnuclear density on a sub-parsec scale around an extragalactic supermassive black hole.

A ROTATIONALLY POWERED MAGNETAR NEBULA AROUND SWIFT J1834.9–0846

Abstract A wind nebula generating extended X-ray emission was recently detected surrounding Swift J1834.9–0846. This is the first magnetar for which such a wind nebula was found. Here, we investigate whether there is a plausible scenario where the pulsar wind nebula (PWN) can be sustained without the need of advocating for additional sources of energy other than rotational. We do this by using a detailed radiative and dynamical code that studies the evolution of the nebula and its particle population in time. We find that such a scenario indeed exists: Swift J1834.9–0846's nebula can be explained as being rotationally powered, as all other known PWNe are, if it is currently being compressed by the environment. The latter introduces several effects, the most important of which is the appearance of adiabatic heating, being increasingly dominant over the escape of particles as reverberation goes by. The need of reverberation naturally explains why this is the only magnetar nebula detected and provides estimates for Swift 1834.9–0846's age.

Learning about the magnetar Swift J1834.9−0846 from its wind nebula

The first wind nebula around a magnetar was recently discovered in X-rays around Swift~J1834.9$-$0846. We study this magnetar's global energetics and the properties of its particle wind or outflows. At a distance of $\sim4\;$kpc, Swift~J1834.9$-$0846 is located at the center of the supernova remnant (SNR) W41 whose radius is $\sim 19\;$pc, an order of magnitude larger than that of the X-ray nebula ($\sim2\;$pc). The association with SNR W41 suggests a common age of $\sim5-100\;$kyr, while its spin-down age is $4.9$~kyr. A small natal kick velocity may partly explain why a wind nebula was detected around this magnetar but not around other magnetars, most of which appear to have larger kick velocities and may have exited their birth SNR. We find that the GeV and TeV source detected by Fermi/LAT and H.E.S.S., respectively, of radius $\sim11\;$pc is most likely of hadronic origin. The dynamics and internal structure of the nebula are examined analytically to explain the nebula's current properties. Its size may naturally correspond to the diffusion-dominated cooling length of the X-ray emitting $e^+e^-$ pairs. This may also account for the spectral softening of the X-ray emission from the nebula's inner to outer parts. Analysis of the X-ray synchrotron nebula implies that (i) the nebular magnetic field is $\gtrsim 11\;\mu$G (and likely $\lesssim30\;\mu$G), and (ii) the nebula is not powered predominantly by the magnetar's quiescent spin-down-powered MHD wind, but by other outflows that contribute most of its energy. The latter are most likely associated with the magnetar's bursting activity, and possibly dominated by outflows associated with its past giant flares. The energy source for the required outflows cannot be the decay of the magnetar's dipole field alone, and is most likely the decay of its much stronger internal magnetic field.

The influence of circumnuclear environment on the radio emission from TDE jets

Dozens of stellar tidal disruption events (TDEs) have been identified at optical, UV and X-ray wavelengths. A small fraction of these, most notably Swift J1644+57, produce radio synchrotron emission, consistent with a powerful, relativistic jet shocking the surrounding circumnuclear gas. The dearth of similar non-thermal radio emission in the majority of TDEs may imply that powerful jet formation is intrinsically rare, or that the conditions in galactic nuclei are typically unfavorable for producing a detectable signal. Here we explore the latter possibility by constraining the radial profile of the gas density encountered by a TDE jet using a one-dimensional model for the circumnuclear medium which includes mass and energy input from a stellar population. Near the jet Sedov radius of 10$^{18}$ cm, we find gas densities in the range of $n_{18} \sim$ 0.1$-$1000 cm$^{-3}$ across a wide range of plausible star formation histories. Using one- and two-dimensional relativistic hydrodynamical simulations, we calculate the synchrotron radio light curves of TDE jets (as viewed both on and off-axis) across the allowed range of density profiles. We find that bright radio emission would be produced across the plausible range of nuclear gas densities by jets as powerful as Swift J1644+57, and we quantify the relationship between the radio luminosity and jet energy. We use existing radio detections and upper limits to constrain the energy distribution of TDE jets. Radio follow up observations several months to several years after the TDE candidate will strongly constrain the energetics of any relativistic flow.

On the possible wind nebula of magnetar Swift J1834.9–0846: a magnetism-powered synchrotron nebula

Recently, the magnetar Swift J1834.9–0846 has been reported to have a possible wind nebula. It is shown that both the magnetar and its wind nebula are understandable in the wind braking scenario. The magnetar's rotational energy loss rate is not enough to power the particle luminosity. The required particle luminosity should be about 1036 erg s−1 to 1038 erg s−1. It is obtained in three different approaches: considering wind braking of Swift J1834.9–0846; the spectral and spatial observations of the wind nebula; and an empirical upper bound on wind nebula X-ray luminosity. The nebula magnetic field is about 10−4 G. The possible wind nebula of Swift J1834.9–0846 should be a magnetar wind nebula. It is powered by the magnetic energy released from the magnetar.

Polarized synchrotron emission in quiescent black hole X-ray transients

We present near-infrared polarimetric observations of the black hole X-ray binaries Swift J1357.2−0933 and A0620−00. In both sources, recent studies have demonstrated the presence of variable infrared synchrotron emission in quiescence, most likely from weak compact jets. For Swift J1357.2−0933, we find that the synchrotron emission is polarized at a level of 8.0 ± 2.5 per cent (a 3.2σ detection of intrinsic polarization). The mean magnitude and rms variability of the flux (fractional rms of 19–24 per cent in KS band) agree with previous observations. These properties imply a continuously launched (stable on long time-scales), highly variable (on short time-scales) jet in the Swift J1357.2−0933 system in quiescence, which has a moderately tangled magnetic field close to the base of the jet. We find that for A0620–00, there are likely to be three components to the optical–infrared polarization; interstellar dust along the line of sight, scattering within the system, and an additional source that changes the polarization position angle in the reddest (H and KS) wavebands. We interpret this as a stronger contribution of synchrotron emission, and by subtracting the line-of-sight polarization, we measure an excess of ∼1.25 ± 0.28 per cent polarization and a position angle of the magnetic field vector that is consistent with being parallel with the axis of the resolved radio jet. These results imply that weak jets in low-luminosity accreting systems have magnetic fields which possess similarly tangled fields compared to the more luminous, hard state jets in X-ray binaries.

No evidence for a low-mass black hole in Swift J1753.5−0127

We present high-resolution, time-resolved optical spectroscopy of the black hole X-ray transient Swift J1753.5-0127. Our optical spectra do not show features that we can associate with the companion star. However we do observe broad, double-peaked emission lines, typical of an accretion disc. We show that the mass of the compact object is likely $>7.4\pm1.2M_{\odot}$, much higher than previous suggestions of a low-mass ($<5M_{\odot}$) black hole.

ASASSN-14li: A MODEL TIDAL DISRUPTION EVENT

ABSTRACT ASASSN-14li is a recently discovered tidal disruption event with an exceptionally rich data set: spectra and lightcurves in soft X-rays, UV, optical, and radio. To understand its emission properties in all these bands, we have extended our model for post-tidal disruption accretion and photon production to estimate both soft X-ray radiation produced by the “prompt” accretion phase and synchrotron emission associated with the bow shock driven through an external medium by the unbound tidal debris, as well as optical and UV light. We find that fiducial values of the stellar mass ( ) and black hole mass ( ) yield quantitative agreement with the optical/UV luminosity, lightcurve, and color temperature; approximate agreement with the soft X-ray spectrum and lightcurve; and quantitative agreement with the radio luminosity, spectrum, and lightcurve. Equipartition analysis of the radio data implies that the radio-emitting region expands with a constant speed, and its magnitude is comparable to the speed expected for the unbound stellar ejecta. Both facts provide strong support to our model. We find that the disruption event took place in 2014 mid-September. Two independent parameters, the magnitude and logarithmic radial gradient of the ambient gas density near the black hole, must be fit to the data to explain the radio emission; their inferred values are comparable to those found near both Sgr A* and the TDE candidate Swift J1644.

SEARCH FOR HIGH-ENERGY GAMMA-RAY EMISSION FROM TIDAL DISRUPTION EVENTS WITH THE FERMI LARGE AREA TELESCOPE

ABSTRACT Massive black holes at galaxy center may tear apart a star when the star passes occasionally within the disruption radius, which is the so-called tidal disruption event (TDE). Most TDEs radiate with thermal emission resulting from the acceleration disk, but three TDEs have been detected in bright nonthermal X-ray emission, which is interpreted as arising from the relativistic jets. A search for high-energy gamma-ray emission from one relativistic TDE (Swift J164449.3+573451) with the Fermi Large Area Telescope (LAT) has yielded nondetection. In this paper, we report the search for high-energy emission from the other two relativistic TDEs (Swift J2058.4+0516 and Swift J1112.2-8238) during the flare period. No significant GeV emission is found, with an upper limit fluence in the LAT energy range being less than 1% of that in X-rays. Compared with gamma-ray bursts and blazars, these TDEs have the lowest flux ratio between GeV emission and X-ray emission. The nondetection of high-energy emission from relativistic TDEs could be due to the fact that the high-energy emission is absorbed by soft photons in the source. Based on this hypothesis, upper limits on the bulk Lorentz factors, , are then obtained for the jets in these TDEs. We also search for high-energy gamma-ray emission from the nearest TDE discovered to date, ASASSN-14li. No significant GeV emission is found, and an upper limit of erg s−1 (at 95% confidence level) is obtained for the first 107 s after the disruption.

Relativistic reverberation in the accretion flow of a tidal disruption event.

Our current understanding of the curved space-time around supermassive black holes is based on actively accreting black holes, which make up only ten per cent or less of the overall population. X-ray observations of that small fraction reveal strong gravitational redshifts that indicate that many of these black holes are rapidly rotating; however, selection biases suggest that these results are not necessarily reflective of the majority of black holes in the Universe. Tidal disruption events, where a star orbiting an otherwise dormant black hole gets tidally shredded and accreted onto the black hole, can provide a short, unbiased glimpse at the space-time around the other ninety per cent of black holes. Observations of tidal disruptions have hitherto revealed the formation of an accretion disk and the onset of an accretion-powered jet, but have failed to reveal emission from the inner accretion flow, which enables the measurement of black hole spin. Here we report observations of reverberation arising from gravitationally redshifted iron Kα photons reflected off the inner accretion flow in the tidal disruption event Swift J1644+57. From the reverberation timescale, we estimate the mass of the black hole to be a few million solar masses, suggesting an accretion rate of 100 times the Eddington limit or more. The detection of reverberation from the relativistic depths of this rare super-Eddington event demonstrates that the X-rays do not arise from the relativistically moving regions of a jet, as previously thought.

THE WIND NEBULA AROUND MAGNETAR SWIFT J1834.9–0846

ABSTRACT We report on the analysis of two deep XMM-Newton observations of the magnetar Swift J1834.9−0846 and its surrounding extended emission taken in 2014 March and October, 2.5 and 3.1 yr after the source went into outburst. The magnetar is only weakly detected in the first observation, with an absorption-corrected flux erg s−1 cm−2 and a upper limit during the second observation of about 3 × 10−14 erg s−1 cm−2. This flux level is more than 3 orders of magnitude lower than the flux measured at the outburst onset in 2011 September. The extended emission, centered at the magnetar position and elongated toward the southwest, is clearly seen in both observations; it is best fit by a highly absorbed power law (PL), with a hydrogen column density of cm−2 and PL photon index . Its flux is constant between the two observations at erg s−1 cm−2. We find no statistically significant changes in the spectral shape or the flux of this extended emission over a period of 9 yr from 2005 to 2014. These new results strongly support the extended emission nature as a wind nebula and firmly establish Swift J1834.9−0846 as the first magnetar to show a surrounding wind nebula. Further, our results imply that such nebulae are no longer exclusive to rotation-powered pulsars and narrow the gap between these two subpopulations of isolated neutron stars. The size and spectrum of the nebula are compatible with those of pulsar-wind nebulae, but its radiative efficiency is markedly high, possibly pointing to an additional wind component in Swift J1834.9−0846.

The origin of seed photons for Comptonization in the black hole binary Swift J1753.5–0127

Aims. The black hole binary SWIFT J1753.5-0127 is providing a unique data set to study accretion flows. Various investigations of this system and of other black holes have not, however, led to an agreement on the accretion flow geometry or on the seed photon source for Comptonization during different stages of X-ray outbursts. We place constraints on these accretion flow properties by studying long-term spectral variations of this source. Methods. We performed phenomenological and self-consistent broad band spectral modeling of Swift J1753.5-0127 using quasi-simultaneous archived data from INTEGRAL/ISGRI, Swift/UVOT/XRT/BAT, RXTE/PCA/HEXTE and MAXI/GSC instruments. Results. We identify a critical flux limit, F \sim 1.5 \times 10^{-8} erg/cm^2/s, and show that the spectral properties of SWIFT J1753.5-0127 are markedly different above and below this value. Above the limit, during the outburst peak, the hot medium seems to intercept roughly 50 percent of the disk emission. Below it, in the outburst tail, the contribution of the disk photons reduces significantly and the entire spectrum from the optical to X-rays can be produced by a synchrotron-self-Compton mechanism. The long-term variations in the hard X-ray spectra are caused by erratic changes of the electron temperatures in the hot medium. Thermal Comptonization models indicate unreasonably low hot medium optical depths during the short incursions into the soft state after 2010, suggesting that non-thermal electrons produce the Comptonized tail in this state. The soft X-ray excess, likely produced by the accretion disk, shows peculiarly stable temperatures for over an order of magnitude changes in flux. Conclusions. The long-term spectral trends of SWIFT J1753.5-0127 are likely set by variations of the truncation radius and a formation of a hot, quasi-spherical inner flow in the vicinity of the black hole. (abridged)