Previous Outbursts Research Articles

Spin-down rate and inferred dipole magnetic field of the soft gamma-ray repeater SGR 1627–41

Abstract Using Chandra data taken in 2008 June, we detected pulsations at 2.594 39(4) s in the soft gamma-ray repeater SGR 1627−41. This is the second measurement of the source spin period and allows us to derive for the first time a long-term spin-down rate of (1.9 ± 0.4) × 10−11 s s−1. From this value, we infer for SGR 1627−41 a characteristic age of ∼2.2 kyr, a spin-down luminosity of ∼4 × 1034 erg s−1 (one of the highest among sources of the same class), and a surface dipole magnetic field strength of ∼2 × 1014 G. These properties confirm the magnetar nature of SGR 1627−41; however, they should be considered with caution since they were derived on the basis of a period derivative measurement made using two epochs only, and magnetar spin-down rates are generally highly variable. The pulse profile, double-peaked and with a pulsed fraction of 13 ± 2 per cent in the 2–10 keV range, closely resembles that observed by XMM–Newton in 2008 September. Having for the first time a timing model for this soft gamma-ray repeater (SGR), we also searched for a pulsed signal in archival radio data collected with the Parkes radio telescope 9 months after the previous X-ray outburst. No evidence for radio pulsations was found, down to a luminosity level ∼10–20 times fainter (for a 10 per cent duty cycle and a distance of 11 kpc) than the peak luminosity shown by the known radio magnetars.

GAS SLOSHING AND BUBBLES IN THE GALAXY GROUP NGC 5098

We present results from Chandra observations of the galaxy pair and associated galaxy group NGC 5098, and find evidence for both gas sloshing and AGN heating. The X-ray brightness images show diffuse emission with a spiral structure, centered on NGC 5098a, and a sharp edge in the diffuse emission surrounding much of the galaxy at about 30 kpc. The spiral structure in the X-ray surface brightness and temperature maps, the offset between the peak of the cool gas and the central AGN, and the structure of the cold front edges all suggest gas sloshing in the core. The most likely perturber is the nearby galaxy NGC 5098b, which has been stripped of its gaseous atmosphere. Detailed images of the core reveal several X-ray cavities, two of which, just north and southeast of the central AGN, correlate with radio emission and have bright X-ray rims, similar to buoyant bubbles seen in the ICM of other systems. We estimate the pressures in the bubbles and rims and show that they are roughly equal, consistent with these being young features, as suggested by their close proximity to the central AGN. We assume that the other X-ray cavities in the core, which show no correlation with existing radio observations, are ghost cavities from previous AGN outbursts. An estimate of the mechanical energy required to inflate the cavities indicates that it is sufficient to offset radiative cooling of the gas for 15 Myr. Therefore, for a typical cycle time of 10^7 yrs, the central AGN energy output is enough to balance cooling over long timescales.

The unusual 2006 dwarf nova outburst of GK Persei

The 2006 outburst of GK Perseii differed significantly at optical and ultraviolet wavelengths from typical outbursts of this object. We present multi-wavelength (X-ray, UV and optical) Swift and AAVSO data, giving unprecedented broad-band coverage of the outburst, allowing us to follow the evolution of the longer-than-normal 2006 outburst across these wavelengths. In the optical and UV we see a triple-peaked morphology with maximum brightness ~1.5 magnitudes lower than in previous years. In contrast, the peak hard X-ray flux is the same as in previous outbursts. We resolve this dichotomy by demonstrating that the hard X-ray flux only accounts for a small fraction of the total energy liberated during accretion, and interpret the optical/UV outburst profile as arising from a series of heating and cooling waves traversing the disc, caused by its variable density profile.

THE JET HEATED X-RAY FILAMENT IN THE CENTAURUS A NORTHERN MIDDLE RADIO LOBE

We present results from a 40 ks {\em XMM-Newton} observation of the X-ray filament coincident with the southeast edge of the Centaurus A Northern Middle Radio Lobe (NML). We find that the X-ray filament consists of five spatially resolved X-ray knots embedded in a continuous diffuse bridge. The spectrum of each knot is well fitted by a thermal model with temperatures ranging from 0.3-0.7 keV and subsolar elemental abundances. In four of the five knots, non-thermal models are a poor fit to the spectra, conclusively ruling out synchrotron or IC/CMB mechanisms for their emission. The internal pressures of the knots exceed that of the ambient ISM or the equipartition pressure of the NML by more than an order of magnitude, demonstrating that they must be short lived ($\sim3\times10^6$ yrs). Based on energetic arguments, it is implausible that these knots have been ionized by the beamed flux from the active galactic nucleus of Cen A or that they have been shock-heated by supersonic inflation of the NML. In our view, the most viable scenario for the origin of the X-ray knots is that they are the result of cold gas shock heated by a direct interaction with the jet. The lifetimes of the X-ray knots in the NML are roughly the same as the age of the strong shock around the SW inner lobe, suggesting they were created in the same AGN outburst. The most plausible model of the NML is that it is a bubble from a previous outburst that is being re-energized by the current outburst. We also report the discovery of a large scale (at least 35 kpc radius) gas halo around Cen A.

DISENTANGLING THE SYSTEM GEOMETRY OF THE SUPERGIANT FAST X-RAY TRANSIENT IGR J11215–5952 WITHSWIFT

IGR J11215-5952 is a hard X-ray transient discovered in 2005 April by INTEGRAL and a member of the new class of HMXB, the Supergiant Fast X-ray Transients (SFXTs). While INTEGRAL and RXTE observations have shown that the outbursts occur with a periodicity of ~330 days, Swift data have recently demonstrated that the true outburst period is ~165 days. IGR J11215-5952 is the first discovered SFXT displaying periodic outbursts, which are possibly related to the orbital period. We performed a Guest Investigator observation with Swift that lasted 20ks and several follow-up Target of Opportunity (ToO) observations, for a total of ~32ks, during the expected "apastron" passage (defined assuming an orbital period of ~330 days), between 2008 June 16 and July 4. The characteristics of this "apastron'' outburst are quite similar to those previously observed during the "periastron'' outburst of 2007 February 9. The mean spectrum of the bright peaks can be fit with an absorbed power law model with a photon index of 1 and an absorbing column of 1E22 cm^-2. This outburst reached luminosities of ~1E36 erg/s (1-10keV), comparable with the ones measured in 2007. The light curve can be modelled with the parameters obtained by Sidoli et al. (2007) for the 2007 February 9 outburst, although some differences can be observed in its shape. The properties of the rise to this new outburst and the comparison with the previous outbursts allow us to suggest that the true orbital period of IGR J11215-5952 is very likely 164.6 days, and that the orbit is eccentric, with the different outbursts produced at the periastron passage, when the neutron star crosses the inclined equatorial wind from the supergiant companion. Based on a ToO observation performed on 2008 March 25-27, we can exclude that the period is 165/2 days. [Abridged]

Broad-band X-ray spectral evolution of GX 339−4 during a state transition★

We report on X-ray and soft γ-ray observations of the black hole candidate GX 339−4 during its 2007 outburst, performed with the RXTE and INTEGRAL satellites. The hardness–intensity diagram of all RXTE/PCA data combined shows a q-shaped track similar to that observed in previous outbursts. The evolution in the diagram suggested that a transition from hard- to soft-intermediate state occurred, simultaneously with INTEGRAL observations performed in March. The transition is confirmed by the timing analysis presented in this work, which reveals that a weak type-A quasi-periodic oscillation (QPO) replaces a strong type-C QPO. At the same time, spectral analysis shows that the flux of the high-energy component shows a significant decrease in its flux. However, we observe a delay (roughly one day) between variations of the spectral parameters of the high-energy component and changes in the flux and timing properties. The changes in the high-energy component can be explained either in terms of the high-energy cut-off or in terms of variations in the reflection component. We compare our results with those from a similar transition during the 2004 outburst of GX 339−4.

Peak Luminosities of the Hard States of GX 339‐4: Implications for the Accretion Geometry, Disk Mass, and Black Hole Mass

We have analyzed observations of the black hole transient {GX 339$-$4} made with the {\it Rossi} X-ray Timing Explorer (RXTE) and the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma-ray Observatory (CGRO). We have found a nearly linear relation between the peak flux during the low/hard (LH) state that occurs at the beginning of an outburst and the time since the flux peak of the latest LH state identified in the previous outburst. Assuming that the rate at which mass accumulates in the accretion disk between these peaks is constant and that any mass that remains in the disk after an outburst has a negligible effect on the next outburst, this nearly linear relation suggests that the peak flux during the LH state that occurs at the beginning of an outburst is related to the mass in the disk, and thus that the entire disk is probably involved in powering these LH states. We have also found a positive correlation between the peak luminosities of the LH state in the three recent outbursts of {GX 339$-$4} and the peak luminosities of the following HS state. This correlation is similar to the correlations reported previously for {Aql X$-$1}, {4U 1705$-$44}, and {XTE J1550$-$564}, providing further support that the accretion flow that powers the LH state is related to the accretion flow that powers the following HS state. Although the luminosity of the LH-to-HS transition varies by up to an order of magnitude, the neutron stars and the black holes are distinguishable in the state transition luminosity. We discuss the implications for the mass determination of the compact stars in the ultraluminous X-ray sources (ULXs).

On the cyclotron line in Cepheus X-4

Context. Accreting X-ray pulsars provide us with laboratories for the study of extreme gravitational and magnetic fields, hence accurate descriptions of their observational properties contribute to our understanding of this group of objects. Aims. We aim to detect a cyclotron resonance scattering feature in the Be/X-ray binary Cep X-4 and to investigate pulse profile and spectral changes through the outburst. Methods. Spectral fitting and timing analysis are employed to probe the properties of Cep X-4 during an outburst in 2002 June. Results. A previously announced cyclotron feature at 30.7 keV is confirmed, while the source shows spectral behaviour and luminosity related changes similar to those observed in previous outbursts. The long-term X-ray lightcurve shows a periodicity at 20.85 d, which could be attributed to the orbit in this Be system.

SwiftObservations of the 2006 Outburst of the Recurrent Nova RS Ophiuchi. I. Early X‐Ray Emission from the Shocked Ejecta and Red Giant Wind

RS Ophiuchi began its latest outburst on 2006 February 12. Previous outbursts have indicated that high velocity ejecta interact with a pre-existing red giant wind, setting up shock systems analogous to those seen in Supernova Remnants. However, in the previous outburst in 1985, X-ray observations did not commence until 55 days after the initial explosion. Here we report on Swift observations covering the first month of the 2006 outburst with the Burst Alert (BAT) and X-ray Telescope (XRT) instruments. RS Oph was clearly detected in the BAT 14-25 keV band from t=0 to $t\sim6$ days. XRT observationsfrom 0.3-10 keV, started at 3.17 days after outburst. The rapidly evolving XRT spectra clearly show the presence of both line and continuum emission which can be fitted by thermal emission from hot gas whose characteristic temperature, overlying absorbing column, $[N_H]_W$, and resulting unabsorbed total flux decline monotonically after the first few days. Derived shock velocities are in good agreement with those found from observations at other wavelengths. Similarly, $[N_H]_W$ is in accord with that expected from the red giant wind ahead of the forward shock. We confirm the basic models of the 1985 outburst and conclude that standard Phase I remnant evolution terminated by $t\sim10$ days and the remnant then rapidly evolved to display behaviour characteristic of Phase III. Around t=26 days however, a new, luminous and highly variable soft X-ray source began to appear whose origin will be explored in a subsequent paper.

Further Evidence for Variable Synchrotron Emission in XTE J1118+480 in Outburst

We present simultaneous multicolor infrared and optical photometry of the black hole X-ray transient XTE J1118+480 during its short 2005 January outburst, supported by simultaneous X-ray observations. The variability is dominated by short timescales, ~10 s, although a weak superhump also appears to be present in the optical. The optical rapid variations, at least, are well correlated with those in X-rays. Infrared JHK_s photometry, as in the previous outburst, exhibits especially large-amplitude variability. The spectral energy distribution (SED) of the variable infrared component can be fitted with a power law of slope α = -0.78 ± 0.07, where F_ν ∝ ν_α. There is no compelling evidence for evolution in the slope over five nights, during which time the source brightness decayed along almost the same track as seen in variations within the nights. We conclude that both short-term variability and longer timescale fading are dominated by a single component of constant spectral shape. We cannot fit the SED of the IR variability with a credible thermal component, either optically thick or thin. This IR SED is, however, approximately consistent with optically thin synchrotron emission from a jet. These observations therefore provide indirect evidence to support jet-dominated models for XTE J1118+480 and also provide a direct measurement of the slope of the optically thin emission, which is impossible, based on the average spectral energy distribution alone.

An asymmetric shock wave in the 2006 outburst of the recurrent nova RS Ophiuchi

Nova outbursts take place in binary star systems comprising a white dwarf and either a low-mass Sun-like star or, as in the case of the recurrent nova RS Ophiuchi, a red giant. Although the cause of these outbursts is known to be thermonuclear explosion of matter transferred from the companion onto the surface of the white dwarf, models of the previous (1985) outburst of RS Ophiuchi failed to adequately fit the X-ray evolution and there was controversy over a single-epoch high-resolution radio image, which suggested that the remnant was bipolar rather than spherical as modelled. Here we report the detection of spatially resolved structure in RS Ophiuchi from two weeks after its 12 February 2006 outburst. We track an expanding shock wave as it sweeps through the red giant wind, producing a remnant similar to that of a type II supernova but evolving over months rather than millennia. As in supernova remnants, the radio emission is non-thermal (synchrotron emission), but asymmetries and multiple emission components clearly demonstrate that contrary to the assumptions of spherical symmetry in models of the 1985 explosion, the ejection is jet-like, collimated by the central binary whose orientation on the sky can be determined from these observations.

IGR J11215–5952: a hard X-ray transient displaying recurrent outbursts

Context. The hard X-ray source IGR J11215–5952 has been discovered with INTEGRAL during a short outburst in 2005 and proposed as a new member of the class of supergiant fast X-ray transients. Aims. We analysed INTEGRAL public observations of the source field in order to search for previous outbursts from this transient, not reported in literature. Methods. Our results are based on a systematic re-analysis of INTEGRAL archival observations, using the latest analysis software and instrument calibrations. Results. We report the discovery of two previously unnoticed outbursts, spaced by intervals of ~330 days, that occurred in July 2003 and May 2004. The 5–100 keV spectrum of IGR J11215–5952 is well described by a cut-off power law, with a photon index of ~0.5, and a cut-off energy ~15–20 keV, typical of High Mass X-ray Binaries hosting a neutron star. A 5–100 keV luminosity of ${\sim}3\times10^{36}$ erg s -1 has been derived (assuming 6.2 kpc, the distance of the likely optical counterpart). Conclusions. The 5–100 keV spectral properties, the recurrent nature of the outbursts, together with the reduced error region containing the blue supergiant star HD 306414, support the hypothesis that IGR J11215–5952 is a member of the class of the Supergiant Fast X-ray Transients.

Variable Quasi Periodic Oscillations during an outburst of the transient X-ray pulsar XTE J1858 + 034

We have investigated the Quasi Periodic Oscillation (QPO) properties of the transient accreting X-ray pulsar XTE J1858 + 034 during the second outburst of this source in April–May 2004. We have used observations made with the Proportional Counter Array (PCA) of the Rossi X-ray Timing Explorer (RXTE) during May 14–18, 2004, in the declining phase of the outburst. We detected the presence of low frequency QPOs in the frequency range of 140–185 mHz in all the RXTE-PCA observations. We report evolution of the QPO parameters with the time, X-ray flux, and X-ray photon energy. Though a correlation between the QPO centroid frequency and the instantaneous X-ray flux is not very clear from the data, we point out that the QPO frequency and the one day averaged X-ray flux decreased with time during these observations. We have obtained a clear energy dependence of the RMS variation in the QPOs, increasing from about 3% at 3 keV to 6% at 25 keV. The X-ray pulse profile is a single peaked sinusoidal, with pulse fraction increasing from 20% at 3 keV to 45% at 30keV. We found that, similar to the previous outburst, the energy spectrum is well fitted with a model consisting of a cut-off power law along with an iron emission line.

M 31-RV evolution and its alleged multi-outburst pattern

The photometric evolution of M 31-RV has been investigated on 1447 plates of the Andromeda galaxy obtained over half a century with the Asiago telescopes. M 31-RV is a gigantic stellar explosion that occurred during 1988 in the Bulge of M 31 and that was characterized by the appearance for a few months of an M supergiant reaching Mbol = −10. The 1988 outburst has been positively detected on Asiago plates, and it has been the only such event recorded over the period covered by the plates (1942−1993). In particular, an alleged previous outburst in 1967 is excluded by the more numerous and deeper Asiago plates, with relevant implication for the interpretative models of this unique event. We outline a close analogy in spectral and photometric evolution with those of V838 Mon which exploded in our Galaxy in 2002. The analogy is found to extend also to the closely similar absolute magnitude at the time of the sudden drop in photospheric temperature that both M 31-RV and V838 Mon exhibited. These similarities, in spite of the greatly differing metallicity, age and mass of the two objects, suggest that the same, universal and not yet identified process was at work in both cases.

On the Possibility of V4444 Sgr as a Recurrent Nova

V4444 Sgr (Nova Sgr 1999) has recently been suggested to be a possible recurrent nova based on the detection of preexisting dust, which may have been formed in a previous outburst. We examined this possibility using the available outburst observations, including the VSNET observations. We also noticed that this nova was recorded in the OGLE II public photometric database, which covered both preoutburst and late-decline phases. The outburst light curve constructed from these observations showed a plateau phase, particularly in the OGLE II $I$-band light curve, which was followed by a more rapid decline. The light curve resembled those of “fast” recurrent novae and candidates in the presence of the distinct plateau phase. The outburst spectrum and the preoutburst magnitude more resemble those of classical novae, but they do not exclude the possibility of a recurrent nova.

Discovery of a Second Millisecond Accreting Pulsar: XTE J1751−305

We report the discovery by the Rossi X-Ray Timing Explorer Proportional Counter Array of a second transient accreting millisecond pulsar, XTE J1751-305, during regular monitoring observations of the Galactic bulge region. The pulsar has a spin frequency of 435 Hz, making it one of the fastest pulsars. The pulsations contain the signature of orbital Doppler modulation, which implies an orbital period of 42 minutes, the shortest orbital period of any known radio or X-ray millisecond pulsar. The mass function, fX = (1.278 ± 0.003) × 10-6 M☉, yields a minimum mass for the companion of between 0.013 and 0.017 M☉, depending on the mass of the neutron star. No eclipses were detected. A previous X-ray outburst in 1998 June was discovered in archival All-Sky Monitor data. Assuming mass transfer in this binary system is driven by gravitational radiation, we constrain the orbital inclination to be in the range 30°-85° and the companion mass to be 0.013-0.035 M☉. The companion is most likely a heated helium dwarf. We also present results from the Chandra High Resolution Camera-S observations, which provide the best-known position of XTE J1751-305.

Hiding in Plain Sight:ChandraObservations of the Quiescent Neutron Star 4U 2129+47 in Eclipse

During a previous outburst phase, the neutron star 4U 2129+47 exhibited evidence for a spatially extended accretion disk corona via broad, partial X-ray eclipses occurring periodically on the 5.24 hr orbit. Since 1983, however, 4U 2129+47 has entered a quiescent state several orders of magnitude fainter. We have performed a 37 ks (≈two binary orbits) Chandra observation of 4U 2129+47 to determine whether an extended coronal structure also exists in quiescence. Total eclipses are found, and the rapidity of the eclipse ingress and egress is used to place upper limits on the size of the X-ray source. The spectrum is found to be comprised of a soft component plus a fairly hard power-law tail. The former is seen to be sinusoidally modulated over the orbital period in a manner consistent with neutral column variations, possibly due to the interaction of an accretion stream with a (small) disk about the neutron star. We fit realistic neutron star atmosphere models to the soft spectra and comment on the consistency of the spectra with cooling neutron star models. It has been previously suggested that the 4U 2129+47 system is a hierarchical triple, with the outer body being an F star. We use differential astrometry to show that the X-ray point source and F star are spatially coincident to within 01. We further compare newly determined upper limits for the extrinsic neutral column to the reddening of the F star. Finally, we discuss how future X-ray observations can further constrain models of quiescent neutron star emission as well as directly verify the triple hypothesis.

Echoes in X-ray binaries

We present a method of analysing the correlated X-ray and optical/UV variability in X-ray binaries, using the observed time delays between the X-ray driving lightcurves and their reprocessed optical echoes. This allows us to determine the distribution of reprocessing sites within the binary. We model the time-delay transfer functions by simulating the distribution of reprocessing regions, using geometrical and binary parameters. We construct best-fit time-delay transfer functions, showing the regions in the binary responsible for the reprocessing of X-rays. We have applied this model to observations of the Soft X-ray Transient, GRO j1655-40. We find the optical variability lags the X-ray variability with a mean time delay of 19.3$pm{2.2}$ seconds. This means that the outer regions of the accretion disc are the dominant reprocessing site in this system. On fitting the data to a simple geometric model, we derive a best-fit disk half-opening angle of 13.5$^{+2.1}_{-2.8}$ degrees, which is similar to that observed after the previous outburst by cite{orosz97}. This disk thickening has the effect of almost entirely shielding the companion star from irradiation at this stage of the outburst.

X‐Ray Observations of XTE J1550−564 during the Decay of the 2000 Outburst. I.ChandraandRXTEEnergy Spectra

We report on the evolution of the X-ray energy spectrum for the black hole candidate (BHC) X-ray transient XTE J1550-564 during the decay of the 2000 outburst. The Rossi X-Ray Timing Explorer (RXTE) and Chandra observations span nearly 5 orders of magnitude in luminosity. The RXTE spectra are dominated by a power-law component, and a comparatively weak soft component was also detected for the first two observations. The source made a transition to the canonical hard state near a luminosity of 9 ? 1036 ergs s-1 over several observations, as evidenced by a drop in the flux of the soft component in the RXTE energy band and a hardening of the power-law component to a photon index near 1.6. The power law did not exhibit this behavior for the previous XTE J1550-564 outburst. For some observations, we detect a high-energy cutoff and find that the cutoff is more significant and at lower energy during the transition than in the hard state. The cutoff in the hard state is at higher energy than has been seen for most previous accreting BHCs. The Chandra spectrum provides evidence for spectral evolution after the hard-state transition. It is well, but not uniquely, described by a power law with a photon index of 2.30 (90% confidence) and interstellar absorption. Advection-dominated accretion flow models predict gradual spectral softening as the luminosity drops, but our observations do not allow us to determine whether the spectral evolution is gradual or sudden. The lowest luminosity we measure for XTE J1550-564 with Chandra is 5 ? 1032 ergs s-1 (0.5-7 keV, for a distance of 4 kpc). Although this is probably not the true quiescent luminosity, it represents a useful upper limit on this quantity.

3–14 Micron Spectroscopy of Nova V445 Puppis

We report 3–14 μm spectroscopy of the nova V445 Puppis on 2001 January 31.45 UT using the Broadband Array Spectrograph System on the Infrared Telescope Facility, approximately 1 month after the object was discovered. The spectrum (W cm-2 μm-1) revealed only a smooth, featureless continuum that decreases monotonically with increasing wavelengths between 3 and 13.6 μm. Its slope is much shallower than the Rayleigh-Jeans tail of a blackbody. The spectrum is consistent with thermal emission from gray (constant) emissivity dust whose temperatures ranged from around 280 K to upward of 1300 K. IR magnitudes were L = 2.8, M = 1.6, and N = -0.27, all ±0.02. The presence of such strong IR continuum emission so early after the nova's outburst may suggest that this object has undergone previous outbursts and that we observed preexisting dust, at least in part