arXiv subjects

Ersin Gogus

Publications and source records attributed to Ersin Gogus.

70 records · Page 4Linked to original sources

arXiv2010

Photospheric radius expansion during magnetar bursts

On August 24th 2008 the new magnetar SGR 0501+4516 (discovered by SWIFT) emitted a bright burst with a pronounced double-peak structure in hard X-rays, reminiscent of the double-peak temporal structure seen in some bright thermonuclear bursts on accreting neutron stars. In the latter case this is due to Photospheric Radius Expansion (PRE): when the flux reaches the Eddington limit, the photosphere expands and cools so that emission becomes softer and drops temporarily out of the X-ray band, re-appearing as the photosphere settles back down. We consider the factors necessary to generate double-peaked PRE events, and show that such a mechanism could plausibly operate in magnetar bursts, despite the vastly different emission process. Identification of the magnetic Eddington limit in a magnetar would constrain magnetic field and distance and could, in principle, enable a measurement of gravitational redshift. It would also locate the emitting region at the neutron star surface, constraining the burst trigger mechanism. Conclusive confirmation of PRE events will require more detailed radiative models for bursts. However for SGR 0501+4516 the predicted critical flux (using the magnetic field strength inferred from timing and the distance suggested by its probable location in the Perseus arm of our Galaxy) is consistent with that observed in the August 24th burst.

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arXiv2007

The Magnetic Fields of Anomalous X-ray Pulsars

Anomalous X-ray Pulsars (AXPs) belong to a class of neutron stars believed to harbor the strongest magnetic fields in the universe, as indicated by their energetic bursts and their rapid spindowns. We have developed a theoretical model that takes into account processes in the atmospheres and magnetospheres of ultramagnetic neutron stars, as well as the effects of their strong gravitational fields on the observable properties. Using this model, we have analyzed the X-ray spectra of a number of AXPs. We find that in all cases, the X-ray spectra are described very well with this emission model. The spectroscopically measured magnetic field strengths of these sources are in close agreement with the values inferred from their spindown properties and provide independent evidence for their magnetar nature. The analysis of spectral data using this physical model also sheds light on the long-term evolution of AXPs.

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arXiv2007

Physical Properties of the AXP 4U 0142+61 from X-ray Spectral Analysis

We analyze archival Chandra and XMM-Newton data of 4U 0142+61 within the context of the Surface Thermal Emission and Magnetospheric Scattering model. We show that 4U 0142+61 spectrum can be fit very well with this physical model that contains only four parameters. The system parameters can be tightly constrained from the fits, yielding a surface magnetic field strength of B=(4.75 \pm 0.02)x 10^{14} G, a surface temperature of kT = 0.309 \pm 0.001$ keV and a scattering optical depth of a few in the magnetosphere. These values do not vary between observations due to the stability of the source within the window of the observations. The detailed fits yield $χ^2$ values that are statistically much better than the traditionally employed blackbody+power-law and two blackbody fits. The spectroscopically measured surface magnetic field strength is higher than, but within, the theoretical uncertainties of the value inferred from the dipole spindown formula.

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arXiv2007

The Magnetar Nature and the Outburst Mechanism of a Transient Anomalous X-ray Pulsar

Anomalous X-ray Pulsars (AXPs) belong to a class of neutron stars believed to harbor the strongest magnetic fields in the universe, as indicated by their energetic bursts and their rapid spindowns. However, an unambiguous measurement of their surface field strengths has not been made to date. It is also not known whether AXP outbursts result from changes in the neutron star magnetic field or crust properties. Here we report a spectroscopic measurement of the surface magnetic field strength of an AXP, XTE J1810$-$197, and solidify its magnetar nature. The field strength obtained from detailed spectral analysis and modeling, B = (2.72$\pm$0.03)$\times10^{14}$ G, is remarkably close to the value inferred from the rate of spindown of this source and remains nearly constant during numerous observations spanning over an order of magnitude in source flux. The surface temperature, on the other hand, declines steadily and dramatically following the 2003 outburst of this source. Our findings demonstrate that heating occurs in the upper neutron star crust during an outburst and sheds light on the transient behaviour of AXPs.

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arXiv2006

Is the Lack of Pulsations in Low Mass X-Ray Binaries due to Comptonizing Coronae?

The spin periods of the neutron stars in most Low Mass X-ray Binary (LMXB) systems still remain undetected. One of the models to explain the absence of coherent pulsations has been the suppression of the beamed signal by Compton scattering of X-ray photons by electrons in a surrounding corona. We point out that simultaneously with wiping out the pulsation signal, such a corona will upscatter (pulsating or not) X-ray emission originating at and/or near the surface of the neutron star leading to appearance of a hard tail of Comptonized radiation in the source spectrum. We analyze the hard X-ray spectra of a selected set of LMXBs and demonstrate that the optical depth of the corona is not likely to be large enough to cause the pulsations to disappear.

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arXiv2006

The Prelude to and Aftermath of the Giant Flare of 2004 December 27: Persistent and Pulsed X-ray Properties of SGR 1806-20 from 1993 to 2005

On 2004 December 27, a highly-energetic giant flare was recorded from the magnetar candidate SGR 1806-20. In the months preceding this flare, the persistent X-ray emission from this object began to undergo significant changes. Here, we report on the evolution of key spectral and temporal parameters prior to and following this giant flare. Using the Rossi X-ray Timing Explorer, we track the pulse frequency of SGR 1806-20 and find that the spin-down rate of this SGR varied erratically in the months before and after the flare. Contrary to the giant flare in SGR 1900+14, we find no evidence for a discrete jump in spin frequency at the time of the December 27th flare (|dnu/nu| < 5 X 10^-6). In the months surrounding the flare, we find a strong correlation between pulsed flux and torque consistent with the model for magnetar magnetosphere electrodynamics proposed by Thompson, Lyutikov & Kulkarni (2002). As with the flare in SGR 1900+14, the pulse morphology of SGR 1806-20 changes drastically following the flare. Using the Chandra X-ray Observatory and other publicly available imaging X-ray detector observations, we construct a spectral history of SGR 1806-20 from 1993 to 2005. The usual magnetar persistent emission spectral model of a power-law plus a blackbody provides an excellent fit to the data. We confirm the earlier finding by Mereghetti et al. (2005) of increasing spectral hardness of SGR 1806-20 between 1993 and 2004. Contrary to the direct correlation between torque and spectral hardness proposed by Mereghetti et al., we find evidence for a sudden torque change that triggered a gradual hardening of the energy spectrum on a timescale of years. Interestingly, the spectral hardness, spin-down rate, pulsed, and phase-averaged of SGR 1806-20 all peak months before the flare epoch.

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arXiv2004

Long Term Spectral and Timing Behavior of the Black Hole Candidate XTE J1908+094

We present the long term X-ray light curves, detailed spectral and timing analyses of XTE J1908+094 using the Rossi X-ray Timing Explorer Proportional Counter Array observations covering two outbursts in 2002 and early 2003. At the onset of the first outburst, the source was found in a spectrally low/hard state lasting for ~40 days, followed by a three day long transition to the high/soft state. The source flux (in 2$-$10 keV) reached $\sim$100 mCrab on 2002 April 6, then decayed rapidly. In power spectra, we detect strong band-limited noise and varying low-frequency quasi periodic oscillations that evolved from ~0.5 Hz to ~5 Hz during the initial low/hard state of the source. We find that the second outburst closely resembled the spectral evolution of the first. The X-ray transient's overall outburst characteristics lead us to classify XTE J1908+094 as a black-hole candidate. Here we also derive precise X-ray position of the source using Chandra observations which were performed during the decay phase of the first outburst and following the second outburst.

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arXiv2002

Chandra Observations of the Anomalous X-ray Pulsar 4U 0142+61

We present X-ray imaging, timing, and phase resolved spectroscopy of the anomalous X-ray pulsar 4U 0142+61 using the Chandra X-ray Observatory. The spectrum is well described by a power law plus blackbody model with power law index = 3.35(2), kT_BB=0.458(3) keV, and N_H=0.91(2) x 10^{22} cm^{-2}$; we find no significant evidence for spectral features (0.5-7.0 keV). Time resolved X-ray spectroscopy shows evidence for evolution in phase in either index, or KT_BB, or some combination thereof as a function of pulse phase. We derive a precise X-ray position for the source and determine its spin period, P=8.68866(30) s. We have detected emission beyond 4 arcsec from the central source and extending beyond 100 arcsec, likely due to dust scattering in the interstellar medium.

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arXiv2002

A Comprehensive Study of Pulse Profile Evolution in SGR 1806-20 & SGR 1900+14 with the RXTE PCA

Soft Gamma Repeaters undergo pulse profile changes in connection with their burst activity. Here we present a comprehensive pulse profile history of SGR 1806-20 and SGR 1900+14 in three energy bands using Rossi X-ray Timing Explorer/Proportional Counter Array observations performed between 1996 and 2001. Using the Fourier harmonic powers of pulse profiles, we quantify the pulse shape evolution. Moreover, we determined the RMS pulsed count rates (PCRs) of each profile. We show that the pulse profiles of SGR 1806$-$20 remain single pulsed showing only modest changes for most of our observing span, while those of SGR 1900+14 change remarkably in all energy bands. Highly significant pulsations from SGR 1900+14 following the 1998 August 27 and 2001 April 18 bursts enabled us to study not only the decay of PCRs in different energy bands but also their correlations with each other.

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arXiv2001

Serendipitous Detections of XTE J1906+09 with the Rossi X-ray Timing Explorer

In 1996 during Rossi X-ray Timing Explorer (RXTE) observations of SGR 1900+14, the 89-s X-ray pulsar XTE J1906+09 was discovered. As a result of monitoring campaigns of SGR 1900+14, XTE J1906+09 was also monitored regularly in 1996 September, 1998 May-June, 1998 August-1999 July, and 2000 March-2001 January. A search for pulsations in these observations resulted in detections of only the two previously reported outbursts in 1996 September and 1998 August- September. Pulsed flux upper limits for the remaining observations indicate that XTE J1906+09 is a transient X-ray pulsar and therefore likely has a Be star companion. XTE J1906+09 was not detected with the RXTE All Sky Monitor. Pulse timing analysis of the second outburst revealed a sinusoidal signature in the pulse frequencies that is likely produced by periastron passage in an orbit. Fits to pulse phases using an orbital model and quadratic phase model have chi^2 minima at orbital periods of 26-30 days for fixed mass functions of 5, 10, 15, and 20 M_sun. The pulse shape showed intensity and energy dependent variations. Pulse phase spectroscopy was used to quantify the energy dependent variations. The phase averaged spectrum, using the pulse minimum spectrum as the background spectrum to eliminate effects from SGR 1900+14 and the galactic ridge, was well fitted by an absorbed power law with a high energy cutoff. Estimated 2-10 keV peak fluxes, corrected for contributions from the galactic ridge and SGR 1900+14, are 6E-12 ergs/cm^2/s and 1.1E-10 ergs/cm^2/s for the 1996 and 1998 outbursts, respectively. XTE J1906+09 may be a member of an unusual class of Be/X-ray binaries that do not lie on the general spin period versus orbital period correlation with the majority of Be/X-ray binaries.

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arXiv2001

Temporal and Spectral Characteristics of Short Bursts from the Soft Gamma Repeaters 1806-20 and 1900+14

We study the temporal and coarse spectral properties of 268 bursts from SGR 1806-20 and 679 bursts from SGR 1900+14, all observed with the Rossi X-Ray Timing Explorer/Proportional Counter Array. Hardness ratios and temporal parameters, such as T_90 durations and tau_90 emission times are determined for these bursts. We find a lognormal distribution of burst durations, ranging over more than two orders of magnitude: T_90 ~ 10^-2 to >~ 1 s, with a peak at ~ 0.1 s. The burst light curves tend to be asymmetrical, with more than half of all events showing rise times t_r < 0.3 T_90. We find that there exists a correlation between the duration and fluence of bursts from both sources. We also find a significant anti-correlation between hardness ratio and fluence for SGR 1806-20 bursts and a marginal anti-correlation for SGR 1900+14 events. Finally, we discuss possible physical implications of these results within the framework of the magnetar model.

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arXiv2001

Evidence for a Sudden Magnetic Field Reconfiguration in SGR 1900+14

We report the detection of large flux changes in the persistent X-ray flux of SGR 1900+14 during its burst active episode in 1998. Most notably, we find a factor ~700 increase in the non-burst X-ray flux following the August 27th flare, which decayed in time as a power-law. Our measurements indicate that the pulse fraction remains constant throughout this decay. This suggests a global flux enhancement as a consequence of the August 27th flare rather than localized heating. While the persistent flux has since recovered to the pre-outburst level, the pulse profile has not. The pulse shape changed to a near sinusoidal profile within the tail of the August 27th flare (in gamma-rays) and this effect has persisted for more than 1.5 years (in X-rays). The results presented here suggest the magnetic field of the neutron star in SGR 1900+14 was significantly altered (perhaps globally) during the giant flare of August 27.

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arXiv2000

Timing Noise in SGR 1806-20

We have phase connected a sequence of RXTE PCA observations of SGR 1806-20 covering 178 days. We find a simple secular spin-down model does not adequately fit the data. The period derivative varies gradually during the observations between 8.1 and 11.7 * 10^-11 s/s (at its highest, ~40% larger than the long term trend), while the average burst rate as seen with BATSE drops throughout the time interval. The phase residuals give no compelling evidence for periodicity, but more closely resemble timing noise as seen in radio pulsars. The magnitude of the timing noise, however, is large relative to the noise level typically found in radio pulsars. Combining these results with the noise levels measured for some AXPs, we find all magnetar candidates have Δ_8 values larger than those expected from a simple extrapolation of the correlation found in radio pulsars. We find that the timing noise in SGR 1806-20 is greater than or equal to the levels found in some accreting systems (e.g., Vela X-1, 4U 1538-52 and 4U 1626-67), but the spin-down of SGR 1806-20 has thus far maintained coherence over 6 years. Alternatively, an orbital model with a period P_orb = 733 days provides a statistically acceptable fit to the data. If the phase residuals are created by Doppler shifts from a gravitationally bound companion, then the allowed parameter space for the mass function (small) and orbital separation (large) rule out the possibility of accretion from the companion sufficient to power the persistent emission from the SGR.

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arXiv2000

Statistical properties of SGR 1806-20 bursts

We present statistics of SGR 1806-20 bursts, combining 290 events detected with RXTE/PCA, 111 events detected with BATSE and 134 events detected with ICE. We find that the fluence distribution of bursts observed with each instrument are well described by power laws with indices 1.43, 1.76 and 1.67, respectively. The distribution of time intervals between successive bursts from SGR 1806-20 is described by a lognormal function with a peak at 103 s. There is no correlation between the burst intensity and either the waiting times till the next burst or the time elapsed since the previous burst. In all these statistical properties, SGR 1806-20 bursts resemble a self-organized critical system, similar to earthquakes and solar flares. Our results thus support the hypothesis that the energy source for SGR bursts is crustquakes due to the evolving, strong magnetic field of the neutron star, rather than any accretion or nuclear power.

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arXiv1999

Statistical properties of SGR 1900+14 bursts

We study the statistics of soft gamma repeater (SGR) bursts, using a data base of 187 events detected with BATSE and 837 events detected with RXTE PCA, all from SGR 1900+14 during its 1998-1999 active phase. We find that the fluence or energy distribution of bursts is consistent with a power law of index 1.66, over 4 orders of magnitude. This scale-free distribution resembles the Gutenberg-Richter Law for earthquakes, and gives evidence for self-organized criticality in SGRs. The distribution of time intervals between successive bursts from SGR 1900+14 is consistent with a log-normal distribution. There is no correlation between burst intensity and the waiting times till the next burst, but there is some evidence for a correlation between burst intensity and the time elapsed since the previous burst. We also find a correlation between the duration and the energy of the bursts, but with significant scatter. In all these statistical properties, SGR bursts resemble earthquakes and solar flares more closely than they resemble any known accretion-powered or nuclear-powered phenomena. Thus our analysis lends support to the hypothesis that the energy source for SGR bursts is internal to the neutron star, and plausibly magnetic.

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arXiv1999

Hard Burst Emission from the Soft Gamma Repeater SGR 1900+14

We present evidence for burst emission from SGR 1900+14 with a power-law high energy spectrum extending beyond 500 keV. Unlike previous detections of high energy photons during bursts from SGRs, these emissions are not associated with high-luminosity burst intervals. Not only is the emission hard, but the spectra are better fit by Band's GRB function rather than by the traditional optically-thin thermal bremsstrahlung model. We find that the spectral evolution within these hard events obeys a hardness/intensity anti-correlation. Temporally, these events are distinct from typical SGR burst emissions in that they are longer (~ 1 s) and have relatively smooth profiles. Despite a difference in peak luminosity of > 1E+11 between these bursts from SGR 1900+14 and cosmological GRBs, there are striking temporal and spectral similarities between the two kinds of bursts, aside from spectral evolution. We outline an interpretation of these events in the context of the magnetar model.

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