LONG-TERM RADIATIVE BEHAVIOR OF SGR 1900+14

Soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs) belong to a class of objects called magnetars—neutron stars whose X-ray emission is likely to be powered by the decay of their extremely strong magnetic fields (Duncan & Thompson 1992; Thompson & Duncan 1996; Thompson et al. 2002). All seven confirmed SGRs and six out of seven confirmed AXPs⁵ have emitted energetic bursts of X-rays/soft gamma rays. Burst active episodes of magnetars last anywhere from few hours to months. During their bursting activity, magnetars also exhibit remarkable temporal and spectral changes in their persistent X-ray output. A detailed description of SGRs and AXPs can be found in Woods & Thompson (2006) and Mereghetti (2008).

In the last seven years, new magnetar candidates have emerged, most prominently through transient outbursts (e.g., XTE J1810–197, Halpern & Gotthelf 2005; CXO J164710.2–455216, Israel et al. 2007; SGR J1833–0832, Göğüş et al. 2010). These sources were too dim to be detected in X-rays during their quiescent phases (usually below our detection sensitivity), but their X-ray fluxes increased by up to a few hundred times as they entered their outburst episodes. In addition, known magnetar sources also exhibit variations (triggered by bursting activity) in their persistent flux, although typically not as dramatic as those in the transient systems (e.g., Woods et al. 2004). These high X-ray luminosities were instrumental in probing the outburst mechanism of magnetars (Özel & Güver 2007; Güver et al. 2007; Ng et al. 2011). In contrast, the low but persistent flux level of most magnetars requires long-term (5–10 years) monitoring to understand (burst-induced) changes in their emission properties (Dib et al. 2009).

SGR 1900+14 has been one of the most prolific SGRs: it was discovered in 1979 (Mazets et al. 1979) and was detected in a bursting mode again in 1992 (Kouveliotou et al. 1993). In 1998 May, the source entered a major outburst episode that lasted about eight months and included the giant flare on 1998 August 27 (Hurley et al. 1999). ASCA and RXTE observations prior to and during the 1998 activation led to the discovery of its 5.16 s spin period (Hurley et al. 1999), its spin-down rate of ∼10⁻¹¹ s s⁻¹, and magnetic field of 2–8 × 10¹⁴ G (Kouveliotou et al. 1999), and thus to the confirmation of its magnetar nature. The source resumed a high level of activity in 2001 April (Guidorzi et al. 2001; Kouveliotou et al. 2001) and again in 2006 March (Vetere et al. 2006). As its burst active phases are well separated from each other, SGR 1900+14 is an excellent source to investigate radiative changes both during bursting behavior as well as in burst quiescence.

The first major enhancement in the persistent X-ray flux of SGR 1900+14 was observed at the onset of the 1998 August 27 giant flare: the flux increased by a factor of ∼700 with respect to its level before the activation (Woods et al. 2001). A detailed spectral analysis of the (much longer) flux decay period was not possible due to the lack of continuous monitoring observations with imaging instruments during the decay phase. Esposito et al. (2007) analyzed nine pointed BeppoSAX observations of SGR 1900+14 spanning about five years from 1997 May to 2002 April and noted that the (2–10 keV) flux measured in the last pointing faded significantly with respect to earlier observations. Following the 2001 April activation, the Chandra X-ray Observatory and XMM-Newton observed SGR 1900+14 at numerous occasions, establishing a valuable data set for understanding the long-term behavior of this source in, particular, and of magnetars, in general.

In this paper, we make use of all archival Chandra and XMM-Newton observations to construct the persistent X-ray flux temporal and spectral history of SGR 1900+14 spanning about seven years following the 2001 April activation. In the next section, we introduce the Chandra and XMM-Newton observations used in this study. In Section 3, we present the results of the spectral analysis and show that the magnetic field strength and the magnetospheric properties remain stable over the period spanned by the observations, while the surface temperature and the inferred emitting area both decline with time following both outbursts. We also show that the source flux shows the same trend in its decline from outburst in both episodes and ultimately reaches the same minimum flux in both cases. We discuss the implications of these results in Section 4.

Between 2001 April 22 and 2008 April 8, SGR 1900+14 was observed 13 times with Chandra and XMM-Newton. Table 1 lists the log of these pointed X-ray observations. We describe below the details of our data reduction.

2.1. Chandra

2.2. XMM-Newton

We used XSPEC v12.5.1n (Arnaud 1996) to analyze all spectra. We assumed a gravitational redshift correction of 0.306, corresponding to a neutron star mass of 1.4 M_☉ and a radius of 10 km. We performed the fits in the 0.8–6.5 keV range, where the lower energy bound is set by the source flux and the higher energy limit is determined by the non-thermal hard X-ray component (Götz et al. 2006) affecting the soft X-ray spectra.

We fit the X-ray spectra using the Surface Thermal Emission and Magnetospheric Scattering (STEMS; Güver et al. 2006) model. STEMS is based on the radiative equilibrium atmosphere calculations presented in Özel (2001, 2003) but also includes the effects of magnetospheric scattering on the photons emitted from the neutron star surface as calculated by Lyutikov & Gavriil (2006). The model parameters consist of the surface magnetic field strength, B, surface temperature, T, the magnetospheric scattering optical depth, τ, and the velocity of the particles in the magnetosphere, β = v/c, where c is the speed of light.

We first fit all 13 spectra simultaneously using STEMS and taking into account the effect of interstellar absorption. We allowed the spectral parameters of all spectra to vary individually. While we did not specify its value a priori, we forced the hydrogen column density to be the same between observations. We found that the magnetic field strength, the scattering optical depth, and the average particle velocity did not vary significantly between different observations. We measured a hydrogen column density of (2.36 ± 0.05) × 10²² cm⁻², assuming solar abundances (Anders & Grevesse 1989). We, therefore, performed all further STEMS fits fixing the hydrogen column density at the above value and forcing the parameters B, τ, and β to be constant between different observations. We still allowed the temperature and the model normalization to vary individually for all spectra. We obtained good fits to all 13 spectra, with a χ²/dof = 2370/2438. In this combined fit, we found a magnetic field strength of B = (5.0 ± 0.3) × 10¹⁴ G, a scattering optical depth of τ = 8.7 ± 0.9, and a particle velocity of β = 0.37 ± 0.01. Below, we discuss the time evolution of the temperature T and the source flux based on these combined fits. Note that errors reported throughout the paper are 1σ.

We also investigated the potential effects of the cross calibration between the Chandra/ACIS in CC mode and the XMM-Newton/EPIC-pn on the determination of spectral parameters. To this end, we used two data sets, observation ID 7593 with Chandra and 0410580101 with XMM-Newton, which were selected because they occurred at comparable source fluxes. We fit both spectra using STEMS as well as the empirical blackbody plus power-law model, accounting for interstellar absorption in both cases. In this analysis, we again forced the absorption parameter to remain constant for both spectra as it is not observed to vary over time. In Figure 1, we present the two spectra along with the best fitting STEMS models. We found all parameters of both continuum models to be consistent between the Chandra and XMM spectra to within 1σ errors. These results ensure that the use of two different instruments does not introduce any systematic biases in the joint spectral analysis.

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We present in Figure 2 the history of the unabsorbed X-ray flux of SGR 1900+14 in the 0.8–6.5 keV band. The first Chandra observation took place four days after the intermediate event (Feroci et al. 2002). The source flux declined rapidly in the period soon after this event, dropping from (1.34 ± 0.02) × 10⁻¹¹ erg cm⁻² s⁻¹ to (1.16 ± 0.02) × 10⁻¹¹ erg cm⁻² s⁻¹ in only about six days. An even faster decline was seen in the contemporaneous BeppoSAX observations: the source flux in the 2–10 keV band was (3.1 ± 0.3) × 10⁻¹¹ erg cm⁻² s⁻¹ on 2001 April 18 and declined to (1.06 ± 0.04) × 10⁻¹¹ erg cm⁻² s⁻¹ in 11 days (Esposito et al. 2007). We observed a similar trend following the 2006 reactivation of the source as its flux drops from (8.6 ± 0.1) × 10⁻¹² erg cm⁻² s⁻¹ to (7.9 ± 0.1) × 10⁻¹² erg cm⁻² s⁻¹ in three days.

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We further studied these long-term persistent flux variations of SGR 1900+14 as follows: we determined the relative times of the first eight observations (which took place before 2006) with respect to the onset of the 2001 April outburst and those of the remaining five observations with respect to the onset of the 2006 March activation. We present in Figure 3 the unabsorbed flux as a function of relative time since each respective outburst onset. We found that in both outbursts the source flux declined rather gradually until about 600 days after the onset and exhibited a sharper decline trend beyond ∼600 days. We also found that SGR 1900+14 was at its lowest X-ray flux level of 6.8 × 10⁻¹² erg cm⁻² s⁻¹ before the 2006 March reactivation. Following the source re-brightening after the 2006 outburst, the flux reached that level again in the last pointed observation.

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To better understand the nature of these flux variations in SGR 1900+14, we investigated a possible correlation between the flux and the only varying STEMS parameter, i.e., the surface temperature. We find that the flux and surface temperature are indeed correlated, with a Spearman's rank order correlation coefficient r = 0.72. The probability of obtaining such a correlation with a random data set is P = 0.005. In Figure 4, we show the history of the surface temperature of the neutron star as well as its long-term flux behavior. The decline in the surface temperature alone (assuming a single temperature) cannot account for the observed flux decline. In particular, the average surface temperature dropped from ∼0.56 keV to 0.53 keV as measured on 2001 April 22 (MJD 52021) and 2005 September 22 (MJD 53635), respectively, which corresponds to a flux decline of about 25% (if the emitting surface area remains constant). However, the source flux declined by about 97% between these two epochs, mostly soon after the rise; the remaining decline over the last 1100 days of the observing period was only about 33%, while the temperature decreased by about 6%.

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We also investigated the long-term behavior of the surface emitting area inferred using the STEMS normalization and surface temperature. We found that, for a uniform surface temperature, the radius of the emitting region is 4.8 ± 0.2 km soon after the 2001 outburst onset (assuming a distance to SGR 1900+14 of 13.5 kpc; Vrba et al. 2000). It then declined to 3.9 ± 0.2 km and remained fairly constant until 2005. The radius emitting surface went up again to 4.4 ± 0.1 km following the 2006 outburst but quickly fell to a marginally higher constant level of 4.1 ± 0.2 km. This change, however, could be the result of neglecting temperature inhomogeneities. Note also that the errors in the surface temperature and normalization, and, consequently, in the radius of the emitting area, are correlated.

Finally, we performed a coarse phase resolved spectral analysis using five Chandra observations (indicated by arrows in Figure 4) to check whether there are significant surface temperature variations over the spin phase, which may affect or bias the spectral determination of the phase-averaged temperature. For each of the selected pointings, we obtained a pulse peak spectrum (spanning 0.30 of the spin phase during the pulse peak) and a pulse minimum spectrum (spanning 0.30 of the spin phase during the pulse minimum) using accurate and contemporaneous pulse period reported in Mereghetti et al. (2006). We show in Figure 5 the pulse profile and pulse phase intervals within which the pulse peak and pulse minimum spectra were obtained in the observation on 2001 April 22 (MJD 52021). We fit the X-ray spectra in the 0.8–6.5 keV using STEMS with surface magnetic field strength and magnetospheric parameters fixed at their phase-averaged values. In Table 2, we list the temperatures and model normalizations (i.e., an indicator of intensity) for the five selected observations. We find that the surface temperature remains constant over the spin phase of the source in the majority of these observations.

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We found that SGR 1900+14 exhibits a monotonic but non-steady flux decline following the X-ray brightening during its 2001 and 2006 outbursts. In both outbursts, its flux dropped rapidly within a few weeks after the onset of the outburst and then at a slower rate for approximately 600 days. After this period, the flux declines again at a much faster rate. A similar decay trend was also seen in SGR 1627–41 (Kouveliotou et al. 2003). The fact that we observe the same trend in successive outbursts from the same source suggests that the long-term effects of outbursts are not stochastic but reproducible.

It is clear that magnetar bursting activity leads to long-term flux enhancements and that the additional energy powering these enhancements is stored in the crust as heat. This heat comes most likely from the energy released in the crust during the bursts or from the energy deposited in the crust by the bombardment with magnetospheric particles during the burst. The crust, then, reradiates this additional heat over a timescale of a few years.

Despite the correlation between the flux and the surface temperature, the variation in the surface temperature alone does not account for the decline in the flux, but it also requires a change in the emitting area over time. This can perhaps be understood if the crust is heated inhomogeneously, as would be expected if the initial heating episode is due to magnetic energy release. Moreover, because the thermal resistance of the crust is dominated by the uppermost layers, where the heat conductivity is strongly affected by the magnetic field, heat coming from the deeper layers of the crust could reach the surface unevenly. This leads to both uneven heating and uneven cooling, which may affect the total inferred emitting area. Furthermore, over time, because the observations are carried out in a limited energy range, cooler parts of the crust may fall out of the observed energy band faster than the hotter regions, again reducing the inferred emitting area. Thus, the observed source flux would decline both due to a decline in temperature as ∝T⁴ and due to a decline in the emitting area.

We find on three different occasions that the X-ray flux of SGR 1900+14 was as low as 6.8 × 10⁻¹² erg cm⁻² s⁻¹, suggesting that this level may correspond to the source persistent X-ray flux in the absence of burst-induced enhancements. The current detection thresholds of imaging instruments are ∼10⁻¹³ erg cm⁻² s⁻¹; it is, therefore, possible that the persistent flux levels of the so-called transient magnetars are much lower than those of the always detectable magnetars. If that is indeed the case, the magnetar engine that is responsible for a source persistent quiescent X-ray emission would seem to power a broad flux range of ≲10⁻¹³ to ∼10⁻¹⁰ erg cm⁻² s⁻¹.

E.G., T.G., and F.Ö. acknowledge EU FP6 Transfer of Knowledge Project Astrophysics of Neutron Stars (MTKD-CT-2006-042722). D.E. acknowledges support from the Israel Science Foundation, the U. S. Israel Binational Science Foundation, and the Joan and Robert Arnow Chair of Theoretical Astrophysics.