A. Tiengo

AN XMM-NEWTON VIEW OF THE SOFT GAMMA REPEATER SGR 1806-20: LONG-TERM VARIABILITY IN THE PRE-GIANT FLARE EPOCH

The low-energy (<10 keV) X-ray emission of the soft gamma repeater SGR 1806-20 has been studied by means of four XMM-Newton observations carried out in the last two years, the latest performed in response to a strong sequence of hard X-ray bursts observed on 2004 October 5. The source was caught in different states of activity; over the 2003-2004 period, the 2-10 keV flux doubled with respect to the historical level observed previously. The long-term rise in luminosity was accompanied by a gradual hardening of the spectrum, with the power-law photon index decreasing from 2.2 to 1.5, and by a growth of the bursting activity. The pulse period measurements obtained in the four observations are consistent with an average spin-down rate of 5.5 × 10-10 s s-1, higher than the values observed in the previous years. The long-term behavior of SGR 1806-20 exhibits the correlation between spectral hardness and spin-down rate previously found only by comparing the properties of different sources (both SGRs and anomalous X-ray pulsars [AXPs]). The best-quality spectrum (obtained on 2004 September 6) cannot be fitted by a single power law but requires an additional blackbody component [kTBB = 0.79 keV, RBB = 1.9 (d/15 kpc)2 km], similar to the spectra observed in other SGRs and AXPs. No spectral lines were found in the persistent emission, with equivalent width upper limits in the range 30-110 eV. Marginal evidence for an absorption feature at 4.2 keV is present in the cumulative spectrum of 69 bursts detected in 2004 September-October.

Three XMM-Newton observations of the anomalous X-ray pulsar 1E 1048.1–5937: Long term variations in spectrum and pulsed fraction

We report the results of a recent (July 2004) XMM-Newton Target of Opportunity observation of the Anomalous X-ray Pulsar 1E 1048.1-5937 together with a detailed re-analysis of previous observations carried out in 2000 and 2003. In July 2004 the source had a 2-10 keV flux of 6.2 × 10 −12 erg cm −2 s −1 and a pulsed fraction PF = 0.68. Comparison of the three data sets shows the presence of an anti-correlation between flux and pulsed fraction, implying that previous estimates of the source energetics based on the assumption of a large and constant pulsed fraction might be significantly underestimated. The source spectrum is well described by a power law plus blackbody model (kT ∼ 0.63 keV, photon index Γ ∼ 2.7-3.5) or, alternatively, by the sum of two blackbodies of which the hotter is Comptonized by relativistic electrons. In this case the temperatures are kT 1 ∼ 0.2-0.3 keV and kT 2 ∼ 0.4-0.5 keV and the emitting area of the cooler component is consistent with the whole neutron star surface. The long-term luminosity variation of a factor >2 is accompanied by relatively small variations in the spectral shape. Phase-resolved spectroscopy indicates a harder spectrum together with the pulse maximum. No spectral features have been detected with 4σ limits on the equivalent width in the range ∼10-220 eV, depending on line energy and width.

Pronounced long term flux variability of the Anomalous X-ray Pulsar 1E 1048.1-5937

We present XMM-Newton and Chandra observations of 1E 1048.1-5937, being the first to show evidence for a significant variation in the X-ray luminosity of this anomalous X-ray pulsar (AXP). While during the first XMM-Newton (2000 December) and Chandra (2001 July) observations the source had a flux consistent with that measured on previous occasions (~5 × 10-12 ergs cm-2 s-1), two more recent observations found it at a considerably higher flux level of 2 × 10-11 ergs cm-2 s-1 (2002 August; Chandra) and 10-11 ergs cm-2 s-1 (2003 June; XMM-Newton). All the spectra are fit by the sum of a blackbody with kT ~ 0.6 keV and a power law with photon index ~3. No significant changes were seen in the spectral parameters, while the pulsed fraction in the 0.6-10 keV energy range decreased from ~90% in 2000 to ~53% in 2003. The spectral invariance does not support the presence of two physically distinct components in the AXP emission. The sparse coverage of the data does not permit us to unambiguously relate the observed variations to the two bursts seen from this source in the fall of 2001.

Correlated Infrared and X-ray variability of the transient Anomalous X-ray Pulsar XTE J1810-197

We report on observations aimed at searching for flux variations from the proposed IR counterpart of the Anomalous X-ray Pulsar (AXP) XTE J1810−197. These data, obtained in March 2004 with the adaptive optics camera NAOS-CONICA at the ESO VLT, show that the candidate proposed by Israel et al. (2004a, ApJ, 603, L97) was fainter by ∆H = 0.7 ± 0.2 and ∆Ks = 0.5 ± 0.1 with respect to October 2003, confirming it as the IR counterpart of XTE J1810−197. We also report on an XMM-Newton observation carried out the day before the VLT observations. The 0.5−10 keV absorbed flux of the source was 2.2 × 10 −11 erg cm −2 s −1 , which is less by a factor of about two compared to the previous XMM-Newton observation on September 2003. Therefore, we conclude that a similar flux decrease took place in the X-ray and IR bands. We briefly discuss these results in the framework of the proposed mechanism(s) responsible for the IR variable emission of AXPs.

XMM-Newton observations of the Soft Gamma Ray Repeater SGR 1627-41 in a low luminosity state

The sky region containing the soft gamma-ray repeater SGR 1627-41 has been observed three times with XMM-Newton in February and September 2004. SGR 1627-41 has been detected with an absorbed flux of ~9x10^{-14} erg cm^{-2} s^{-1} (2-10 keV). For a distance of 11 kpc, this corresponds to a luminosity of ~3x10^{33} erg s^{-1}, the smallest ever observed for a Soft Gamma Repeater and possibly related to the long period of inactivity of this source. The observed flux is smaller than that seen with Chandra in 2001-2003, suggesting that the source was still fading and had not yet reached a steady quiescent level. The spectrum is equally well fit by a steep power law (photon index ~3.2) or by a blackbody with temperature kT~0.8 keV. We also report on the INTEGRAL transient IGR J16358-4726 that lies at ~10 from SGR 1627-41. It was detected only in September 2004 with a luminosity of ~4x10^{33} erg s^{-1} (for d=7 kpc), while in February 2004 it was at least a factor 10 fainter.The sky region containing the soft gamma-ray repeater SGR 1627-41 has been observed three times with XMM-Newton in February and September 2004. SGR 1627-41 has been detected with an absorbed flux of similar to 9 x 10(-14) erg cm(-2) s(-1) ( 2-10 keV). For a distance of 11 kpc, this corresponds to a luminosity of similar to 3 x 10(33) erg s-1, the smallest ever observed for a Soft Gamma Repeater and possibly related to the long period of inactivity of this source. The observed flux is smaller than that seen with Chandra in 2001-2003, suggesting that the source was still fading and had not yet reached a steady quiescent level. The spectrum is equally well fit by a steep power law ( photon index similar to 3.2) or by a blackbody with temperature kT similar to 0.8 keV. We also report on the INTEGRAL transient IGR J16358-4726 that lies at similar to 10 from SGR 1627-41. It was detected only in September 2004 with a luminosity of similar to 4 x 10(33) erg s(-1) ( for d = 7 kpc), while in February 2004 it was at least a factor 10 fainter.

The X-ray afterglow of GRB 030329

We report on XMM-Newton u2000and Rossi-XTE u2000observations of the bright (fluence ~10 -4 xa0ergu2009cm -2 ) and nearby (

The Island State of the Atoll Source 4U 1820–30

z=0.1685

A First Look with Chandra at SGR 1806?20 after the Giant Flare: Significant Spectral Softening and Rapid Flux Decay

) Gamma-Ray Burst GRBxa0030329 associated to SN2003dh. The first Rossi-XTE u2000observation, 5 hours after the burst, shows a fluxu2029decreasing with time as a power law with index

Late evolution of the X-ray afterglow of GRB 030329

0.9pm0.3

The calm after the storm: XMM-Newton observation of SGR 1806–20 two months after the Giant Flare of 2004 December 27

. Such a decay law is only marginally consistent with a further Rossi-XTE measurement (at