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Accurate X-ray position of the Anomalous X-ray Pulsar XTE J1810-197 and identification of its likely IR counterpart

We report the accurate sub-arcsec X-ray position of the new Anomalous X-ray Pulsar (AXP) XTE J1810-197, derived with a Chndra-HRC Target of Opportunity observation carried out in November 2003. We also report the discovery of a likely IR counterpart based on a VLT (IR band) Target of Opportunity observation carried out in October 2003. Our proposed counterpart is the only IR source (Ks=20.8) in the X-ray error circle. Its IR colors as well as the X-ray/IR flux ratio, are consistent with those of the counterparts of all other AXPs (at variance with field star colors). Deep Gunn-i band images obtained at the 3.6m ESO telescope detected no sources down to a limiting magnitude of 24.3. Moreover, we find that the pulsed fraction and count rates of XTE J1810-197 remained nearly unchanged since the previous Chandra and XMM-Newton observations (2003 August 27th and September 8th, respectively). We briefly discuss the implications of these results. In particular, we note that the transient (or at least highly variable) nature of this AXP might imply a relatively large number of hidden members of this class.


INTRODUCTION
Despite their limited number, Anomalous X-ray Pulsars (AXPs) are one of the most intensively studied galactic high energy sources. Since these sources were first proposed as a separate class (Mereghetti & Stella 1995, van Paradijs et al. 1995, their number has grown slowly and there are now five confirmed AXPs plus two candidates. These sources are relatively slowly-rotating, spinning-down, radio-quiet X-ray pulsars with no evident signature for binary motion, and X-ray luminosity exceeding the rotational energy loss by a large factor (see Mereghetti et al. 2002 for a review). AXPs have been linked to SGRs because of similar timing properties, namely spin periods (P in the 5-12s range) and large period derivatives (Ṗ ∼ 10 −11 − 10 −12 s/s). It is now commonly believed that Soft γ-ray Repeaters (SGRs) are magnetars, isolated neutron stars powered by the decay of theirstrong, super-critical magnetic fields of B>10 14 Gauss (Duncan & Thompson 1992; Thompson & Duncan 1995;Kouveliotou et al. 1998). The same model has been applied to AXPs. Nonetheless, there is a growing group of radio pulsars McLaughlin et al. 2003) with comparably long periods and inferred magnetic field strengths approaching 10 14 G. These radio pulsars possess no special attribute linking them to either AXPs (no steady bright quiescent X-ray emission; Pivovaroff, Kaspi & Camilo 2000) or SGRs (no bursting episodes). Thus periodicity alone does not appear to be a sufficient attribute for classification. Correspondingly, a very high magnetic field strength cannot be the sole factor governing whether a neutron star is a magnetar, a radio pulsar or an accreting neutron star.
The recent detection of X-ray bursts from 1E 2259+586 and 1E 1048.1-5937 has strengthened the connection of AXPs with SGRs (Kaspi & Gavriil 2002;Gavriil et al. 2002). At the same time, the study of the former sources resulted in the identification of new observational properties. In the case of 1E 2259+584, IR variability of the counterpart was detected a few days after an episode of strong X-ray bursting activity . IR variability of the counterpart to 1E 1048.1-5937 was also detected; this might be related to the X-ray variability which was observed from this source . These findings have opened a new perspective in the field, challenging the predictions of current models. It is yet clear which specific physical parameter(s) differentiate(s) AXPs from SGRs (if any).
In this paper, we present the results from Chandra and VLT Target of Opportunity observations of XTE J1810−197 that we obtained in the fall of 2003. These led to a subarcsec accurate X-ray position for XTE J1810−197 and to the identification of its likely IR counterpart. Preliminary results from this study were reported in Israel et al 2003c(posted on 2003. We found that the optical/IR colours and broad band spectrum of this pulsar are similar to those of AXPs, strongly suggesting that XTE J1810−197 belongs to the same class. Based on these findings and on the X-ray variability previously reported for this pulsar (Ibrahim et al. 2003), we conclude that XTE J1810−197 represents the first confirmed AXP showing a transient behavior (in the X-ray band). Data were reduced with CIAO 3.0.1 and analysed with standard software packages for X-ray data (Ximage, Xronos, etc.). Only one source was detected in the HRC-I (see Israel et al. 2002 for details on the detection algorithm). The source has the following coordinates: R.A. = 18 h 09 m 51. s 08, DEC. = -19 • 43 ′ 51. ′′ 74 (statistical uncertainty of 0. ′′ 13; equinox J2000), with a total uncertainty circle radius of 0. ′′ 7 (90% confidence level; Israel et al. 2003c). The position is consistent with that of the previous Chandra observation (G03). Photon arrival times were extracted from a circular region with a radius of 1. ′′ 5, including more than 90% of the source photons, and corrected to the barycenter of the solar system. Coherent pulsations at a period of about 5.5 s were detected, confirming that the source was indeed XTE J1810−197 . To refine the period determination we adopted a phase fitting technique. The best pulse period was determined to be P=5.5391±0.0004 s (90% confidence level). The pulsed fraction in the HRC-I energy band was 49±2% (semi-amplitude of modulation divided by the mean source count rate). The latter value is consistent with that reported by G03 (46±3% using our pulsed fraction definition). The source count rate was 1.04±0.04 ct s −1 , marginally lower than the previous Chandra observations carried out 66 days earlier (0.96±0.04 ct s −1 ; G03).
Finally, the spatial profile was found to be in good agreement with the expected Chandra Point Spread Function (PSF) for an on-axis source (see Israel et al. 2002 for details).
3. OPTICAL/IR OBSERVATIONS Data were acquired at VLT-UT4 Yepun with the Nasmyth Adaptive Optics System and the High Resolution Near IR Camera (NAOS-CONICA) on 2003 October 7th. The pixel size of the camera is 0.027 ′′ and the FWHM is approximately 6 pixels. Images have been reduced with instrument-specific pipelines and then co-added. A total of 21 images (effective exposure time of each frame is 120 s obtained by averaging three exposures of 40 s on the chip) in H and Ks filters were used for the analysis. Zero-points were obtained by using the standard S234-E from the catalog of Persson et al. (1998) obtaining the following values: ZP(H)=23.80, ZP(Ks)=22.91. Thanks to adaptive optics we obtained an on-axis source PSF of ∼0. ′′ 15 and we inferred a limiting magnitude (S/N∼3) of 22.5 (H) and 21.5 (Ks). A preliminary catalog for the objects in the field was obtained by co-adding H and Ks images. Magnitudes and positions of detected stars in the coadded images were derived by means of aperture photometry and centroid determination. (Stetson 1990).
Optical observations were carried out on 2003 October 12th with EFOSC2 at the 3.6m ESO telescope (La Silla). The night was clear with variable seeing (between 0.7" and 1.5"). The XTE J1810−197 field was imaged with 21 exposures of 240s each using the Gunn-i filter (central wavelength 803.4nm; FWHM of 151nm). The zero point for the night is extracted from the tabulated Johnson and Cousins I magnitudes for the standard field, and its value is: ZP(I)=23.85 (Landolt 1992). Data were flat field corrected and fringe pattern subtracted. Using a S/N=3 threshold, a PSF area of 11 pixels and the average of the tabulated extinction values for La Silla, the I limiting magnitude was 24.30±0.03.
To register the Chandra coordinates of XTE J1810−197 on our IR images, we computed the image astrometry using, as a reference, the positions of stars selected from the GSC2.2 and 2MASS catalogs which has an intrinsic absolute accuracy of about 0. ′′ 2-0. ′′ 4 (for GSC2.2 and depending on magnitude and sky position of stars). After taking into account the uncertainties in the source X-ray coordinates (0. ′′ 7), the rms error of our astrometry (0. ′′ 06) and the propagation of the intrinsic absolute uncertainties on the catalog coordinates (we assumed a value of ∼0. ′′ 3), we estimated an accuracy of about 0. ′′ 8 to be attached to the XTE J1810−197 position. Figure 1 shows a region of 18 ′′ ×18 ′′ around the XTE J1810−197 position in the Ks band VLT image (the 1σ confidence level uncertainty circle reported by G03 is also superimposed).

DISCUSSION
The 3.6m ESO data show no object within the new Chandra uncertainty circle down to a I limiting magnitude of 24.3 (S/N∼3). On the other hand, only one faint point-like object is present in the VLT H and Ks frames (marked X1 in Figure 1; H=22.0±0.1, Ks=20.8±0.1). Note that the probability of finding by chance an object unrelated to the X-ray source within the Chandra uncertainty region is of the order of ∼50% (number of detected sources normalised to the area of the Chandra error circle). No further object was detected in the Chandra circle down to a limiting Ks magnitude of about 22.5 (S/N∼1.5). The colors of the IR source, H − Ks = 1.2 and i − Ks > 3.5 are at variance with those of field stars which lie far from nearby stars in a color-color diagram. On the contrary the IR colors are to those of other AXP counterparts. As an example, the persistent optical/IR emission of 4U 0142+614 has H − Ks = 1.1 and I − Ks while the "outbursting" IR emission of 1E 1048.1−5937 has H − Ks = 1.4 (Wang & Chakrabarty 2002; note however that the extinction in the direction of the three objects is different). These findings make the association of this object with XTE J1810−197 quite probable.
In order to further test the hypothesis of the AXP nature of XTE J1810−197 , we studied the IR-to-X-ray spectrum. We retrieved the XMM-Newton archival data on XTE J1810−197 obtained on 2003 September 8th and extracted and fitted the EPIC-PN spectrum by adopting power-law plus blackbody spectral model described by G03. We plotted the IR/optical through X-ray data of XTE J1810−197 in Figure 2. A value of A V =5.9±0.3 was used to infer the unabsorbed IR fluxes and their uncertainties (this was derived from the N H inferred from the XMM-Newton spectra and A V = N H / (1.79×10 21 cm −2 ); Predehl & Schmitt 1995). Note that the count rate, and thus flux, of the second Chandra dataset of XTE J1810−197 was nearly unchanged with respect to the previous Chandra and XMM-Newton observations; we are thus justified in combining the IR-to-X-ray measurements plotted in Figure 2 (we assumed that the spectral parameters did not change, as suggested also by the constant pulsed fraction level between the two Chandra observations). It is evident from Figure 2 that the F X over F IR ratio is larger than 10 3 and similar to those of other AXPs (for a comparison see Figure 2 of Israel et al. 2003b; note however that the F X /F IR ratio is also consistent with that of low-mass X-ray binaries as in the case of all the other AXPs).
The relatively high pulsed fraction of XTE J1810−197 is not unusual for AXPs: in fact 1E 1048.1−5937 has a higher pulsed fraction and additionally, shows flux variability both in the Xray and IR bands (Oosterbroek et al. 1998). All the above findings and similarities with known members of the AXP class, clearly indicate that XTE J1810−197 is an AXP, the one possessing the highest degree of X-ray flux variability seen so far (a factor of about 100 between quiescent and outburst peak fluxes). The candidate AXP AX J1844−0258 might be another example of variable/transient AXP (TAXP; Torii et al. 1998;Gotthelf & Vasisht 1998). However, AX J1844−0258 was caught in a high state only once and noṖ measurement is available in order to definitively assess the AXP nature of this source. We note that the quiescent XMM-Newton and BeppoSAX spectrum of AX J1844−0258 has a 0.5-10 keV absorbed flux of ∼3×10 −13 erg cm −2 s −1 (Israel et al 2003b), quite similar to that of XTE J1810−197 as seen by ROSAT in 1993 (∼5×10 −13 erg cm −2 s −1 ; G03). Additionally, as in the case of XTE J1810−197 , also for AX J1844−0258 no pulsations were detected in the quiescent phase (although poor statistics prevented to set any sensitive upper limit; Israel et al. 2003d).
Regardless of whether or not AX J1844−0258 is an AXP, the existence of at least one TAXP clearly points to a larger number of hidden members of the AXP class in the Galaxy. As suggested by G03, part of them might be the radio-quite X-ray unpulsed Central Compact Objects (CCOs) found in an increasing number of SNRs. Other AXPs might spend a large fraction of the time in a quiescent state, and therefore might remain unidentified as AXPs. There are at least two important new facts that should be taken into account in the comparison with models: (i) the X-ray flux variability of more than two orders of magnitude, and (ii) the non-detection of X-ray pulsations in the quiescent state of XTE J1810−197 during a 1996 ROSAT observation. Variations in the persistent X-ray emission are common in neutron stars accreting from a companion (White et al. 1995). The IR measurements presented in this paper for XTE J1810−197 rule out any hypothetical main sequence companion star from O to F spectral-types, and are comparable to those set for other, more "standard", AXPs Wilson et al 1999). However, a lighter companion cannot be ruled out (as in the case of all the other AXPs) and would imply an extremely small and virtually undetectable Doppler shift in the pulsations (similar e.g. to the 42 minutes orbital period binary system hosting the 7s pulsar 4U 1626-67; Chakrabarty 1998).
If the two transient objects discovered so far do belong to the same class of SGRs and AXPs, then one would like to interpret the observations within the context of the magnetar model. The magnetar model as currently formulated does not make specific predictions for on/off states of the pulsars as well as for their IR fluxes. Variations in the quiescent emission have been observed following an X-ray burst in 1E 2259+586 . Soon after the burst, the persistent X-ray flux was a factor of about 20 higher, the temperature also higher, and the blackbody radius much smaller than in the low quiescent state. Some of the 1E 2259+586 properties in the quiescent emission before and after the burst are consistent (at least qualitatively), with the those of XTE J1810−197 in its low and high state. Moreover, the slow decay of the "high-state" X-ray emission of XTE J1810−197 is found to be in the range of those of SGR 1627-41 and SGR 1900+14 (Ibrahim et al. 2003;Kouveliotou et al. 2003) for a power-law decay; therefore we cannot rule out that XTE J1810−197 is a SGR. In this respect, we note that if XTE J1810−197 were proven to be a SGR our proposed IR candidate would be the first ever for a SGR.
If XTE J1810−197 were an isolated object, then (a hypothetical) accretion would have to proceed through a fall-back disk (Chatterjee et al. 2000, Alpar 2001). While short-term, small scale fluctuations are expected in this case, by analogy with most accreting neutron star systems, large scale variations might require special conditions. In particular, as the object spins down, it might occasionally switch from a propeller phase to an accretion phase. During the propeller phase, accretion is inhibited, and the star should be bright in X-rays due to its thermal emission. Indeed, in its low state, XTE J1810−197 had an X-ray luminosity of the order of a few ×10 33 d 5 erg/s, which is typical of other known thermal emitting sources (e.g. Becker & Trümper 1997). Moreover, in its low state the pulsed fraction of XTE J1810−197 (if any) decreased considerably, suggesting emission from the whole surface of the star as expected in a cooling object (G03). Note that in a more general scenario, the CCOs would be AXPs in the above described state. If the high state is due to resumed accretion onto the magnetic poles, then larger pulsed fractions would be naturally explained. The accretion luminosity could be much higher than the thermal emission, and would dominate the emitted spectrum. These properties appear consistent with those of XTE J1810−197 , and the properties of the IR counterpart could be explained in terms of a fall-back disk of size a few ×10 10 cm (using the spectral models of . However, the X-ray flux decay reported by Ibrahim et al. (2003) would be hardly accounted by the fall-back model unless the accretion rate were decreasing very rapidly. Moreover, the above active phase is expected to occur just once in the AXP life (with a duration longer than the about one year observed in XTE J1810−197 and possibly AX J1844−0258 : Chatterjee et al. 2000).
The pronounced long-term X-ray flux and pulsed fraction variability of XTE J1810−197 might be more easily explained in the framework of a pulsar in a binary system with a light companion. In fact, we note that the quiescent luminosity of XTE J1810−197 (and AX J1844−0258 ) is similar to that already observed from transient binary system pulsars (Campana et al. 2002), together with the pulsed fraction decrease in the on/off transitions (Campana et al. 2001). However, in the binary scenario, the short X-ray bursts displayed by AXPs and, especially, SGRs would be difficult to explain (see Mereghetti et al. 2002 for more details).
We thank Harvey Tananbaum for making the Chandra HRC-I observation possible through the Chandra Director's Discretionary Time program, and Michael Juda for help during the Chandra scheduling phase. We also thank the ESO Director's Discretionary Time Committee for accepting to extend our original program allowing us to trigger the VLT observations, and Lowell Tacconi-Garman for his help and patience during the OB submission phase. This work is partially supported through ASI and Ministero dell'Università e Ricerca Scientifica e Tecnologica (MURST-COFIN) grants.