A strongly magnetized pulsar within grasp of the Milky Way's supermassive black hole

I. Introduction

Among the large variety of Galactic neutron stars, magnetars constitute the most energetic and unpredictable class (Mereghetti 2008). They are X-ray pulsars rotating at relatively long periods (0.3–12 s, with $\dot{P}\sim 10^{-15}$–$10^{-10}$ s s-1), and persistent X-ray luminosities of $L_{\rm X}\sim 10^{33}$–$10^{35}$ ${\rm erg\,s}^{-1}$. They exhibit flaring activity, generally classified as giant flares ($\sim 10^{46}$–$10^{47}$ erg emitted in several minutes), intermediate flares ($10^{42}$–$10^{45}$ erg in a few minutes) or short X-ray bursts ($10^{38}$–$10^{40}$ erg in less than a second), as well as large increases of the persistent flux (outbursts) which can last about 1 yr (Rea & Esposito 2011). The powerful emission observed from these objects has been attributed to their exceptionally high magnetic field ($B\sim 10^{14}$–$10^{15}$ G at the star surface), hence the name magnetars (Duncan & Thompson 1992; Thompson & Duncan 1993).

Analysis of stellar orbits has demonstrated that a supermassive black hole (SMBH) of about $4.3\times 10^{6}$ $M_{\odot}$ resides at the dynamic center of our Galaxy (Ghez et al. 2008; Gillessen et al. 2009). The black hole is associated with Sgr A∗, a compact source with non-thermal emission in radio, infrared, and X-rays (Melia & Falcke 2001; Baganoff et al. 2003; Genzel et al. 2010). X-ray fluorescence lines from numerous molecular clouds near the Galactic center have been interpreted as evidence of interaction with a light echo tracing a past bright state of Sgr A∗ (Ponti et al. 2013).

On 2013 April 24, Swift detected powerful X-ray emission from the direction of Sgr A∗ (Degenaar et al. 2013; Kennea et al. 2013), initially interpreted as an X-ray flare from the SMBH. One day later, a short X-ray burst was observed from a position consistent with that of Sgr A∗, very similar in fluence, duration and spectrum to those commonly observed from magnetars. The magnetar picture received further support when NuSTAR observed the region, and detected a persistent source with periodic modulation at 3.76 s (Mori et al. 2013), a typical value for magnetar spin periods.

In this Letter we report on X-ray (Chandra and Swift) and radio (Robert C. Byrd Green Bank Telescope (GBT) and Parkes Radio Telescope) observations of SGR J1745–2900, which point to a likely physical connection of the source with Sgr A∗.

II. X-ray observations

Chandra (Weisskopf et al. 2003) observed SGR J1745–2900 for the first time on 2013 April 29 with the High Resolution Camera (HRC-S), and three other times in the following three months with the Advanced CCD for Imaging Spectrometer (ACIS-S; see Table 1). The HRC-S instrument (timing mode with a 0.14” pixel size) observed SGR J1745–2900 for about 10 ks. The three ACIS-S observations were performed in faint data mode with the 1/8 chip sub-array (the source was positioned in the back-illuminated ACIS-S3 CCD at the nominal target position; time resolution 0.441 s). The data were reduced following standard procedures using the Chandra Interactive Analysis of Observations software (CIAO, version 4.5) and the calibration database CALDB 4.5.6.

After the discovery of SGR J1745–2900, Swift observed its field almost daily (see also Degenaar et al. 2013; Kennea et al. 2013). In this work we use all the observations performed between TJD 16406 and 16480, taken in both photon counting (PC) and windowed timing (WT) modes. Only the WT observations could be used for the timing analysis of SGR J1745–2900 (Section II.2; readout time of 1.7 ms), owning to the slow read out time of the PC mode (2.5 s). The data were processed and filtered with standard criteria using Ftools within HEAsoft software package (v6.12).

For the ACIS observation, the spectra, ancillary response files and spectral redistribution matrices were created using the CIAO script specextract. For the XRT observations, we used the latest available spectral redistribution matrix in CALDB (v013/v014), while the ancillary response files were generated with xrtmkarf, and they account for different extraction regions, vignetting and PSF corrections.

We have converted all photon arrival times to the Barycentric Dynamical Time (TDB) system, using the accurate position derived from the Chandra observations (see below).

In the ACIS-S observations a number of sources were detected besides SGR J1745–2900. In particular, four known sources present in an X-ray catalog (Muno et al. 2009) were located within $\sim$$30^{\prime\prime}$ from the SGR and could be employed to refine the absolute astrometry. We used the four source positions, calculated with wavdetect, to register the ACIS-S images on the catalog by optimizing a roto-translation (using the CIAO script reproject_aspect). The fit yielded an average rms of $\sim$80 mas. The estimated coordinates (taking into account also the accuracy of the reference positions) of SGR J1745–2900 are: $\rm RA=17^{h}45^{m}40\fs 169$, $\rm Dec=-29^{\circ}00^{\prime}29\farcs 84$ (J2000.0) with a 95% confidence level uncertainty radius of $0\farcs 3$ (see Fig. 1).

II.2. X-ray timing analysis

Because of the complex shape and variability of the three peaked pulse profile (Fig. 2), we decided not to use a pulse template for the timing analysis (which might artificially affect the phase shift), using instead two parallel methods: fitting a sinusoid to the profile at the fundamental period, and fitting the highest peak in all the observations with a Gaussian. We used 12 phase bins to produce a folded pulse profile. We built up our timing solution from the phases of the second Chandra pointing which has the highest number of counts. The resulting best-fit period for this Chandra observation was $P=3.763554(2)$ s (at TJD 16424.5509871; TJD = JD – 2440000.5 days). The accuracy on this measure of the period, 2 $\mu$s, is enough to coherently phase-connect adjacent observations. At each step, we checked that the timing solution was accurate enough to determine univocally the phase of the following observation. To this end, we propagated the error affecting the best-fitting parameters of the solution obtained at any step, to the epoch of the newly added observation. We never obtained a phase uncertainty larger than 0.4, which allowed us to tentatively maintain the phase connection. The phase-coherent solution obtained considering the phases of the best-fit fundamental harmonic component has a period $P=3.7635537(2)$ s and period derivative $\dot{P}=6.61(4)\times 10^{-12}$ s s-1 (epoch TJD 16424.5509871), with a reduced chi-squared of 0.85 for 5 degrees of freedom (dof). On the other hand, when comparing this results with the timing solution derived considering the phases of the best-fit Gaussian to the main peak, we find compatible numbers for the period and the period derivative, but with a marginal evidence for a cubic component (resulting in a possible $|\ddot{P}|=4(3)\times 10^{-19}$ s s-2).

II.3. X-ray spectral analysis

In the Chandra observations, the pileup in the ACIS-S3 detector was $\sim$5–15$\%$, therefore an annular region with $0.75^{\prime\prime}$ inner radius (1.5 pixels) and $2^{\prime\prime}$ outer radius was selected to extract the source spectrum. For the background we used an annular region of maximum radius $8^{\prime\prime}$ around the source. The point source spectrum was rebinned to have at least 50 counts per energy bin.

The X-ray flux decay can be modeled with an exponential function of the form $F(t)=F_{\circ}\,e^{(-(t-t_{\circ})/\tau)}$ ($\chi^{2}_{\nu}=1.44$ for 80 dof). We fixed $t_{\circ}$ at the time of the first burst detected; the resulting best-fit parameters are $F_{\circ}=1.72(3)\times 10^{-11}$ erg cm-2 s-1 and $e$-folding time $\tau=$144(8) days. This is a rather slow flux decay as compared with other outbursts (Fig. 1).

III. Radio observations

We have also started a monitoring campaign in the radio band using the 64-m Parkes (NSW, Australia) and the 100-m GBT (WV, USA) radio telescopes to study the magnetar’s radio emission (Eatough et al. 2013). Given the very high electron column density expected in the central regions of the Galaxy we chose to observe at 3.1 GHz (over a bandwidth of 1024 MHz split into 512 frequency channels) at Parkes, and at 2 and 8.9 GHz (with a bandwidth of 800 MHz split into 2048 frequency channels) at GBT (see Table 1). At Parkes observations were performed using the ATNF digital filterbank (DFB4; Ferris & Saunders 2004) in search mode, 2-bit sampling the data every 125 $\mu$s, while at GBT the Green Bank Ultimate Pulsar Processing Instrument (GUPPI; DuPlain et al. 2008) was used in search mode, 8-bit sampling the data every 64 $\mu$s. For a faster analysis, GBT data were downsampled in time by a factor of 4 and in frequency by a factor of 8. Data show that at 2–3 GHz, the magnetar switched on between 2013 April 28 and 29. During the Parkes observation the magnetar pulse profile showed a two peaks profile, that evolved into a single one in all subsequent detections (Fig. 2). From the 10-cm Parkes observations, we obtained a dispersion measure $\rm{DM}=1750\pm 50$ pc cm-3 measured using the Tempo2 software package (consistent with the value derived by Eatough et al. 2013; Shannon & Johnston 2013).

IV. discussion

Radio observations performed with Parkes and GBT detected the magnetar radio emission (Eatough et al. 2013), with the flux and pulse profile variability typical of radio magnetars. The dispersion measure derived from Parkes observations at 10 cm, $DM=1750\pm 50$ pc cm-3, is the highest ever measured for a radio pulsar, and implies a distance of $\sim$8.3 kpc (Cordes & Lazio 2002). The relatively high $\dot{E}\sim 5\times 10^{33}$ ${\rm erg\,s}^{-1}$of SGR J1745–2900 compared with the deep limit of its quiescent emission derived by several Ms of Chandra observations ($\sim 10^{32}$ ${\rm erg\,s}^{-1}$in the 2–10 keV range; Muno et al. 2005) supports the predictions of the fundamental plane for radio magnetars (Rea et al. 2012).

On the other hand, estimates of the neutron star population in the Galaxy (Freitag et al. 2006) suggest that there are about $2\times 10^{4}$ neutron stars within 1 pc from the Galactic center. Since this is the total population born during the entire lifetime of the Galaxy (about a Gyr), a new neutron star is born in the same region every $\sim 10^{5}$ yr. Even on an observational ground alone, about 80 ordinary radio pulsars (Wharton et al. 2012) are expected to be visible with 1 pc from Sgr A∗, compatible with us seeing only one young radio pulsar with a characteristic age $<$10 kyr . The magnetar nature of this young radio pulsar so close to Sgr A∗ suggests that there might possibly be as many magnetars as ordinary pulsars in the Galaxy (Rea et al. 2010), and/or the Galactic center region is a favorable place for magnetar formation, possibly because of its high density of very massive stars.

The projected position of SGR J1745–2900 places it within the disk of young and massive stars observed within 0.5 pc of Sgr A∗ (Paumard et al. 2006; Lu et al. 2009), which is most likely its birthplace (see Fig. 4). SGR J1745–2900 is probably the end product of one of the young massive stars born during a recent star formation activity within the dense gaseous disk around Sgr A∗, now accreted into the black hole (Levin & Beloborodov 2003).

IV.2. The possible imprint of SGR J1745–2900’s past activity on the Galactic center Fe fluorence

V. conclusions

SGR J1745–2900 is the first pulsar discovered at a parsec distance from a supermassive black hole with a non-negligible probability of being in a bound orbit. Future measurements of the magnetar proper motion and, possibly, spin-down variability (the contribution to the magnetar observed $\dot{P}$ due to the acceleration imparted by Sgr A∗’s gravitational well can be up to $\sim 8\times 10^{-13}$ s s-1), will be key in observing the effects of the supermassive black hole’s gravitational potential on the magnetar’s evolution .

References