Astron. Nachr. /AN 335, No. 3, 296 – 300 (2014) / DOI 10.1002/asna.201312035

A classification scheme for magnetars

E. Gogus1,�

Sabancı University, Faculty of Engineering and Natural Sciences, Orhanlı− Tuzla, Istanbul, 34956 Turkey

Received 2013 Dec 20, accepted 2014 Jan 14Published online 2014 Mar 16

Key words gamma rays: bursts – stars: neutron – stars: pulsars – X-rays: bursts

We investigate cumulative burst occurrence trends of five soft gamma repeaters (SGRs) and introduce an indicative mea-sure for the duration of their outburst phases. We find a clear distinction between the durations of the active episodes ofpersistent and transient magnetars: The outburst episodes of the persistent sources are significantly longer (>

∼100 days)

than those of transient sources (<∼

10 days). We also investigate burst-induced changes in the radiative behavior of transientsources and suggest a mechanism for the constant flux trend seen in the early phases of the outburst of SGRs with lowburst rates.

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1 Introduction

Soft gamma repeaters (SGRs) are neutron stars that arecharacterized by emitting unique bursts in hard X-rays andsoft γ-rays. SGRs occasionally enter burst active episodes,during which they can emit anywhere from a few to thou-sands of short bursts; these typically last for a fraction of asecond but the energy released during such a short time isvery large, ranging from ∼1037 erg to 1040 erg (see, e.g.,Gogus et al. 2001; van der Horst et al. 2011). On very rarecircumstances, SGRs emit extremely energetic giant flareswhich last a few hundreds of seconds and having isotropi-cally released energy in excess of 1044 erg. Only three giantflares have been observed to date (Mazets et al. 1979; Hur-ley et al. 1999; Palmer et al. 2005). SGRs can also emit in-termediate events whose energy is in between typical shortbursts and giant flares (Lenters et al. 2003; Mazets et al.1999; Gogus et al. 2010). During burst active phases, someSGRs exhibit increased X-ray emission in conjunction withtheir outburst.

Anomalous X-ray pulsars (AXPs) are also neutron stars,resembling SGRs in timing and X-ray spectral behaviorof their persistent emission, as well as occasional emis-sion of energetic bursts (Gavriil et al. 2002; Kaspi et al.2002). Bursts from AXPs, however, are usually short du-ration (<∼ few seconds), seen in X-rays and less energeticthan those of SGRs (see, e.g., Gavriil et al. 2004). Similarto SGRs, burst activity in AXPs may elevate their persistentX-ray emission (Kaspi et al. 2002; Israel et al. 2006), whilesome AXPs do not exhibit any significant persistent X-rayincrease after emitting burst (Lin et al. 2011).

Both steady X-ray emission and energetic bursts are ex-pected from extremely magnetized neutron stars (or mag-

� Corresponding author: ersing@sabanciuniv.edu

netars.1 Within the context of the magnetar model, the de-cay of very strong magnetic fields (10

14–1015 G) can power

the persistent emission from SGRs and AXPs (Thompson& Duncan 1996; Thompson, Lyutikov & Kulkarni 2002),while the observed bursts can be either due to cracking ofthe neutron star crust that is strained by magnetic stress(Thompson & Duncan 1995) or to magnetic field line re-connection (Lyutikov 2002).

Since August 2008, six new magnetar candidates havebeen discovered. Five of these new sources have intrigu-ing differences with the previous members of the magne-tar family: (i) They were all discovered by emitting typicalSGR-like short bursts, but they became inactive after oneor two events; (ii) at least two sources possess an inferreddipole magnetic field lower than the quantum critical limitof 4.4×1013 G and the inferred magnetic field strengths ofother three are near the lowest value among SGRs. In Fig. 1we present the P -P diagram of radio pulsars, RRATS, mag-netars and these five sources. Finally, (iii) their X-ray spec-tra are different than the rest of the magnetar sources; theyare typically well described by a single blackbody func-tion with a temperature around 1 keV. These new sourceswere also shown to have transient nature (Rea & Esposito2011): their quiescent flux levels are usually near or be-low our detection capabilities. They are identified as tran-sient magnetars and detected only when they undergo anoutburst episode. During these outbursts, their X-ray powercan be enhanced by >

∼ 100 fold and subsequently declinesto pre-outburst level over a time frame of months to fewyears. Their positions on the P -P diagram, as well as theirspectral and temporal characteristics suggest that these fivemagnetar candidates represent a subgroup which emerges

1 Soft gamma repeaters and anomalous X-ray pulsars form the familyof magnetars; see Woods & Thompson (2006) or Mereghetti (2008) for adetailed review.

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Astron. Nachr. /AN 335, No. 3 (2014) 297

10-2 10-1 1 10Period (s)

10-18

10-16

10-14

10-12

10-10

Per

iod

Der

ivat

ive

(s/s

)

RPRRATSGRAXP

Fig. 1 P -P distribution of radio pulsars (RP), anomalous X-raypulsars (AXP), soft gamma repeaters (SGR), and rotating radiotransients (RRAT). The upper and lower solid lines show the con-stant magnetic field lines of 1014 and 1013 G, respectively. Encir-cled SGRs are the five recently discovered sources mentioned inthe text.

as a crucial link for our understanding of the evolutionarypath between magnetars and various diverse sub-groups ofneutron stars.

Here, we investigate episodes of burst activity in a setof persistent (SGR 1806–20, SGR 1900+14) and transientmagnetars (SGR 1627–41, SGR J0501+4516, SGR J1550–5418). In particular, we define a new measure to estimatethe duration of a bursting phase of a source, and classifythem according to the duration of their outburst lengths.

2 Duration of burst active episodes

We define the 90 % of duration (D90) of the burst activeepisode of a magnetar as the duration over which 90 % ofthe all observed bursts are detected. In this scheme, the onsetof an active episode is set if there were more than five shortbursts observed from the same source within 24 hours. Allsubsequent bursts with waiting times less than one monthwere treated to be events within the same active episode.The D90 duration spans from the onset of the burst activ-ity to the 90th percentile of bursts. Note that we employthe 90th percentile to avoid consequences of rather arbitraryend setting of activity phases. We also required each activeepisode to contain at least ten bursts. We describe below ma-jor burst active episodes of a set of SGRs and present theircorresponding D90 durations.

Fig. 2 The cumulative burst counts vs. time for the 1983, 1996,1998, and 2004 active episodes of SGR 1806–20. The vertical dot-ted lines marks the D90 duration of each burst active episode.

2.1 SGR 1806–20

SGR 1806–20 is one of the most burst prolific magnetars: Itexhibited a major bursting episode in 1983, with which softgamma-ray repeaters were established as a group (Laros etal. 1987). The source was in major burst action again in1996, 1998, and 2004 prior to the emission of a giant flareon 2004 December 27. Here, we analyzed 93 bursts start-ing from 1983 October 20 with ICE, 53 bursts starting from1996 October 30 and 52 bursts from 1998 July 25 both de-tected with CGRO/BATSE and 318 bursts from 2004 May2 detected with the IPN instruments. We find the D90 ofduration of the 1983 activity as 136.8 days, the 1996 ac-tive episode as 311.2 days, the 1998 active episode as 291.9days and the 2004 burst active episode as 112.8 days. Wepresent in Fig. 2, the cumulative burst counts for each ofthese four activity periods as a function of time since theircorresponding onset.

2.2 SGR 1900+14

SGR 1900+14 is another prolific burster: Its major burst ac-tivity was observed in 1998, prior to the giant flare on 1998August 27. The source was also in major action in 2001,2002, and 2006, while no burst has been detected fromSGR 1900+14 since 2006 March. In this study, we exam-ined 204 bursts starting from 1998 May 26 detected withCGRO/BATSE and 32 bursts starting from 2002 August 30with the IPN instruments. We find the D90 of duration ofthe former bursting episode as 183.6 days, and that of the

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298 E. Gogus: Magnetar classification

Fig. 3 The cumulative burst counts vs. time for the 1998 and2002 active episodes of SGR 1900+14. The vertical dotted linesmark the D90 duration of each burst active episode.

latter episode as 86.1 days. We present the cumulative burstnumbers for these two activity episodes in Fig. 3.

2.3 SGR 1627–41

SGR 1627–41 was discovered in 1998 June with a stormof soft γ-ray bursts (Kouveliotou et al. 1998). The sourcewent into another burst active episode about 10 years af-ter the first observed one in 2008. We constructed the cu-mulative burst counts distribution for this SGR using 99bursts detected between 1998 June 15 and August 4 withCGRO/BATSE. We obtain the D90 of duration of the 1998active episode of SGR 1627–41 as 9.1 days (see Fig. 4).

2.4 SGR J0501+4516

SGR J0501+4516 was discovered on 2008 August 22 withbursts detected with Swift/BAT. Later, Fermi/GBM detecteda total of 29 bursts over the time frame of 12 days follow-ing the onset of the activity. In Gogus et al. (2010), we havenoted that this source was active on 1993 July 25, duringwhich it emitted two bursts on the same day. Here, we used29 bursts collected with GBM and found the D90 dura-tion of its latest active episode as 6.3 days. In Fig. 5, wepresent the trend of cumulative burst numbers vs. time forSGR J0501+4516.

2.5 SGR J1550–5418

SGR J1550–5418 was identified as an AXP based on itsspectral similarities to magnetars and its galactic loca-

Fig. 4 The cumulative burst counts vs. time for the 1998 activeepisode of SGR 1627–41. The vertical dotted line marks the D90duration of the burst active episode.

Fig. 5 The cumulative burst counts vs. time for the 2008 activeepisode of SGR J0501+4516. The vertical dotted line marks theD90 duration of the burst active episode.

tion (Gelfand & Gaensler 2007). SGR J1550–5418 wasalso the first AXP from which pulsed radio signals weredetected (Camilo et al. 2007). The source went into abrief episode of burst activity in 2008 October 8 as de-tected with Swift/BAT (Israel et al. 2010) and Fermi/GBM

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Astron. Nachr. /AN 335, No. 3 (2014) 299

Fig. 6 The cumulative burst counts vs. time for the 2009 activeepisode of SGR J1550–5418 . The vertical dotted line marks theD90 duration of the burst active episode.

(von Kienlin et al. 2012). The major bursting episode ofSGR J1550–5418 started on 2009 January 22, when morethan 400 bursts were detected on the day of the onset. Herewe investigated 534 burst data collected with Fermi/GBMfrom the onset of its 2009 outburst until 2009 February 24.We find the D90 duration of its 2009 January active episodeas 5.6 days (see Fig. 6).

3 Activity based classification of magnetars

Based on our investigations in the previous section, wefind a clear distinction in the burst occurrence trend amongSGRs. Here, we classify SGRs in the light of our findingsabove. We summarize this magnetar classification schemein Table 1.

3.1 Prolific bursters

SGR 1806–20 and SGR 1900+14 are considered as pro-lific bursters as their major bursting episodes last relativelylong, >

∼ 100 days. In between major activity periods, thesesources may exhibit short duration bursting phases or justisolated bursts. We note the fact these short bursts would notmake any significant impact on the persistent X-ray emis-sion of these sources. However, a large scale burst, such asa giant flare can cause a significant radiative enhancement(Woods et al. 2001). We include SGR 0526+66 in this classas it is the source of a giant flare (Mazets et al. 1979) andexhibited large number of short bursts in the aftermath ofthe March 5th giant flare.

3.2 Prolific transients

We classify SGR 1627–41, SGR J0501+4516, and SGRJ550–5418 as prolific transient since they exhibit significantnumber of bursts (>∼ 10) while their burst active periods arerelatively short, <

∼ 10 days. What makes these sources tran-sient is the fact that they experience remarkable radiative en-hancements in conjunction with the bursting. The enhancedflux then decays back to the pre-outburst level on a timescale of months to years. Moreover, these sources have rel-atively lower inferred dipole magnetic fields (2−3×1014 G)than the prolific magnetars.

3.3 AXPs with SGR-like bursts

At least six out of 11 AXPs emitted SGR-like typical shortbursts to date. Outburst from 1E 2259+586 in its 2002 ac-tive episode resembles the peak of the burst active phaseof a prolific SGR, while the outburst in 1E 2259+586 wasaccompanied with significant flux enhancement, persistentX-ray spectral variations and a strong timing glitch (Woodset al. 2004). This is, however, not a general behavior forall AXPs in action as 1E 1841–045 exhibits bursts but notsignificant flux increase associated with them. Bursts fromAXPs are usually detected only in X-ray band, indicatingthat their energy budget is, on average, lower than that ofSGRs.

3.4 Transient SGRs with low burst rates

The latest members of the magnetar family were alldiscovered with the emission of short bursts with rela-tively small energy budget (see, e.g., van der Horst et al.2009; Gogus et al. 2010). These sources, however, arenot efficient bursters: only one burst was detected fromSGR J1833-0832, two bursts from SGR J0418+5729 andSwift J1822.3–1606, a few bursts were detected from eachof Swift J1834.9–0846 and SGR 1745−29. These sourcesexperience large persistent X-ray flux increase associatedwith the onset of the bursting episode, similar to the be-havior in the prolific transients. Their elevated X-ray flux,however, remains fairly constant for about 10−20 days af-ter the outburst onset (see Fig. 4 in Kennea et al. 2013), andthen starts to decline, while the flux decline in AXPs andprolific transients start much earlier. Similar to the prolifictransients, these sources have, on average, lower inferreddipole magnetic fields than the prolific bursters.

4 Discussion and conclusions

We investigated nine burst active episodes of five magne-tars and introduced an estimate for the duration of their out-burst phases. Based on this analysis, we find a clear dis-tinction between the 90 % durations of the active episodesof persistent and transient SGRs: The outburst episodes ofthe persistent sources are long lasting (>∼ 100 days) whilethose of transient sources last much shorter (<∼ 10 days). We

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300 E. Gogus: Magnetar classification

Table 1 Activity based reclassification of magnetars.

Prolific Prolific AXPs with SGRs with LowBursters Transients SGR-like Bursts Burst Rates

SGR 1806–20 SGR 1627–41 1E 2259+586 SGR J0418+5729SGR 1900+14 SGR J0501+4516 1E 1048.1–5937 SGR J1833–0832SGR 0526+66 SGR J1550–5418 4U 0142+61 Swift J1822.3–1606

1E 1841–045 Swift J1834.9–0846XTE J1810–197 SGR 1745–29CXOU J164710.2–455216

therefore suggest that sources with long activity episodesare prolific bursters, and sources with burst active episodesless than about ten days are prolific transients.

Not all transients are efficient burst emitters. Namely,magnetars discovered in recent years tend to emit a smallnumber of bursts (one or a few) with modest energy, whilethey experience associated large flux enhancements simi-lar to the prolific transients. A striking characteristics ofthe SGRs with low bursting rate is that their enhanced fluxwould not start declining right after the outburst onset, butremain constant for about ten days. According to the magne-tar model, bursts are likely due to the build up of stress in thestellar crust from the evolving magnetic field and the even-tual release of this stress when the crust fractures (Thomp-son & Duncan 1995). Therefore, a common trigger mech-anism is likely to take place, and in turn result in observedbursts in all sources, as the temporal and spectral proper-ties of short bursts from all magnetars closely resemble eachother. It is possible that neutron star with slightly differentcrustal characteristics, in particular the lattice strain, wouldrespond slightly differently when stressed with a large mag-netic pressure.

Alternatively, prolific magnetars might be the sourceswhich are able to radiate the burst energy away from theirmagnetospheres efficiently, while SGRs with low burstingrates cannot do so as efficiently, even if the same mechanismtriggers the outbursts in both categories. In this case, pairplasma that is injected into the magnetosphere but not radi-ated away might remain trapped within the system, mightend up returning back and heating the neutron star surface.Such implosive heating could be the reason for fairly con-stant enhanced flux seen in the first ∼ 10 days of outburstepisodes of SGRs with low bursting rates.

Acknowledgements. E.G. appreciates Yuki Kaneko and Lin Linfor fruitful discussions and careful reading of the manuscript.

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