High Frequency Radio Observations of Two Magnetars, PSR J1622-4950 and 1E 1547.0-5408
Che-Yen Chu, C.-Y. Ng, Albert K. H. Kong, Hsiang-Kuang Chang
MMNRAS , 1–7 (2021) Preprint 5 February 2021 Compiled using MNRAS L A TEX style file v3.0
High Frequency Radio Observations of Two Magnetars, PSRJ1622 − − Che-Yen Chu, ★ C.-Y. Ng, Albert K. H. Kong, Hsiang-Kuang Chang, Institute of Astronomy, National Tsing Hua University, Hisnchu, Taiwan Department of Physics, The University of Hong Kong, Hong Kong, China
Accepted XXX. Received YYY.
ABSTRACT
We investigated the radio spectra of two magnetars, PSR J1622 − − − − ± − − − Key words: stars: magnetars – stars: neutron – pulsars: individual: PSR J1622 − − Magnetars are isolated neutron stars with strong magnetic fields (cid:38) G (Duncan & Thompson 1992). Unlike rotation-poweredpulsars, their X-ray luminosities cannot be solely due to rotationalenergy loss. Instead, their strong magnetic fields provide the energyto power the observed X-ray emission. Magnetars manifest them-selves as soft gamma repeaters (SGRs) or anomalous X-ray pulsars(AXPs) in observations. SGRs and AXPs were first discovered inX-rays as neutron stars with slow rotation periods (1–12 s) show-ing burst activities. Currently, there are 24 confirmed magnetars(Olausen & Kaspi 2014) , and only five of them show pulsed ra-dio emission, of which the radio spectra are either flat or inverted(Camilo et al. 2006, 2007a; Levin et al. 2010; Eatough et al. 2013;Shannon & Johnston 2013; Karuppusamy et al. 2020).An X-ray outburst of a magnetar is an event of sudden increasein X-ray flux and sometimes comes with multiple X-ray short burstsin millisecond to second timescale. Sudden magnetar crustal ac-tivities can twist the magnetosphere resulting in an X-ray outburstas observed (Thompson 2008; Beloborodov 2009). The magneto- ★ Contact e-mail: [email protected] McGill Online Magnetar Catalog sphere gradually becomes untwisted so that the X-ray flux decreases.The flux of an outburst usually decays in different time scales, withone rapid decay within hours to a day followed by a slow decay inseveral months to years (e.g., Woods et al. 2004). Most outbursts areassociated with some radiative phenomena such as changes in pulseprofile, pulsed fraction and multi-wavelength emission, and X-rayspectral hardening. Multi-wavelength studies of radio-loud magne-tars suggest that an X-ray outburst may trigger radio emission (e.g.,Halpern et al. 2005; Camilo et al. 2007a; Anderson et al. 2012).The magnetar 1E 1547.0 − − − − Swift /BAT,
Fermi /GBM and
IN-TEGRAL /SPI detected two more outbursts in 2008 October (Israelet al. 2010; Kaneko et al. 2010) and in 2009 January (Mereghettiet al. 2009; Savchenko et al. 2010; Kaneko et al. 2010). The 2009X-ray outburst was more energetic but less variable than the 2008one (Israel et al. 2010; Ng et al. 2011; Scholz & Kaspi 2011). There © a r X i v : . [ a s t r o - ph . H E ] F e b C.-Y. Chu et al. were also some short burst activities in 2009 March and 2010 Jan-uary (Kaneko et al. 2010; von Kienlin et al. 2012; Kuiper et al.2012). However, after 2010, there has been no X-ray burst report for1E 1547.0 − − 𝛼 = .
71 (Levin et al. 2010). Thepositive index is very rare as radio pulsars generally have indexesranging from − −
1. However, in later observations, it turnedout that the spectrum is not inverted from 17 to 24 GHz (Keithet al. 2011). PSR J1622 − − − −
197 is the first radio detected magnetar (Camilo et al. 2006)that was discovered after its X-ray outburst (Ibrahim et al. 2004).The radio emission disappeared in 2008 and then reactivated dur-ing the 2019 X-ray outburst. SGR J1745 − − − ∼ . × G), PSR J1846 − − − − Table 1.
Details of ATCA and ALMA observations.Telescope Band Frequency Date in Observing FluxArray (GHz) 2017 time (min) calibratorATCA 16cm 2.1 Jun 24 15 PKS 1934 − − − − − − − − We observed the two target magnetars with the Australia TelescopeCompact Array (ATCA) which consists of six 22-m antennas withthe longest baseline of 6 km. The observations were made in the3mm and 7mm bands for 1E 1547.0 − − −
638 were observedas flux calibrators for 3mm band and all other bands, respectively.Observation details are described in Table 1.The data reduction were carried out with MIRIAD (Sault et al.1995) using standard techniques described in the ATCA UsersGuide . After applying bandpass, gain, and flux calibrations, thetask mfclean was used to clean the entire primary beam and then theflux densities were obtained with the task imfit . The noise levelsof all frequency band observations are smaller than 0.3 mJy/beamand the beam sizes are around 4". We observed PSR J1622 − − − tclean . TheBand 3 image taken in May has a beam size of 1.1" and rms noiseof 24 𝜇 Jy/beam. The September one has a beam size of 0.16" andnoise of 31 𝜇 Jy/beam. The Band 6 image from May has a beam See ATCA User Guide
MNRAS , 1–7 (2021) igh Frequency Radio Observations of Two Magnetars Table 2.
Results of observations on magnetars.Magnetar Date in Frequency Bandwidth Flux density ∗ − ± ± ± ± − ± ± ± ± ± ± ± Note. ∗ The upper limit in flux density is 3 𝜎 detection limit. F l u x d e n s i t y ( m J y ) Pearlman et al. (2017)This work
Figure 1.
Radio spectra of PSR J1622 − − ± size of 0.45" and rms noise of 21 𝜇 Jy/beam. All the noise levels areconsistent with theoretical values.We did not detect PSR J1622 − ± 𝜇 Jy. − The results of ALMA and ATCA observations of PSR J1622 − − ± F l u x d e n s i t y ( m J y ) Camilo et al. (2008)This work
Figure 2.
Radio spectra of 1E 1547.0 − ± at 97.5 and 233 GHz showed non-detection with a 3 𝜎 upper limitof 0.08 mJy on May 19. However, the flux density at 97.5 GHzincreased to 0.19 mJy after 4 months.The black points in Figure 1 are taken from Pearlman et al.(2017). Their observations on May 23 in 2017 yield a flux densityof 3.8/0.41 mJy at 2.3/8.4 GHz, respectively. Our observation takenone month later, June 24, however shows a flux density of 10.8 and3.5 mJy at 5.5 and 9.0 GHz, respectively. The cm band flux densityincreased by nearly an order of magnitude in only one month. OurALMA observations in Band 3 also show an increase in flux density(see Section 4.1 for discussion). − Our ATCA measurements of 1E 1547.0 − ± − The radio emission of magnetars is known to be variable (e.g.,Camilo et al. 2006; Levin et al. 2010) and it has different evolutionthan the X-ray emission (Lynch et al. 2015; Pennucci et al. 2015).We obtained radio and X-ray data from literature and found that
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C.-Y. Chu et al.
Modified Julian Date − − − X - r a y f l u x ( e r g c m − s − ) X-ray 0.3-10 keV 051015202530 R a d i o f l u x d e n s i t y ( m J y ) Radio 8.3-9.1 GHzRadio 8.5-9.3 GHzRadio 8.1-8.6 GHz
Year
Figure 3.
X-ray and radio light curves of SGR J1745 − the radio emission has longer rising time than X-ray emission. Themost obvious evidence is the 2013 outburst of SGR J1745 − − ∼
400 days after theoutburst, giving a radio rising time of 400 days. In contrast, theX-ray outburst is an event of sudden increase in the persistent X-rayflux and it decayed rapidly after the first day. The rising time of radioemission from SGR J1745 − − − − − (cid:38)
150 days,which is much longer than the X-ray rising time in outburst (Huet al. 2020).For 1E 1547.0 − − − − − − ∼ MNRAS , 1–7 (2021) igh Frequency Radio Observations of Two Magnetars Modified Julian Date 01020304050 R a d i o f l u x d e n s i t y ( m J y ) Radio 3.0 GHzRadio 1.4 GHz10 X - r a y f l u x ( e r g c m s ) X-ray 0.3-10 keV
Year
Figure 4.
X-ray and radio light curves of PSR J1622 − et al. 2016a). On August 9, the radio emission re-activated but theflux density was significantly weaker than the pre-outburst value(Burgay et al. 2016b). After the re-activation, the radio flux densitygradually increased to its highest value of ∼ ∼ − − − The observed radio spectra of 1E 1547.0 − − − − − ∼ −
197 showsan inverted spectrum (Dai et al. 2019). Compared to its past higherfrequency observations, XTE J1810 −
197 is likely to have GPS fea-ture as well with a peak at ∼ − − − −
497 shows noevidence of association with other source (Gotthelf et al. 2004). Ifthe 0.7–4 GHz band spectrum of XTE J1810 −
197 is actually a GPS,the absorption could happen in the molecular clouds in the line ofsight or the reason for the GPS from magnetars may be differentfrom that of the radio pulsars.Moreover, the spectra of radio magnetars could have not onlya single peak at GHz but also a second peak at few hundred GHz.Observations of SGR J1745 − ∼ ∼
200 GHz. Comparing spectra of SGR J1745 − MNRAS000
200 GHz. Comparing spectra of SGR J1745 − MNRAS000 , 1–7 (2021)
C.-Y. Chu et al. epochs from 2014 to 2015, the cm band flux density decreased byabout 5 times while sub-mm band flux density increased about 4times (Torne et al. 2015, 2017). This evolution difference supportsthe theory of a double-component radio spectrum.In addition, the observed inverted spectrum of 1E 1547.0 − − − − − ∼ − − − −
197 in 2006 and 2007 alsoshow no hint of hundred-gigahertz-peaked feature (Camilo et al.2007b). Since the hundred-gigahertz peak of SGR J1745 − ∼ − −
197 was not dominate at the time oftheir observations.
Previously observed radio emission from magnetars can all be as-sociated with an X-ray outburst except for the archival radio dataof PSR J1622 − −
197 and PSR J1622 − − − −
197 also suggeststhat the radio disappearance is most likely due to the decrease ofX-ray flux. But they emphasize that the X-ray flux change is only20%. More observations on X-ray and radio and more timing and/orspectral analysis are needed to confirm the radio turn-off criteria.
We performed analysis of radio observations of two magnetars, 1E1547.0 − − − − − ACKNOWLEDGEMENTS
The Australia Telescope Compact Array is part of the Aus-tralia Telescope National Facility which is funded by the Aus-tralian Government for operation as a National Facility managedby CSIRO. This paper makes use of the following ALMA data:ADS/JAO.ALMA
DATA AVAILABILITY
The data underlying this article are available in the article. ATCAdata are available through Australia Telescope Online Archive .ALMA data are available through ALMA Science Archive . REFERENCES
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