High Frequency Radio Observations of Two Magnetars, PSR J1622−4950 and 1E 1547.0−5408
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MNRAS 000,1–7 (2021) Preprint 5 February 2021 Compiled using MNRAS LATEX style file v3.0 High Frequency Radio Observations of Two Magnetars, PSR J1622−4950 and 1E 1547.0−5408 Che-Yen Chu,1¢ C.-Y. Ng,2 Albert K. H. Kong,1 Hsiang-Kuang Chang,1 1Institute of Astronomy, National Tsing Hua University, Hisnchu, Taiwan 2Department of Physics, The University of Hong Kong, Hong Kong, China Accepted XXX. Received YYY. ABSTRACT We investigated the radio spectra of two magnetars, PSR J1622−4950 and 1E 1547.0−5408, using observations from the Australia Telescope Compact Array and the Atacama Large Millimeter/submillimeter Array taken in 2017. Our observations of PSR J1622−4950 show a steep spectrum with a spectral index of −1.3 ± 0.2 in the range of 5.5–45 GHz during its re-activating X-ray outburst in 2017. By comparing the data taken at different epochs, we found significant enhancement in the radio flux density. The spectrum of 1E 1547.0−5408 was inverted in the range of 43–95 GHz, suggesting a spectral peak at a few hundred gigahertz. Moreover, we obtained the X-ray and radio data of radio magnetars, PSR J1622−4950 and SGR J1745−2900, from literature and found two interesting properties. First, radio emission is known to be associated with X-ray outburst but has different evolution. We further found that the rising time of the radio emission is much longer than that of the X-ray during the outburst. Second, the radio magnetars may have double peak spectra at a few GHz and a few hundred GHz. This could indicate that the emission mechanism is different in the cm and the sub-mm bands. These two phenomenons could provide a hint to understand the origin of radio emission and its connection with the X-ray properties. Key words: stars: magnetars – stars: neutron – pulsars: individual: PSR J1622−4950 – pulsars: individual: 1E 1547.0−5408 – radio continuum: stars 1 INTRODUCTION sphere gradually becomes untwisted so that the X-ray flux decreases. The flux of an outburst usually decays in different time scales, with Magnetars are isolated neutron stars with strong magnetic fields one rapid decay within hours to a day followed by a slow decay in 1014 G(Duncan & Thompson 1992). Unlike rotation-powered & several months to years (e.g., Woods et al. 2004). Most outbursts are pulsars, their X-ray luminosities cannot be solely due to rotational associated with some radiative phenomena such as changes in pulse energy loss. Instead, their strong magnetic fields provide the energy profile, pulsed fraction and multi-wavelength emission, and X-ray to power the observed X-ray emission. Magnetars manifest them- spectral hardening. Multi-wavelength studies of radio-loud magne- selves as soft gamma repeaters (SGRs) or anomalous X-ray pulsars tars suggest that an X-ray outburst may trigger radio emission (e.g., (AXPs) in observations. SGRs and AXPs were first discovered in Halpern et al. 2005; Camilo et al. 2007a; Anderson et al. 2012). X-rays as neutron stars with slow rotation periods (1–12 s) show- ing burst activities. Currently, there are 24 confirmed magnetars The magnetar 1E 1547.0−5408 was discovered in a supernova arXiv:2102.02466v1 [astro-ph.HE] 4 Feb 2021 (Olausen & Kaspi 2014)1, and only five of them show pulsed ra- remnant (SNR) G327.24−0.13 (Lamb & Markert 1981) and was dio emission, of which the radio spectra are either flat or inverted confirmed to be an AXP later in X-ray and radio observations with a (Camilo et al. 2006, 2007a; Levin et al. 2010; Eatough et al. 2013; spin period of 2.1 s (Gelfand & Gaensler 2007; Camilo et al. 2007a). Shannon & Johnston 2013; Karuppusamy et al. 2020). In previous centimeter observations, 1E 1547.0−5408 showed a flat An X-ray outburst of a magnetar is an event of sudden increase spectrum (Camilo et al. 2008) which is possible a gigahertz peaked in X-ray flux and sometimes comes with multiple X-ray short bursts spectrum (GPS; Kijak et al. 2013). The 2007 X-ray observations in millisecond to second timescale. Sudden magnetar crustal ac- suggest that there was an X-ray outburst, which could have trig- tivities can twist the magnetosphere resulting in an X-ray outburst gered the radio emission of 1E 1547.0−5408 between late 2006 and as observed (Thompson 2008; Beloborodov 2009). The magneto- early 2007 (Halpern et al. 2008). Swift/BAT, Fermi/GBM and IN- TEGRAL/SPI detected two more outbursts in 2008 October (Israel et al. 2010; Kaneko et al. 2010) and in 2009 January (Mereghetti ¢ Contact e-mail: [email protected] et al. 2009; Savchenko et al. 2010; Kaneko et al. 2010). The 2009 1 McGill Online Magnetar Catalog http://www.physics.mcgill.ca/ X-ray outburst was more energetic but less variable than the 2008 ~pulsar/magnetar/main.html one (Israel et al. 2010; Ng et al. 2011; Scholz & Kaspi 2011). There © 2021 The Authors 2 C.-Y. Chu et al. were also some short burst activities in 2009 March and 2010 Jan- Table 1. Details of ATCA and ALMA observations. 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 for Telescope Band Frequency Date in Observing Flux 1E 1547.0−5408. Array (GHz) 2017 time (min) calibrator ATCA 16cm 2.1 Jun 24 15 PKS 1934−638 PSR J1622−4950 is the first magnetar discovered in radio. At 4cm 5.5, 9.0 Jun 24 15 PKS 1934−638 the time of discovery, it showed an inverted spectrum from 1.4 to 15mm 16.7, 21.2 Jun 24 40 PKS 1934−638 9.0 GHz with a spectral index U = 0.71 (Levin et al. 2010). The 7mm 43.0, 45.0 Jun 24 50 PKS 1934−638 positive index is very rare as radio pulsars generally have indexes Aug 22 100 PKS 1934−638 ranging from −2 to −1. However, in later observations, it turned 3mm 93.0, 95.0 Aug 22 120 Mars out that the spectrum is not inverted from 17 to 24 GHz (Keith ALMA 3 97.5 May 19 8.6 J1617−5848 et al. 2011). PSR J1622−4950 shows a GPS similar to the spectra Sep 15 8.8 J1617−5848 of 1E 1547.0−5408 observed in 2007 (Kijak et al. 2013). The X- 6 233 May 19 18.6 J1617−5848 ray data during the epoch of radio discovery of PSR J1622−4950 suggest that an X-ray outburst could have happened before 2007 April (Anderson et al. 2012). Follow up radio observations show 2 OBSERVATIONS AND DATA REDUCTION that the radio emission decreased to an undetectable level in 2015 2.1 ATCA (Scholz et al. 2017) until the re-activating X-ray outburst event in 2017 March (Pearlman et al. 2017; Camilo et al. 2018). We observed the two target magnetars with the Australia Telescope Compact Array (ATCA) which consists of six 22-m antennas with the longest baseline of 6 km. The observations were made in the For other magnetars with radio emission detected, XTE 3mm and 7mm bands for 1E 1547.0−5408 on 2017 August 22 with − J1810 197 is the first radio detected magnetar (Camilo et al. 2006) the array configuration EW352. For PSR J1622−4950, we observed that was discovered after its X-ray outburst (Ibrahim et al. 2004). in the 7mm, 15mm, 4cm and 16cm bands on 2017 June 24 with the The radio emission disappeared in 2008 and then reactivated dur- array configuration H214. Mars and PKS 1934−638 were observed − ing the 2019 X-ray outburst. SGR J1745 2900 is a radio magnetar as flux calibrators for 3mm band and all other bands, respectively. discovered near Sargittarius A* in 2013 (Kennea et al. 2013; Mori Observation details are described in Table1. et al. 2013) and it showed the highest frequency radio detection of The data reduction were carried out with MIRIAD (Sault et al. a magnetar at 291 GHz (Torne et al. 2017). The newly confirmed 1995)2 using standard techniques described in the ATCA Users − magnetar, Swift J1818.0 1607, was discovered on 2020 March 12 Guide3. After applying bandpass, gain, and flux calibrations, the with a spin period of 1.36 s (Evans et al. 2020; Enoto et al. 2020; task mfclean was used to clean the entire primary beam and then the Esposito et al. 2020) and was found to have pulsed radio emisson flux densities were obtained with the task imfit. The noise levels few days later (Karuppusamy et al. 2020; Esposito et al. 2020). The of all frequency band observations are smaller than 0.3 mJy/beam − radio emission of Swift J1818.0 1607 was detected in a wide band and the beam sizes are around 4". from 0.65 to 154 GHz. There is another special magnetar, SGR 1935+2154, which was confirmed to be a magnetar in 2014 (Israel et al. 2014). It entered an active state with a forest of short X-ray 2.2 ALMA bursts on 2020 April 27 (Palmer 2020). Later on April 28, a short − radio burst was detected from SGR 1935+2154 (Collaboration et al. We observed PSR J1622 4950 with the Atacama Large Millime- 2020; Bochenek et al. 2020). However, pulsed radio emission was ter/submillimeter Array (ALMA) in Band 3 (97.5 GHz) and Band detected only at 2 epochs (Burgay et al. 2020; Zhu et al. 2020). 6 (233 GHz) on 2017 May 19 and September 15.