1809.02140.pdf

CCEPTED FOR PUBLICATION IN

STROPHYSICAL

OURNAL

, 2018 August 30

Preprint typeset using L

X style emulateapj v. 01/23/15

PULSE MORPHOLOGY OF THE GALACTIC CENTER MAGNETAR PSR J1745–2900

ARON

B. P

EARLMAN

1,4,5

, W

ALID

A. M

AJID

2,1

, T

HOMAS

A. P

RINCE

1,2

, J

ONATHON

OCZ

AND

HINJI

ORIUCHI

Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA; aaron.b.pearlman@caltech.edu

Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

CSIRO Astronomy and Space Science, Canberra Deep Space Communications Complex, P.O. Box 1035, Tuggeranong, ACT 2901, Australia

Accepted for publication in The Astrophysical Journal on 2018 August 30

ABSTRACT

We present results from observations of the Galactic Center magnetar, PSR J1745–2900, at 2.3 and 8.4 GHz

with the NASA Deep Space Network 70 m antenna, DSS-43. We study the magnetar’s radio profile shape, flux

density, radio spectrum, and single pulse behavior over a

∼

1 year period between MJDs 57233 and 57621.

In particular, the magnetar exhibits a significantly negative average spectral index of

〈

〉

–1.86

0.02 when

the 8.4 GHz profile is single-peaked, which flattens considerably when the profile is double-peaked. We have

carried out an analysis of single pulses at 8.4 GHz on MJD 57479 and find that giant pulses and pulses with

multiple emission components are emitted during a significant number of rotations. The resulting single pulse

flux density distribution is incompatible with a log-normal distribution. The typical pulse width of the com-

ponents is

∼

1.8 ms, and the prevailing delay time between successive components is

∼

7.7 ms. Many of the

single pulse emission components show significant frequency structure over bandwidths of

∼

100 MHz, which

we believe is the first observation of such behavior from a radio magnetar. We report a characteristic single

pulse broadening timescale of

〈

〉

6.9

0.2 ms at 8.4 GHz. We find that the pulse broadening is highly vari-

able between emission components and cannot be explained by a thin scattering screen at distances

1 kpc.

We discuss possible intrinsic and extrinsic mechanisms for the magnetar’s emission and compare our results to

other magnetars, high magnetic field pulsars, and fast radio bursts.

Key words:

Galaxy: center – pulsars: individual (PSR J1745–2900) – scattering – stars: magnetars – stars:

neutron

INTRODUCTION

Magnetars are a class of slowly rotating neutron stars, with

spin periods between

∼

2 and 12 s, that are thought to be pow-

ered by their decaying ultra-strong magnetic fields (Duncan

& Thompson 1992; Thompson & Duncan 1995, 1996). More

than

∼

2600 pulsars have now been found, but only 31 mag-

netars or magnetar candidates are currently known (Kaspi &

Beloborodov 2017; see the McGill Magnetar Catalog

). Most

of these are Galactic magnetars, many of which are located in

the inner region of the Milky Way (Olausen & Kaspi 2014).

Typical surface dipolar magnetic fields of magnetars range

between

∼

and 10

G, which exceed the

∼

G fields

of rotation-powered pulsars. Transient X-ray and gamma-ray

outbursts are hallmark features of magnetar emission and have

led to the discovery of the majority of new magnetars.

PSR J1745–2900 is one of only four magnetars with de-

tectable radio pulsations (Camilo et al. 2006, 2007a; Levin

et al. 2010; Eatough et al. 2013b; Shannon & Johnston 2013).

It is unique among the population of magnetars because of

its close proximity to the 4

black hole, Sagittar-

ius A

∗

(Sgr A

∗

), at the Galactic Center (GC). The discov-

ery of a rare magnetar near the GC may suggest that the

environment around Sgr A

∗

is more conducive for magne-

tar formation (Dexter & O’Leary 2014). Observations of

PSR J1745–2900 also provide a valuable probe of the inter-

stellar medium (ISM) near the GC (e.g., Eatough et al. 2013b;

Bower et al. 2014; Spitler et al. 2014; Desvignes et al. 2018),

which may shed light on why previous searches for radio

pulsars within

∼

10 arcmin of Sgr A

∗

have been unsuccess-

NDSEG Research Fellow.

NSF Graduate Research Fellow.

http://www.physics.mcgill.ca/

∼

pulsar/magnetar/main.html.

ful (Kramer et al. 2000; Johnston et al. 2006; Deneva et al.

2009; Macquart et al. 2010; Bates et al. 2011; Eatough et al.

2013a; Siemion et al. 2013). It is widely believed that these

searches may have been hindered by scattering-induced pulse

broadening of the pulsed radio emission as a result of large

electron densities along the line of sight.

The GC magnetar was serendipitiously discovered by the

Swift

Burst Alert Telescope (BAT) following an X-ray

flare near Sgr A

∗

and is the most recent addition to

the radio magnetar family (Eatough et al. 2013b; Ken-

nea et al. 2013; Shannon & Johnston 2013). Subsequent

observations with the

NuSTAR

X-ray telescope uncovered

X-ray pulsations at a period of

3.76 s and a spin-down

rate of

6.5

–12

s s

–1

(Mori et al. 2013).

Assum-

ing a dipolar magnetic field, this implies a surface mag-

netic field of

surf

≈

1.6

G, spin-down luminosity of

≈

erg s

–1

, and characteristic age of

≈

9 kyr. A

series of

Chandra

and

Swift

observations were later per-

formed, which localized the magnetar to an angular distance

of 2.4 arcsec from Sgr A

∗

(Rea et al. 2013). The proper mo-

tion of PSR J1745–2900 was measured relative to Sgr A

∗

us-

ing the Very Long Baseline Array (VLBA), which yielded a

transverse velocity of 236 km s

–1

at a projected separation of

0.097 pc (Bower et al. 2015).

Radio pulsations have been detected from PSR J1745–2900

at frequencies between 1.2 and 291 GHz, and its radio spec-

trum is relatively flat (Eatough et al. 2013b; Spitler et al.

2014; Torne et al. 2015, 2017). Multifrequency radio obser-

vations established that the GC magnetar has the largest dis-

persion measure (DM

1778

3 pc cm

–3

) and Faraday rota-

The

Swift

Gamma-Ray Burst Explorer was renamed the “Neil Gehrels

Swift

Observatory” in honor of Neil Gehrels,

Swift’s

principal investigator.

arXiv:1809.02140v2 [astro-ph.HE] 25 Oct 2018

PEARLMAN ET AL.

tion measure (RM

–66,960

50 rad m

–2

) of any known pul-

sar (Eatough et al. 2013b). Schnitzeler et al. (2016) found that

its RM had increased to –66,080

24 rad m

–2

approximately

2 years later, and recent measurements by Desvignes et al.

(2018) showed that its linear polarization fraction and RM

were both significantly variable over a time span of roughly

4 years.

Single pulse radio observations of PSR 1745–2900 have

been performed at 8.7 GHz by Lynch et al. (2015) with the

Green Bank Telescope (GBT) and at 8.6 GHz by Yan et al.

(2015) using the Shanghai Tian Ma Radio Telescope (TMRT).

Lynch et al. (2015) showed that the magnetar experienced a

transition from a stable state to a more erratic state early in

2014. During this period, significant changes in the magne-

tar’s flux density, radio profile shape, and single pulse prop-

erties were observed. Yan et al. (2015) presented single pulse

observations between 2014 June 28 and October 13, and they

performed an analysis of pulses detected during an erratic pe-

riod on MJD 56836. Yan et al. (2018) recently reported on

single pulse observations at 3.1 GHz with the Parkes radio

telescope, which showed that the magnetar was in a stable

state between MJDs 56475 and 56514.

Temporal scatter broadening measurements were per-

formed by Spitler et al. (2014) using single pulses

and average pulse profiles from PSR J1745–2900 between

1.19 and 18.95 GHz. They derived a pulse broadening spec-

tral index of

–3.8

0.2 and a pulse broadening timescale

1.3

0.2 s at 1 GHz, which is several orders of mag-

nitude lower than the value predicted by the NE2001 elec-

tron density model (Cordes & Lazio 2002). Observations

with the VLBA and phased array of the Karl G. Jansky Very

Large Array (VLA) were subsequently performed to mea-

sure the angular broadening of PSR J1745–2900 (Bower et al.

2014). Bower et al. (2014) argued that the observed scat-

tering is consistent with a single thin screen at a distance

∆

5.8

0.3 kpc from the GC. A secondary scattering

screen, located

∼

0.1 pc in front of the magnetar, was recently

proposed by Desvignes et al. (2018) to explain the magnetar’s

depolarization at low radio frequencies.

In this paper, we present results from simultaneous observa-

tions of PSR J1745–2900 at 2.3 and 8.4 GHz with the NASA

Deep Space Network (DSN) antenna, DSS-43. The obser-

vations and data reduction procedures are described in Sec-

tion 2. In Section 3, we provide measurements of the magne-

tar’s profile shape, flux density, and radio spectrum. We also

carry out a detailed single pulse analysis at 8.4 GHz and study

the morphology of individual pulses from the magnetar. We

discuss and summarize our results in Section 4. In this sec-

tion, we consider the implication of our results on scattering

through the ISM toward the GC. We also compare the emis-

sion properties of the GC magnetar to other magnetars and

high magnetic field pulsars. Lastly, we describe the similari-

ties between the single pulse emission from this magnetar and

fast radio bursts (FRBs).

OBSERVATIONS

High frequency radio observations of PSR J1745–2900

were carried out during four separate epochs between

2015 July 30 and 2016 August 20 using the NASA DSN 70 m

antenna (DSS-43) in Tidbinbilla, Australia. A detailed list of

these radio observations is provided in Table 1. Simultane-

ous dual circular polarization

-band and

-band data, cen-

tered at 2.3 and 8.4 GHz, were recorded during each epoch

with a time sampling of 512

s. The data were channelized,

with a frequency spacing of 1 MHz, in a digital polyphase

filterbank with 96 and 480 MHz of bandwidth at

-band and

-band, respectively. Polarimetric measurements are not pro-

vided since data from a polarimetry calibrator was unavail-

able.

These observations were performed at elevation angles be-

tween 12

◦

and 21

◦

, and the antenna gain was

∼

1 K/Jy. The

total system temperature was calculated at

-band for each

epoch using:

sys

rec

atm

(1)

where

rec

is the receiver noise temperature,

atm

is the atmo-

spheric contribution, and

is the contribution from the GC.

The atmospheric component was determined from the eleva-

tion angle, atmospheric optical depth, and atmospheric tem-

perature during each epoch. In Table 1, we list the sum of the

instrumental and atmospheric components of the system tem-

perature for each epoch at

-band, where we have assumed

15% uncertainties on these values. We modeled

using

the following empirical relationship derived by Rajwade et al.

(2017) from calibrated continuum maps of the GC (Law et al.

2008):

(

) = 568

(

GHz

)

−

1

13

(2)

where

denotes the observing frequency. At the

-band

central frequencies, the GC adds 227/52 K to the sys-

tem temperature, giving an average system temperature of

262(3)/78(2) K.

2.1.

Data Reduction

The raw filterbank data are comprised of power spectral

measurements across the band and can include spurious sig-

nals due to radio frequency interference (RFI). The first step

in the data reduction procedure was to remove data that

were consistent with either narrowband or wideband RFI. We

searched the data using the

rfifind

tool from the

PRESTO

pulsar search package (Ransom 2001), which produced a

mask for filtering out data identified as RFI and resulted in

the removal of less than 3% of the data from each epoch.

Next, we flattened the bandpass response and removed low

frequency variations in the baseline of each frequency chan-

nel by subtracting the moving average from each data point,

which was calculated using 10 s of data around each time sam-

ple. The sample times were corrected to the solar system

barycenter using the

TEMPO

timing analysis software, and

the data were then incoherently dedispersed at the magnetar’s

nominal DM of 1778 pc cm

–3

RESULTS

3.1.

Average Pulse Profiles

A blind search for pulsations was performed between

3.6 and 3.9 s using the

PRESTO

pulsar search package.

Barycentric period measurements are provided in Table 2 and

were derived from the

-band data, where the pulsations

were strongest. Average

-band and

-band pulse profiles,

shown in Figure 1, were obtained after applying barycentric

corrections, dedispersing at the magnetar’s nominal DM, and

folding the data on the barycentric periods given in Table 2.

These pulse profiles were produced by combining data from

See https://www.cv.nrao.edu/

∼

sransom/presto.

See http://tempo.sourceforge.net.

PULSE MORPHOLOGY OF PSR J1745–2900

TABLE 1

ADIO

BSERVATIONS OF

PSR J1745–2900

Epoch

Date

Time

Date

Duration

rec

atm

rec

atm

(hh:mm:ss)

(MJD)

(hr)

(K)

2015 Jul 30

16:15:00

57233.67708

1.2

2015 Aug 15

15:25:22

57249.64262

1.3

2016 Apr 01

12:35:42

57479.52479

0.4

2016 Aug 20

15:26:28

57620.64338

1.0

OTE

. —

Start time of the observation (UTC).

Start time of the observation.

Sum of the instrumental and atmospheric components of the system temperature at

-band.

Sum of the instrumental and atmospheric components of the system temperature at

-band.

both circular polarizations in quadrature. The top panels show

the integrated pulse profiles in units of peak flux density and

signal-to-noise ratio (S/N). The S/N was calculated by sub-

tracting the off-pulse mean from the pulse profiles and divid-

ing by the off-pulse root mean square (RMS) noise level,

off

The bottom panels show the strength of the pulsations as a

function of time and pulse phase. The pulse profiles have

been aligned such that the peak of the

-band pulse profile

lies at the center of the pulse phase window.

The

-band pulse profiles in Figure 1 display a narrow

emission component during each epoch, and the

-band pulse

profiles from epochs 1–3 show broader peaks that are nearly

coincident in phase with the

-band peaks.

-band pulsa-

tions were only marginally detected during epoch 4. From

Figure 1, we see that the pulsed emission was stronger at

-band compared to

-band during epochs 2–4, but epoch 1

showed slightly more significant pulsations at

-band. The

pulsations also became noticably fainter toward the end of

epoch 1, and we found that the pulsed emission was weaker in

the right circular polarization (RCP) channel compared to the

left circular polarization (LCP) channel during this particular

epoch.

3.2.

Mean Flux Densities and Spectral Indices

Measurements of the magnetar’s mean flux density were

calculated from the average

-band and

-band pulse profiles

in Figure 1 using the modified radiometer equation (Lorimer

& Kramer 2012):

β T

sys

(

pulse

total

)

√

∆

ν n

obs

(3)

where

is a correction factor that accounts for system imper-

fections such as digitization of the signal,

sys

is the effective

system temperature given by Equation (1),

pulse

is the area

under the pulse,

is the telescope gain,

total

√

bin

off

the total RMS noise level of the profile,

bin

is the total num-

ber of phase bins in the profile,

∆

is the observing band-

width,

is the number of polarizations, and

obs

is the total

observation time. Errors on the mean flux densities were de-

rived from the uncertainties in the flux calibration parameters.

In Table 3, we provide a list of mean flux density measure-

ments at 2.3 and 8.4 GHz for each epoch. An upper limit is

given for the

-band mean flux density during epoch 4 since

pulsations were only marginally detected.

The

-band mean flux densities measured on 2015 July 30

and August 15 were smaller by a factor of

∼

7.5 compared to

measurements made roughly 5 months earlier by Torne et al.

(2017). Observations performed on 2016 April 1 and Au-

gust 20 indicate that the magnetar’s

-band mean flux den-

sity more than doubled since 2015 August 15. The

-band

mean flux density was noticably variable, particularly dur-

ing epoch 4 when a significant decrease in pulsed emission

strength was observed. This behavior is not unusual, as large

changes in radio flux densities have also been observed from

other magnetars on short timescales (e.g., Levin et al. 2012).

The spectral index,

, was calculated for each epoch us-

ing our simultaneous mean flux density measurements at

2.3 and 8.4 GHz, assuming a power-law relationship of the

form

∝

. These spectral index measurements are listed

in Table 3.

A wide range of spectral index values have

been reported from multifrequency radio observations of

this magnetar (Eatough et al. 2013b; Shannon & Johnston

2013; Pennucci et al. 2015; Torne et al. 2015, 2017). Torne

et al. (2017) measured a spectral index of

= +

0.4

0.2

from radio observations between 2.54 and 291 GHz be-

tween 2015 March 4 and 9, approximately 5 months prior

to our observations. However, the radio spectrum derived

by Torne et al. (2017) was considerably steeper between

2.54 and 8.35 GHz. We performed a nonlinear least squares

fit using their total average flux densities in this frequency

range and found a spectral index of

–0.6

0.2. Our

spectral index measurements (see Table 3) indicate that the

magnetar exhibited a significantly negative average spec-

tral index of

〈

〉

–1.86

0.02 during epochs 1–3 when its

8.4 GHz profile was single-peaked. The spectral index flat-

tened to

α>

–1.12 during epoch 4 when the profile became

double-peaked (see Figure 2). While our spectral index val-

ues suggest a much steeper spectrum than is typical for the

other three known radio magnetars, which have nearly flat or

inverted spectra (Camilo et al. 2006, 2008; Lazaridis et al.

2008; Levin et al. 2010; Keith et al. 2011), a comparably steep

spectrum has previously been observed from this magnetar

between 2 and 9 GHz (Pennucci et al. 2015).

3.3.

Rotation-resolved Pulse Profiles

The

-band rotation-resolved pulse profiles in Figure 2

were produced by folding the barycentered and dedispersed

time series data on the barycentric periods given in Table 2

and combining the data from both circular polarizations in

quadrature. A time resolution of 512

s was used to define the

spacing between neighboring phase bins. The bottom panels

show the single pulse emission during each individual pulsar

rotation as a function of pulse phase, and the integrated pulse

profiles are shown in the top panels. In Figure 2, we show a

restricted pulse phase interval (0.45–0.55) around the

-band

pulse profile peak from each epoch and reference pulse num-

bers with respect to the start of each observation.

-band

rotation-resolved pulse profiles are not shown since the sin-

PEARLMAN ET AL.

TABLE 2

ARYCENTRIC

ERIOD

EASUREMENTS OF

PSR J1745–2900

Epoch

ref

(s)

(s s

–1

)

(MJD)

3.76531(1)

–8

57233.682318131

3.765367(8)

–8

57249.646691855

3.76603(2)

–7

57479.527055687

3.76655(1)

–8

57620.646961446

OTE

. — Period measurements were derived from the

barycentered

-band data.

Barycentric reference time of period measurements.

TABLE 3

LUX

ENSITIES AND

PECTRAL

NDICES OF

PSR J1745–2900

Epoch

2.3

8.4

(mJy)

1.18

0.02

0.078

0.004

–2.08

0.04

0.79

0.02

0.085

0.004

–1.70

0.04

1.92

0.04

0.18

0.01

–1.80

0.04

0.84

0.19

0.01

–1.12

OTE

. —

Mean flux density at 2.3 GHz.

Mean flux density at 8.4 GHz.

Spectral index between 2.3 and 8.4 GHz.

gle pulse emission was significantly weaker at 2.3 GHz.

The integrated profiles from epochs 1–3, shown in

Figures 2(a)–(c), exhibit a single feature with an approxi-

mately Gaussian shape, similar to previous observations near

this frequency by Spitler et al. (2014). Finer substructure is

also seen in the profiles, particularly during epoch 3 when the

single pulse emission is brightest. Two main emission peaks

are observed in the integrated profile from epoch 4, shown

in Figure 2(d), with the secondary component originating

from separate subpulses delayed by

∼

65 ms from the primary

peak. Yan et al. (2015) also found subpulses that were co-

herent in phase over many rotations during observations with

the TMRT at 8.6 GHz between 2014 June and October. We

note that the shape of the average profile is mostly Gaussian

during epoch 1 when the magnetar’s radio spectrum is steep-

est and displays an additional component during epoch 4 after

the spectrum has flattened (see Table 3), which may suggest a

link between the magnetar’s radio spectrum and the structure

of its pulsed emission.

3.4.

Single Pulse Analysis

3.4.1.

Identification of Single Pulses

We carried out a search for

-band and

-band single

pulses from each epoch listed in Table 1. In this paper,

we focus primarily on

-band single pulses detected during

epoch 3 since the single pulse emission was brightest during

this epoch. The data were first barycentered and dedispersed

at the magnetar’s nominal DM of 1778 pc cm

–3

after mask-

ing bad data corrupted by RFI and applying the bandpass

and baseline corrections described in Section 2.1. The full

time resolution time series data were then searched for sin-

gle pulses using a Fourier domain matched filtering algorithm

available through

PRESTO

, where the data were convolved

with boxcar kernels of varying widths.

We used 54 boxcar templates with logarithmically spaced

widths up to 2 s, and events with S/N

≥

5 were recorded for

further analysis. If a single pulse candidate was detected with

different boxcar widths from the same section of data, only

the highest S/N event was stored in the final list. The S/N of

each single pulse candidate was calculated using:

S/N

∑

(

−

)

√

(4)

where

is the time series value in bin

of the boxcar func-

tion,

and

are the local mean and RMS noise after normal-

ization, and

is the boxcar width in number of bins. The time

series data were detrended and normalized such that

≈

0 and

≈

1. We note that the definition of S/N in Equation (4) has

the advantage of giving approximately the same result irre-

spective of how the input time series is downsampled, pro-

vided the pulse is still resolved (Deneva et al. 2016).

3.4.2.

-band Single Pulse Morphology

3.4.2.1.

Multiple Emission Components

An analysis was performed on the

-band single pulse

events from epoch 3 that were both detected using the Fourier

domain matched filtering algorithm described in Section 3.4.1

and showed resolvable dispersed pulses in their barycentered

dynamic spectra. We measured the times of arrival (ToAs) of

the emission components comprising each single pulse event

by incoherently dedispersing the barycentered dynamic spec-

tra at the magnetar’s nominal DM of 1778 pc cm

–3

and then

searching for local maxima in the integrated single pulse pro-

files after smoothing the data by convolving the time series

with a one-dimensional Gaussian kernel. The Gaussian ker-

nel used in this procedure is given by:

(

;

) =

√

πσ

exp

(

−

2

2

)

(5)

PULSE MORPHOLOGY OF PSR J1745–2900

S/N

Epoch 1

(

-band)

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Time (hr)

Epoch 2

(

-band)

0.0

0.2

0.4

0.6

0.8

1.0

Pulse Phase

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Epoch 3

(

-band)

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.1

0.2

0.3

0.4

Epoch 4

(

-band)

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Pulse Time (s)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Peak Flux Density (mJy)

S/N

Epoch 1

(

-band)

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Time (hr)

Epoch 2

(

-band)

0.0

0.2

0.4

0.6

0.8

1.0

Pulse Phase

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Epoch 3

(

-band)

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.1

0.2

0.3

0.4

Epoch 4

(

-band)

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Pulse Time (s)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Peak Flux Density (mJy)

. 1.— Average pulse profiles of PSR J1745–2900 at (top row)

-band and (bottom row)

-band during epochs 1–4 after combining data from both circular

polarizations in quadrature. The data were folded on the barycentric period measurements given in Table 2. The top panels show the integrated pulse profiles

using 64/128 phase bins at

-band, and the bottom panels show the strength of the pulsations as a function of phase and time, where darker bins correspond

to stronger pulsed emission.

PEARLMAN ET AL.

S/N

(a)

Epoch 1

0.45

0.47

0.49

0.51

0.53

0.55

100

200

300

400

500

600

700

800

900

1000

1100

Pulse Number (

)

(b)

Epoch 2

0.45

0.47

0.49

0.51

0.53

0.55

Pulse Phase

100

200

300

400

500

600

700

800

900

1000

1100

1200

(c)

Epoch 3

0.45

0.47

0.49

0.51

0.53

0.55

100

200

300

(d)

Epoch 4

0.45

0.47

0.49

0.51

0.53

0.55

100

200

300

400

500

600

700

800

900

0.00

0.07

0.14

0.21

0.28

0.35

0.00

0.07

0.14

0.21

0.28

0.35

Relative Pulse Time (s)

0.00

0.07

0.14

0.21

0.28

0.35

0.00

0.07

0.14

0.21

0.28

0.35

120

150

100

Peak Flux Density (mJy)

. 2.— Rotation-resolved pulse profiles of PSR J1745–2900 at

-band during (a) epoch 1, (b) epoch 2, (c) epoch 3, and (d) epoch 4 after folding the data

on the barycentric period measurements given in Table 2 and combining data from both circular polarizations in quadrature. The data are shown with a time

resolution of 512

s. The integrated profiles are displayed in the top panels, and the bottom panels show the distribution and relative strength of the single pulses

as a function of pulse phase for each individual pulsar rotation, with darker bins signifying stronger emission. Pulse numbers are referenced with respect to the

start of each observation.

where

is the scale of the Gaussian kernel and

corresponds

to the sample time in the time series. A modest Gaussian ker-

nel scale of 819

s was used to smooth the data, which did

not hinder our ability to distinguish between narrow, closely

spaced peaks. Individual emission components were identi-

fied as events displaying a dispersed feature in their dynamic

spectrum along with a simultaneous peak in their integrated

single pulse profile.

The structure and number of

-band single pulse emission

components varied significantly between consecutive pulsar

rotations (e.g., Figure 2). These changes were observed on

timescales shorter than the magnetar’s 3.77 s rotation period.

An example is shown in the top row of Figure 3 from pulse

cycle

239 of epoch 3, where at least six distinct emis-

sion components can be resolved. While the overall struc-

ture of this particular single pulse is similar in the LCP and

RCP channels, the emission components at later pulse phases

are detected more strongly in the RCP data. Measurements

performed near this epoch at 8.35 GHz with the Effelsberg

telescope indicate that the magnetar likely had a high linear

polarization fraction (Desvignes et al. 2018). This suggests

that some of the magnetar’s emission components may be

more polarized than others.

Other single pulse events contained fewer emission compo-

nents. The middle row of Figure 3 shows a single pulse event

detected during pulse cycle

334 of epoch 3 with four inde-

pendent emission components in the LCP and RCP channels.

The two brightest components are separated by

∼

6.8 ms and

∼

8.6 ms in the LCP and RCP data, respectively. Another ex-

ample from pulse cycle

391 of epoch 3 is provided in the

bottom row of Figure 3, which shows two emission compo-

nents in the LCP data and three components in the RCP data.

Using the threshold criteria described in Section 3.4.1, sin-

gle pulse emission components were significantly detected

in at least one of the polarization channels during 72%

of the pulse cycles in epoch 3 and were identified in the

LCP/RCP data during 69%/50% of the pulse cycles. Faint

emission components were often seen in many of the single

pulses, but at a much lower significance level. In Figure 4,

we show the distribution of the number of significantly de-

tected emission components during these pulse cycles. More

than 72%/87% of the single pulses in the LCP/RCP data con-

tained either one or two distinct emission components. The

number of single pulses with either one or two emission com-

ponents was approximately equal in the LCP data, and 59%

more single pulses in the RCP channel were found to have one

emission component compared to the number of events with

two components.

Most of the

-band single pulses detected during

epochs 1 and 2 displayed only one emission component,

whereas the single pulses from epochs 3 and 4 showed

multiple emission components. Single pulses with multiple

emission components have also been previously detected at

8.7 GHz with the phased VLA (Bower et al. 2014) and at

8.6 GHz with the TMRT (Yan et al. 2015). In both studies,

the number of emission components and structure of the sin-

gle pulses were found to be variable between pulsar rotations.

3.4.2.2.

Frequency Structure in Emission Components

Many of the

-band single pulse emission components

from epoch 3 displayed frequency structure in their dynamic

spectra. These events were characterized by a disappearance

or weakening of the radio emission over subintervals of the

frequency bandwidth. The typical scale of these frequency