1810.10720.pdf

RAFT VERSION

CTOBER

26, 2018

Preprint typeset using L

X style AASTeX6 v. 1.0

AN EMBEDDED X-RAY SOURCE SHINES THROUGH THE ASPHERICAL AT2018COW:

REVEALING THE INNER WORKINGS OF THE MOST LUMINOUS FAST-EVOLVING OPTICAL TRANSIENTS

R. M

ARGUTTI

, B. D. M

ETZGER

, R. C

HORNOCK

, I. V

URM

, N. R

OTH

5,6

, B. W. G

REFENSTETTE

, V. S

AVCHENKO

, R. C

ARTIER

9,10

J. F. S

TEINER

11,12

, G. T

ERRERAN

, G. M

IGLIORI

13,14

, D. M

ILISAVLJEVIC

, K. D. A

LEXANDER

1,12

, M. B

IETENHOLZ

16,17

, P. K.

LANCHARD

, E. B

OZZO

, D. B

RETHAUER

, I. V. C

HILINGARIAN

18,19

, D. L. C

OPPEJANS

, L. D

UCCI

8,20

, C. F

ERRIGNO

, W. F

ONG

D. G

ÖTZ

, C. G

UIDORZI

, A. H

AJELA

, K. H

URLEY

, E. K

UULKERS

, P. L

AURENT

, S. M

EREGHETTI

, M. N

ICHOLL

26,18

, D.

ATNAUDE

, P. U

BERTINI

, J. B

ANOVETZ

, N. B

ARTEL

, E. B

ERGER

, E. R. C

OUGHLIN

2,12

, T. E

FTEKHARI

, D. D. F

REDERIKS

A. V. K

OZLOVA

, T. L

ASKAR

29,30

, D. S. S

VINKIN

, M. R. D

ROUT

31,32

, A. M

ADYEN

AND

K. P

ATERSON

Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University, Evanston,

IL 60208.

Department of Physics and Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA.

Astrophysical Institute, Department of Physics and Astronomy, 251B Clippinger Lab, Ohio University, Athens, OH 45701, USA.

Tartu Observatory, University of Tartu, Tõravere 61602, Tartumaa, Estonia.

Department of Astronomy, University of Maryland, College Park, MD 20742, USA.

Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA.

Cahill Center for Astrophysics, 1216 E. California Boulevard, California Institute of Technology, Pasadena, CA 91125, USA.

ISDC, Department of Astronomy, University of Geneva, Chemin d’Écogia, 16 CH-1290 Versoix, Switzerland.

Millennium Institute of Astrophysics, Casilla 36-D, Santiago, Chile.

Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile.

MIT Kavli Institute for Astrophysics and Space Research, MIT, 70 Vassar Street, Cambridge, MA 02139, USA.

Einstein Fellow.

Dipartimento di Fisica e Astronomia, Alma Mater Studiorum, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy.

INAF Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy.

Department of Physics and Astronomy, Purdue University, 525 Northwester Avenue, West Lafayette, IN 47907, USA.

Hartebeesthoek Radio Observatory, PO Box 443, Krugersdorp, 1740, South Africa.

Department of Physics and Astronomy, York University, Toronto, M3J 1P3, Ontario, Canada.

Harvard-Smithsonian Center for Astrophysics, 60 Garden St. Cambridge MA 02138.

Sternberg Astronomical Institute, M.V.Lomonosov Moscow State University, Universitetsky prospect 13, Moscow, 119234, Russia.

Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Eberhard Karls Universitat, Sand 1, 72076, Tubingen, Germany.

CEA Saclay - Irfu/D’epartement d’Astrophysique, Orme des Merisiers, Bat. 709, F91191 Gif-sur-Yvette.

Department of Physics and Earth Science, University of Ferrara, via Saragat 1, I–44122, Ferrara, Italy.

University of California at Berkeley, Space Sciences Laboratory, 7 Gauss Way, Berkeley, CA 94720.

European Space Astronomy Centre (ESA/ESAC), Science Operations Department, 28691 Villanueva de la Canada, Madrid, Spain.

INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, Via E. Bassini 15, 20133 Milano, Italy.

Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK.

Istituto di Astrofisica e Planetologia Spaziali, INAF Via Fosso del Cavaliere 100, 00133 Rome, Italy.

Ioffe Physical-Technical Institute, Politekhnicheskaya 26, St. Petersburg 194021, Russia.

National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA.

Department of Astronomy, University of California, 501 Campbell Hall, Berkeley, CA 94720-3411, USA.

The Observatories of the Carnegie Institution for Science, 813 Santa Barbara St., Pasadena, CA 91101, USA.

Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario, M5S 3H4 Canada.

Center for Cosmology and Particle Physics, New York University, 726 Broadway, New York, NY 10003, USA.

ABSTRACT

We present the first extensive radio to

-ray observations of a fast-rising blue optical transient (FBOT),

AT 2018cow, over its first

∼

100 days. AT 2018cow rose over a few days to a peak luminosity

∼

erg s

−

exceeding those of superluminous supernovae (SNe), before declining as

∝

−

. Initial spectra

15 days were mostly featureless and indicated large expansion velocities

∼

and temperatures

arXiv:1810.10720v1 [astro-ph.HE] 25 Oct 2018

ARGUTTI ET AL

reaching

∼

K. Later spectra revealed a persistent optically-thick photosphere and the emergence of

H and He emission features with

∼

4000 km s

−

with no evidence for ejecta cooling. Our broad-band mon-

itoring revealed a hard X-ray spectral component at

≥

10 keV, in addition to luminous and highly variable

soft X-rays, with properties unprecedented among astronomical transients. An abrupt change in the X-ray de-

cay rate and variability appears to accompany the change in optical spectral properties. AT 2018cow showed

bright radio emission consistent with the interaction of a blastwave with

∼

with a dense environment

(

̇

∼

−

for

= 1000 km s

−

). While these properties exclude

Ni-powered transients, our

multi-wavelength analysis instead indicates that AT 2018cow harbored a "central engine", either a compact

object (magnetar or black hole) or an embedded internal shock produced by interaction with a compact, dense

circumstellar medium. The engine released

∼

−

erg over

∼

−

s and resides within low-

mass fast-moving material with equatorial-polar density asymmetry (

fast

3 M

). Successful SNe from

low-mass H-rich stars (like electron-capture SNe) or failed explosions from blue supergiants satisfy these con-

straints. Intermediate-mass black-holes are disfavored by the large environmental density probed by the radio

observations.

Keywords:

transients — relativistic processes

INTRODUCTION

Recent high-cadence surveys have uncovered a plethora of

rapidly-evolving transients with diverse observed properties

that challenge our current notions of stellar death (e.g., Drout

et al. 2014; Arcavi et al. 2016; Tanaka et al. 2016; Pursi-

ainen et al. 2018 for recent sample compilations). Such rapid

evolution is generally attributed to a small mass of ejecta

. However, the wide range of observed properties

(i.e., luminosities, energetics, chemical composition and en-

vironments), reveals them to be an extremely heterogeneous

class and likely reflects a diverse range of intrinsic origins.

Fast evolving transients can be either rich or poor in hy-

drogen, and span a wide range of peak luminosities. Some

are less luminous than normal H-stripped core-collapse SNe

(i.e., Ibc SNe, e.g., SN 2005E, Perets et al. 2010; SN 2008ha,

Valenti et al. 2009; Foley et al. 2009) or populate the low-

end of the luminosity function of Ibc SNe (e.g., SNe 2005ek,

2010X; Drout et al. 2013; Kasliwal et al. 2010). The rel-

atively old stellar environments of some of these transients

and their low luminosities have inspired connections with

models of He-shell detonations on white dwarf (WD) pro-

genitors (“Ia” SNe, Shen et al. 2010). However, the oxygen-

dominated ejecta of SN 2005ek and the young stellar en-

vironments of other rapidly-evolving transients are instead

more readily explained as the explosions of massive stars

which have been efficiently stripped of their envelopes by bi-

nary interaction (Drout et al. 2013; Tauris et al. 2013; Kleiser

& Kasen 2014; Tauris et al. 2015; Suwa et al. 2015; Moriya

et al. 2017), or “cooling envelope" emission from the explo-

sion of radially-extended red supergiant stars (Tanaka et al.

2016).

Some rapidly-evolving transients can compete in luminos-

ity with Ibc-SNe (e.g., SN 2002bj ; Poznanski et al. 2010) or

even outshine normal core-collapse SNe (Arcavi et al. 2016).

The short timescales, high peak luminosities and lack of UV

line blanketing observed in many of these transients are in

tension with traditional SN models powered by the radioac-

tive decay of

Ni (e.g., Poznanski et al. 2010; Drout et al.

2014; Pursiainen et al. 2018; Rest et al. 2018). These objects

typically show blue colors and have been referred to in the

literature as “Fast Evolving Luminous Transients” (FELTs,

Rest et al. 2018) or “Fast Blue Optical Transients” (FBOTs,

Drout et al. 2014). Here we adopt the “FBOT” acronym.

The non-radioactive sources of energy needed to explain

FBOTs fall into two broad categories: (i) Interaction of the

explosion’s shock wave with a dense circumstellar environ-

ment or extended progenitor atmosphere (Chevalier & Irwin

2011; Balberg & Loeb 2011; Ginzburg & Balberg 2014).

This class of models has been applied to a variety of FBOTs

with and without direct evidence for interaction in their spec-

tra (e.g., Ofek et al. 2010; Drout et al. 2014; Pastorello et al.

2015; Shivvers et al. 2016; Rest et al. 2018. In this scenario

the high luminosities of FBOTs are the result of efficient con-

version of ejecta kinetic energy into radiation, as the explo-

sion shock interacts with a dense external shell, while the

rapid time-scale is attributed to the relatively compact radius

of the shell. (ii) Models involving prolonged energy injec-

tion from a central compact object, such as a magnetar with

a millisecond rotation period (Yu et al. 2013; Metzger & Piro

2014; Hotokezaka et al. 2017), an accreting neutron star (NS;

e.g. following a WD-NS merger; Margalit & Metzger 2016),

or an accreting stellar-mass (Kashiyama & Quataert 2015) or

supermassive black hole (BH e.g., Strubbe & Quataert 2009;

Cenko et al. 2012a).

Until recently, progress in understanding the intrinsic na-

ture of FBOTs was hampered by their low discovery rate and

typically large distances (

≥

500 Mpc), which limited op-

portunities for spectroscopic and multi-wavelength follow-up

observations. Here we present extensive radio-to-

-ray ob-

servations of the Astronomical Transient AT 2018cow over

its first

∼

100 days of evolution. AT 2018cow was discov-

ered on June 16, 2018 by the ATLAS survey as a rapidly

evolving transient located within a spiral arm of the dwarf

star-forming galaxy CGCG 137-068 at 60 Mpc (Smartt et al.

SPHERICAL

OW WITH A

ENTRAL

NGINE

2018; Prentice et al. 2018). Prentice et al. (2018); Perley

et al. (2018); Rivera Sandoval et al. (2018) and Kuin et al.

(2018) presented the UV/optical/NIR and soft X-ray proper-

ties of AT 2018cow (as detected by

Swift

) in the first

∼

days since discovery. We present our UV/optical/NIR pho-

tometry and spectroscopy in §2.1 and §2.2. Broad-band soft-

to-hard X-ray data from coordinated follow up with INTE-

GRAL, NuSTAR,

Swift

-XRT and XMM are presented and

analyzed in §2.3, 2.4 and §2.5, while our radio observations

with VLA and VLBA are described in §2.6. We present the

search for prompt

-ray emission from AT 2018cow with the

Inter-Planetary Network in §2.7. In §3 we derive multi-band

inferences on the physical properties of AT 2018cow and we

discuss the intrinsic nature of AT 2018cow in §4. We con-

clude in §5.

Uncertainties are provided at the 1

confidence level (c.l.)

and we list 3

c.l. upper limits unless explicitly stated oth-

erwise. Throughout the paper we refer times to the time of

optical discovery, which is 2018-06-16 10:35:02 UTC, cor-

responding to MJD 58285.44 (Smartt et al. 2018; Prentice

et al. 2018). AT 2018cow is located in the host galaxy CGCG

137-068 (

= 0

0141) and we adopt a distance of 60 Mpc as

in Smartt et al. (2018); Prentice et al. (2018); Perley et al.

(2018). We assume

= 0

Ω

= 0

Ω

Λ

= 0

OBSERVATIONS AND DATA ANALYSIS

2.1.

UV-Optical-NIR Photometry

The UV Optical Telescope (UVOT, Roming et al. 2005)

on board the Neil Gehrels

Swift

Observatory (Gehrels et al.

2004) started observing AT 2018cow on 2018 June 19 (

∼

days since discovery) with six filters

2 and

in the wavelength range

= 1928 Å (

filter) –

= 5468 Å

(

filter, central wavelength). We extracted aperture photom-

etry following standard prescriptions by Brown et al. (2009),

with the updated calibration files and revised zero points by

Breeveld et al. (2011). Each individual frame has been vi-

sually inspected and quality flagged. Observations with in-

sufficient exposure time have been merged to obtain higher

signal-to-noise ratio (S/N) images from which we extracted

the final photometry. We used a 3

′′

source region of extrac-

tion to minimize the effects of the contamination from the

underlying host-galaxy flux and we manually corrected for

imperfections of the astrometric solution of the automatic

UVOT pipeline re-aligning the frames. In the absence of

template images, we estimated the host galaxy contribution

by measuring the host galaxy emission at a similar distance

from the nucleus. The results from our method are in ex-

cellent agreement with Perley et al. (2018). We note that at

50 days this method is likely to overestimate the UV flux

of the transient, as the images show the presence of a bright

knot of UV emission underlying AT 2018cow that can only

be properly accounted for with template images obtained in

the future.

Ground-based optical photometry has been obtained from

ANDICAM, mounted on the 1.3-m telescope

at Cerro

Tololo Interamerican Observatory (CTIO), the Low Resolu-

tion Imaging Spectrometer (LRIS; Oke et al. 1995), and the

DEep Imaging Multi-Object Spectrograph (DEIMOS; Faber

et al. 2003), mounted on the Keck telescopes. Images from

the latter were reduced following standard bias and flat-field

corrections. Data from ANDICAM, instead, came already

reduced by their custom pipeline

. Instrumental magnitudes

were extracted using the point-spread-function (PSF) fitting

technique, performed using the SNO

package. Abso-

lute calibrations were achieved measuring zero points and

color terms for each night, estimated using as reference the

magnitudes of field stars, retrieved from the Sloan Digital

Sky Survey

(SDSS; York et al. 2000) catalog (DR9). SDSS

magnitudes of the field stars were then converted to John-

son/Cousins, following Chonis & Gaskell (2008). Our

BVRI

PSF photometry agrees well with the host-galaxy subtracted

photometry presented by Perley et al. (2018).

We obtained near-IR imaging observations in the

JHK

bands with the Wide-field Camera (WFCAM; Casali et al.

2007) mounted on the 3.8-m United Kingdom Infrared Tele-

scope (UKIRT) spanning

∼

−

42 days. We obtained

pre-processed images from the WFCAM Science Archive

(Hamly et al. 2008) which are corrected for bias, flat-field,

and dark current by the Cambridge Astronomical Survey

Unit

. For each epoch and filter, we co-add the images and

perform astrometry relative to 2MASS using a combination

of tasks in Starlink

and

IRAF

. For

-band, we obtain a

template image from the UKIRT Hemispheres Survey DR1

(Dye et al. 2018), and use the HOTPANTS software pack-

age (Becker 2015) to perform image subtraction against this

template to produce residual images. We perform aperture

photometry using standard tasks in

IRAF

, photometrically

calibrated to 2MASS. In the absence of a template image in

and

-bands, we performed aperture photometry of the

transient and host galaxy complex centered on the core of the

host galaxy. We used standard procedures in

IRAF

and 2

full-width half-maximum apertures. At

15 days the host

galaxy contribution is negligible, but dominates the

pho-

tometry at

30 days. Single epochs of

JHK

-band photom-

etry were obtained 2018 June 26 (

∼

86 days) using the

WIYN High-resolution Infrared Camera (WHIRC; Meixner

et al. 2010) mounted on the 3.5 m WIYN telescope, and 2018

Operated by the SMARTS Consortium.

https://github.com/SMARTSconsortium/ANDICAM

Cappellaro, E. (2014).

SNO

PY: a package for SN photometry,

http://sngroup.oapd.inaf.it/snoopy.html

http://www.sdss.org

http://casu.ast.cam.ac.uk/

http://starlink.eao.hawaii.edu/starlink

ARGUTTI ET AL

18cow

decay

SLSNe

(a)

(b)

(c)

(d)

Swift

Ground

10 arcsec

18cow

δ

t=62.3 d

Figure 1

Panel (a)

: AT 2018cow maintains observed blue colors (

−

)

0 until late times, while the UV/optical/NIR flux rapidly decays.

Panel (b)

: Filled circles: extinction-corrected, host-galaxy subtracted flux densities derived from

Swift

-UVOT observations. Filled squares:

extinction-corrected flux densities derived from our CTIO photometry (

BVRI

50 days), Keck photometry (

BVRI

50 days),

UKIRT and WIYN photometry (

JHK

). For the NIR bands, empty symbols mark the times when significant contamination from the host

galaxy emission is present.

Inset:

RGB false-color image of AT 2018cow and its host galaxy obtained on 2018 August 17 with DEIMOS

mounted on Keck-II.

Panels (c-d)

: optical light-curve properties of AT 2018cow in the context of other stellar explosions and FBOTs from the

literature. AT 2018cow shows an extremely rapid rise time of a few days (as constrained by Perley et al. 2018; Prentice et al. 2018), and a decay

significantly faster than

Ni-powered decays (orange dashed line in panel(d)). AT 2018cow rivals in luminosity the most luminous

normal

SNe

in the

-band (d-panel), but it is more luminous than some SLSNe when its bolometric output is considered (panel (c)) due to its remarkably

blue colors. Following Gal-Yam (2012), we show in panel (d) prototypical events for each class: PTF 09cnd (SLSN-I, Quimby et al. 2011),

SN 2006gy (SLSN-II), “Nugent template” for normal type-Ia SN, SN 2005cl (SN IIn, Kiewe et al. 2012), the average type Ibc light curve from

Drout et al. (2011), SN 2011dh (SN IIb, Arcavi et al. 2011; Soderberg et al. 2012), and the prototypical type II-P SN 1999em (Leonard et al.

2002). Other references: Hamuy (2003); Campana et al. (2006); Taubenberger et al. (2006); Valenti et al. (2008); Botticella et al. (2009); Cobb

et al. (2010); Kasliwal et al. (2010); Ofek et al. (2010); Poznanski et al. (2010); Andrews et al. (2011); Chomiuk et al. (2011); Arcavi et al.

(2012); Bersten et al. (2012); Valenti et al. (2012); Drout et al. (2013); Inserra et al. (2013); Lunnan et al. (2013); Drout et al. (2014); Margutti

et al. (2014); Vinkó et al. (2015); Nicholl et al. (2016); Arcavi et al. (2016); Pursiainen et al. (2018).

August 31 (

∼

7 days) with the MMT and Magellan In-

frared Spectrograph (MMIRS; McLeod et al. 2012), mounted

on the MMT telescope. These data were processed using

similar methods. AT 2018cow is not detected against the

host-galaxy NIR background in our final observation. After

subtracting the bright sky contribution we estimated the in-

strumental NIR magnitudes via PSF-fitting. We calibrate our

NIR photometry relative to 2MASS

(Skrutskie et al. 2006).

No color term correction was applied to the NIR data.

UV, optical, and NIR photometry have been corrected

for Galactic extinction with

(

−

) = 0

07 mag, (Schlafly

http://www.ipac.caltech.edu/2mass/

& Finkbeiner 2011) and no extinction in the host galaxy.

Our final photometry is presented in Tables A7,A8,A9,A10.

The UV/optical/NIR emission from AT 2018cow is shown in

Fig. 1.

2.2.

Optical and NIR Spectroscopy

We obtained 5 spectra of AT 2018cow using the Goodman

spectrograph (Clemens et al. 2004) mounted on the SOAR

telescope in the time range

∼

−

2 days. We used the

red camera and the 400 lines mm

−

and 600 lines mm

−

grat-

ings, providing a resolution of

∼

5 Å and

∼

3 Å at 7000 Å, re-

spectively. We reduced Goodman data following usual steps

including bias subtraction, flat fielding, cosmic ray rejection

(see van Dokkum 2001), wavelength calibration, flux cali-

SPHERICAL

OW WITH A

ENTRAL

NGINE

Figure 2

. Optical spectral evolution of AT 2018cow (

left panel

), with a zoom-in to the H

region of the spectrum in velocity space (

right panel

20 days the spectrum exhibits only extremely broad features with

∼

, in addition to narrow emission lines from the host galaxy.

20 days He I and H features start to develop with velocities of a few 1000 km s

−

and a redshifted line profile. In the H

panel on the

right, we clipped the strong narrow line emission from the host galaxy in our latest spectrum at

= 85

8 days for display purposes.

bration, and telluric correction using our own custom

IRAF

routines.

On 2018 July 9 (

∼

4 days), we acquired a spec-

trum with the Low Dispersion Survey Spectrograph (LDSS3)

mounted on the 6.5 m Magellan Clay telescope using the

VPH-all grism and a 1

′′

slit. We obtained a spectrum with the

Inamori-Magellan Areal Camera and Spectrograph (IMACS)

mounted on the 6.5 m Magellan Baade telescope on 2018

August 6 (

∼

51 days), using the f/4 camera and 300 l/mm

grating with a 0.9

′′

slit. The data were reduced using stan-

dard procedures in

IRAF

and

PyRAF

to bias-correct, flat-

field, and extract the spectrum. Wavelength calibration was

achieved using HeNeAr comparison lamps, and relative flux

calibration was applied using a standard star observed with

the same setup.

We observed AT 2018cow on 2018 August 29 (

∼

IRAF

is distributed by the National Optical Astronomy Observatories,

which are operated by the Association of Universities for Research in As-

tronomy, Inc., under cooperative agreement with the National Science Foun-

dation.

74 days) with DEIMOS. We used a 0.7

′′

slit and the

600 lines mm

−

grating with the GG400 filter, resulting in

∼

3 Å resolution over the range 4500

−

8500 Å. We ac-

quired a spectrum with LRIS on 2018 September 9 (

∼

85.8

days). We used the 1.0

′′

slit with the 400 lines mm

−

grat-

ing, achieving a resolution of

∼

6 Å and spectral coverage

of 3200–9000 Å. Due to readout issues, we lost a portion of

the spectrum between 5800 and 6150 Å. Reduction of these

spectra were done using standard

IRAF

routines for bias sub-

traction and flat-fielding. Wavelength and flux calibration

were performed comparing the data to arc lamps and stan-

dard stars respectively, acquired during the night and using

the same setups. A final epoch of

BVRI

photometry was ac-

quired with LRIS on 2018 October 5 (

∼

112 days).

We acquired one epoch of low-resolution NIR spec-

troscopy spanning 0

−

m with the MMT using

MMIRS on 2018 July 3 (

∼

6 days). Observations were

performed using a 1

′′

slit width in two configurations: zJ

filter (0.95 - 1.50

m) + J grism (

∼

2000), and HK3 fil-

ter (1.35-2.3

m) + HK grism (

∼

1400). For each of the

configurations the total exposure time was 1800 s, and the

slit was dithered between individual 300 s exposures. We

ARGUTTI ET AL

1.2

1.4

1.6

1.8

2.2

MMT+MMIRS

2018-07-03

+ constant

rest wavelength [

1.05

1.1

1.15

He I 1.083

휹

t~16.6 days

μ

μ

Figure 3

. A NIR spectrum of AT 2018cow acquired at

∼

17 days

shows the emergence of He

emission with a characteristic red-

shifted line profile, as observed at optical wavelengths (Fig. 2). The

gray bands mark regions of strong telluric absorption.

used the standard MMIRS pipeline (Chilingarian et al. 2015)

to process the data and to develop wavelength calibrated 2D

frames from which 1D extractions were made.

Figures 2–3 show our spectral series. These figures show

the drastic evolution of AT 2018cow from an almost feature-

less spectrum around optical peak with very broad features,

to the clear emergence of H and He emission with asymmet-

ric line profiles skewed to the red and significantly smaller

velocities of a few 1000 km s

−

. In Table A1 we summarize

our NIR/optical spectroscopic observations.

2.3.

Soft X-rays: Swift-XRT and XMM

The X-Ray Telescope (XRT) on board the Neil Gehrels

Swift Observatory (Gehrels et al. 2004; Burrows et al. 2005)

started observing AT 2018cow on 2018 June 19 (

∼

3 days

following discovery). We reduced the

Swift

-XRT data with

HEAsoft v. 6.24 and corresponding calibration files, apply-

ing standard data filtering as in Margutti et al. (2013a). A

bright X-ray source is detected at the location of the optical

transient, with clear evidence for persistent X-ray flaring ac-

tivity on timescales of a few days (§ 2.9), superimposed on

an overall fading of the emission (Fig. 4).

A time-resolved spectral analysis reveals limited spectral

evolution. Fitting the 0.3–10 keV data with an absorbed

power-law model within XSPEC, we find that the XRT spec-

tra are well described by a photon index

∼

5 and no evi-

dence for intrinsic neutral hydrogen absorption (Fig. 4, upper

panel). We employ Cash statistics and derive the parame-

ter uncertainties from a series of MCMC simulations. We

Figure 4

. Temporal evolution of AT 2018cow at soft (black, 0.3-

10 keV) and hard (orange and red, 20-200 keV) X-ray energies, as

captured by

Swift

-XRT, XMM, NuSTAR and INTEGRAL. Soft X-

rays are well modeled with a power-law spectrum with photon index

∼

5 and limited temporal evolution (upper panel). Above

∼

keV an additional transient spectral component appears at

days. Orange dots: total luminosity in the 20-200 keV band. Red

stars: contribution of the additional hard X-ray energy component

above the extrapolation of the power-law component from lower

energies. Dashed gray lines: reference

−

and

−

power-law decays

to guide the eye.

adopt a Galactic neutral hydrogen column density in the di-

rection of AT 2018cow

= 0

−

(Kalberla

et al. 2005). With a different method based on X-ray after-

glows of GRBs, Willingale et al. (2013) estimate

−

. In particular, the earliest XRT spectrum

extracted between 3

−

5 days since discovery can be fitted

with

= 1

05 and can be used to put stringent con-

straints on the amount of neutral material in front of the

emitting region, which is

int

−

(we adopt

solar abundances from Asplund et al. 2009 within XSPEC).

The material is thus either fully ionized or absent (§ 3.3.2).

The results from the time-resolved

Swift

-XRT analysis are

reported in Table A2.

The total XRT spectrum collect-

ing data in the time interval 3–60 days can be fitted with

an absorbed power-law with

= 1

04 and

int

−

. From this spectrum we infer a 0.3–10 keV

count-to-flux conversion factor of 4

−

erg cm

−

(absorbed), 4

−

erg cm

−

(unabsorbed), which we

use to flux-calibrate the XRT light-curve (Fig. 4). At the dis-

tance of

∼

60 Mpc, the inferred 0.3–10 keV isotropic X-ray

luminosity at 3 days is

∼

erg s

−

. AT 2018cow is sig-

nificantly more luminous than normal SNe and shows a lumi-

nosity similar to that of low-luminosity GRBs (Fig. 5). The

spectrum also shows evidence for positive residuals above

∼

8 keV, which are connected to the hard X-ray component

of emission revealed by NuSTAR and INTEGRAL (§2.4).

We triggered deep XMM observations of AT 2018cow on

SPHERICAL

OW WITH A

ENTRAL

NGINE

Figure 5

. X-ray emission from AT 2018cow (red circles) in the con-

text of long GRBs at cosmological distances (shades of gray), long

GRBs in the local Universe (shades of blue), tidal disruption events

(TDEs, orange diamonds), and normal core-collapse SNe (black ar-

row and circle), which later show

erg s

−

. The upper lim-

its on the X-ray emission from the very rapidly declining type-Ic

SN 2005ek and the fast-rising and luminous transient “Dougie” are

marked with empty circles. AT 2018cow is significantly more lumi-

nous than normal SNe and competes in luminosity with local GRBs.

References: Margutti et al. (2013a,b); Drout et al. (2013); Vinkó

et al. (2015); Margutti et al. (2017); Ross & Dwarkadas (2017);

Eftekhari et al. (2018).

2018 July 23 (

∼

5 days, exposure time

∼

32 ks, imag-

ing mode, PI Margutti), in coordination with our NuSTAR

monitoring. We reduced and analyzed the data of the Eu-

ropean Photon Imaging Camera (EPIC)-pn data using stan-

dard routines in the Scientific Analysis System (SAS ver-

sion 17.0.0) and the relative calibration files, and used MOS1

data as a validation check. After filtering data for high back-

ground contamination the net exposure times are 24.0 and

31.5 ks for the pn and MOS1, respectively. An X-ray source

is clearly detected at the position of the optical transient. We

extracted a spectrum from a circular region of 30

′′

radius cen-

tered at the source position. Pile-up effects are negligible as

we verified with the task

epatplot

. The background was

extracted from a source-free region on the same chip. We

estimate a 0.3–10 keV net count rate of 0

519

005 c/s.

The X-ray data were grouped to a minimum of 15 counts

per bin. The 0.3–10 keV spectrum is well fitted by an ab-

sorbed power-law model with best-fitting

= 1

02 and

marginal evidence for

int

∼

−

at the 3

c.l.

for

= 0

−

. Given that the uncertainty on

is also

∼

−

, we consider this value as

an upper limit on

int

at 36.5 days.

We acquired a second epoch of deep X-ray observations

with XMM on 2018 September 6 (

∼

82 days, PI Margutti).

The net exposure times are 30.5 ks and 36.8 ks, for the pn and

MOS1, respectively. AT 2018cow is clearly detected with net

0.3–10 keV count-rate (6

−

c s

−

. We used a

source region of 20

′′

to avoid contamination by a faint unre-

lated source located 36.8

′′

south-west from our target (at ear-

lier times AT 2018cow is significantly brighter and the con-

tamination is negligible). The spectrum of AT 2018cow is

well fitted by a power-law model with

= 1

20 with

unabsorbed 0.3–10 keV flux

∼

−

erg cm

−

. We

find no evidence for intrinsic neutral hydrogen absorption.

Finally we note that comparing the two XMM observations,

we find no evidence for a shift of the X-ray centroid, from

which we conclude that X-ray emission from the host galaxy

nucleus, if present, is subdominant and does not represent

a significant source of contamination. The complete 0.3–10

keV X-ray light-curve of AT 2018cow is shown in Fig. 4.

2.4.

Hard X-rays: NuSTAR and INTEGRAL

INTEGRAL started observing AT 2018cow on 2018 June

22 18:38:00 UT until July 8 04:50:00 UT (

∼

−

22 days)

as part of a public target of opportunity observation. The total

on-source time is

∼

900 ks (details are provided in Table A3).

A source of hard X-rays is clearly detected at the location of

AT 2018cow at energies

∼

−

100 keV with significance 7.2

∼

6 days. The source is no longer detected at

days (Fig. 6). After reconstructing the incident photon en-

ergies with the latest available calibration files, we extracted

the hard X-ray spectrum from the ISGRI detector (Lebrun

et al. 2003) on the IBIS instrument (Ubertini et al. 2003)

of INTEGRAL (Winkler et al. 2003) for each of the

∼

2 ks

long individual pointing of the telescope dithering around

the source. We used the Off-line Scientific Analysis Soft-

ware (OSA) with a sky model comprising only AT 2018cow,

which is the only significant source in the field of view. The

energy binning was chosen to have 10 equally spaced loga-

rithmic bins between 25 and 250 keV, the former being the

current lower boundary of ISGRI energy window. We com-

bined the spectra acquired in the same INTEGRAL orbit. We

use these spectra in §2.5 to perform a time-resolved broad-

band X-ray spectral analysis of AT 2018cow.

We acquired a detailed view of the hard X-ray properties

of AT 2018cow between 3–80 keV with a sequence of four

NuSTAR observations obtained between 7.7 and 36.5 days

(PI Margutti, Table A4). The NuSTAR observations were

processed using

NuSTARDAS

v1.8.0 along with the NuS-

TAR CALDB released on 2018 March 12. We extracted

source spectra and light curves for each epoch using the

nuproducts

FTOOL using a 30

′′

extraction region cen-

troided on the peak source emission. For the background

ARGUTTI ET AL

Figure 6

. Broad-band X-ray spectral evolution of AT 2018cow. Co-

ordinated observations of

Swift

-XRT, XMM, INTEGRAL and NuS-

TAR revealed the presence of a hard X-ray emission component that

dominates the spectrum above

∼

15 keV at early times

15 days

(dashed light-blue line in panel (a)). The hard X-ray component

later subsides. At

15 days the broad-band X-ray spectrum is

well described by an absorbed power-law with negligible intrinsic

absorption (thick blue line in panels (b)-(d)). Black dotted line:

early time hard X-ray “bump”. Inset of panel (a): zoom-in into the

region of positive residuals around 6-10 keV.

spectra and light curves we extracted the data from a larger

region (

∼

′′

) located on the same part of the focal plane.

We produced response files (RMFs and ARFs) for each FPM

and for each epoch using the standard

nuproducts

flags

for a point source.

AT 2018cow is well detected at all epochs. The first NuS-

TAR spectrum at 7.7 days shows a clear deviation from a pure

absorbed power-law model with

∼

5, and reveals instead

the presence of a prominent excess of emission above

∼

keV, which matches the level of the emission captured by

INTEGRAL, together with spectral features around 7–9 keV.

By day 16.5 the hard X-ray bump of emission disappeared

and the spectrum is well modeled by an absorbed power-law

(Fig. 6). We model the evolution of the broad-band X-ray

spectrum as detected by

Swift

-XRT, XMM, NuSTAR, and

INTEGRAL in §2.5.

2.5.

Joint soft X-ray and hard X-ray spectral analysis

Our coordinated

Swift

-XRT, XMM, NuSTAR and INTE-

GRAL monitoring of AT 2018cow allows us to extract five

epochs of broad-band X-ray spectroscopy (

∼

0.3–100 keV)

from 7.7 days to 36.5 days. We performed joint fits of data

acquired around the same time, as detailed in Table A5. Our

results are shown in Fig. 6. We find that the soft X-rays at

energies

7 keV are always well described by an absorbed

simple power-law model with photon index

≈

1.5–1.7 with

no evidence for absorption from neutral material in addition

to the Galactic value. Our most constraining limits from the

time-resolved analysis are

int

−

04)

−

(Table A5).

Remarkably, at

∼

7.7 days, NuSTAR and INTEGRAL data

15 keV reveal the presence of a prominent component

of emission of hard X-rays that dominates over the power-law

component.

We model the hard X-ray emission component

with a strongly-absorbed cutoff power-law model (light-blue

dashed line in Fig. 6, top panel). This is a purely phenomeno-

logical model that we use to quantify the observed properties

of the hard emission component. A cutoff power-law is pre-

ferred to a simple power-law model, as a simple power-law

would overpredict the highest energy data points at 7.7 days.

From this analysis the luminosity of the hard X-ray compo-

nent at

∼

7 days is

hard

∼

erg s

−

(20–200 keV).

A joint analysis of

Swift

-XRT+INTEGRAL data at

∼

days indicates that the component of hard X-ray emission

became less prominent, and then disappeared below the level

of the soft X-ray power-law by

∼

5 days, as revealed

by the coordinated

Swift

-XRT, XMM and NuSTAR monitor-

ing (Fig. 6). We derive upper limits on the luminosity of

the undetected hard X-ray emission component at

≥

It is interesting to note in this respect the faint hard X-ray emission

detected by

Swift

-BAT in the first 15 days, with flux consistent with the

NuSTAR observations, see Fig. 1 in Kuin et al. (2018).